Nutrition of Florida

Citrus Trees

Second Edition

Edited by Thomas A. Obreza 

and Kelly T. Morgan

SL 253

1

SL 253

The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, 

educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, 

or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. 

Florida Cooperative Extension Service/Institute of Food and Agricultural Sciences/University of Florida/Larry Arrington, Dean.

Nutrition of Florida 

Citrus Trees 2nd Edition1

Thomas A. Obreza and Kelly T. Morgan2, Editors

1. This document is SL 253, a publication of the Soil and Water Science Department, Florida Cooperative Extension Service, Institute of Food 

and Agricultural Sciences, University of Florida. Original publication date: January 2008. The first edition of this document was SP 169, 

Nutrition of Florida Citrus Trees. Visit the EDIS Web Site at http://edis.ifas.ufl.edu.

2. Thomas A. Obreza, Professor, Soil and Water Science Department; Kelly T. Morgan, Assistant Professor, Soil and Water Science Department, 

Southwest Florida Research and Education Center – Immokalee; Florida Cooperative Extension Service, Institute of Food and Agricultural 

Sciences, University of Florida, Gainesville, 32611.
2 

SL 253 Nutrition of Florida Citrus Trees, 2nd Edition.

Edited by Thomas A. Obreza and Kelly T. Morgan.

This publication replaces UF-IFAS SP 169.

COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORIDA, INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES, Larry R. Arrington, Director, in cooperation with the United States Department of Agriculture, 

publishes this information to further the purpose of the May 8 and June 30, 1914 Acts of Congress; and is authorized to provide research, educational information, and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political 

opinions, or affiliations. This information was published January 2008 as SL 253, Florida Cooperative Extension Service.

Dedication

The first edition of this publication was dedicated to emeritus citrus research scientists Drs. Robert C. J. Koo, Ivan Stewart, and Herman Reitz, Univ. of Florida, Citrus Research and Education Center (CREC), Lake Alfred and Paul Smith, 

USDA-Agricultural Research Service (ARS), Orlando. Their research contributions laid the foundation for citrus 

nutrition programs in Florida. The editors of the second edition dedicate it to an additional group of emeritus citrus research scientists, extension specialists, and educators: Drs. David P. H. Tucker, Larry K. Jackson, and T. Adair Wheaton, 

CREC-Lake Alfred; Dr. David V. Calvert, Indian River Research and Education Center, Ft. Pierce; and Dr. Heinz 

Wutscher, USDA-ARS, Orlando. Their many years of devoted service furthered our knowledge of nutrient management 

as the Florida citrus industry entered the era of Best Management Practices.

Contributing Authors

L. Gene Albrigo University of Florida, Citrus Research and Education Center

Brian J. Boman University of Florida, Indian River Research and Education Center

Mary E. Collins University of Florida, Soil and Water Science Department

James J. Ferguson University of Florida, Horticultural Sciences Department

Stephen H. Futch University of Florida, Multi-County Citrus Extension

Edward A. Hanlon University of Florida, Southwest Florida Research and Education Center

Kelly T. Morgan University of Florida, Southwest Florida Research and Education Center

Thomas A. Obreza University of Florida, Soil and Water Science Department

Lawrence R. Parsons University of Florida, Citrus Research and Education Center

Arnold W. Schumann University of Florida, Citrus Research and Education Center

Mongi Zekri University of Florida, Multi-County Citrus Extension

With Additional Contributions By

Ashok K. Alva US Department of Agriculture – Agricultural Research Service

David V. Calvert University of Florida, Indian River Research and Education Center (retired)

Frederick S. Davies University of Florida, Horticultural Sciences Department

Larry K. Jackson University of Florida, Horticultural Sciences Department (retired)

David P. H. Tucker University of Florida, Citrus Research and Education Center (retired)

T. Adair Wheaton University of Florida, Citrus Research and Education Center (retired)

Heinz Wutscher US Department of Agriculture – Agricultural Research Service (retired)

Acknowledgements

The editors gratefully acknowledge the reviewers of this publication (Mark Colbert, Fred Davies, Hugh English, John 

Jackson, Chris Oswalt, Mark Ritenour, Steve Smith, Jerry Southwell, Jack Zorn, and all contributing authors) for their 

time, effort, and suggestions for improvement.

Funding provided by the Florida Department of Agriculture and Consumer Services to print this document is also 

gratefully acknowledged.

Front and rear cover photos by Thomas S. Wright.
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1. Introduction – Thomas A. Obreza and Kelly T. Morgan ................................................................................................................................................................7

1.1. Preface ......................................................................................................................................................................................................................................................................................7

1.2. Nutrition of Florida citrus trees – A historical perspective ..........................................................................................................................................7

1.3. Florida enters the BMP era ...............................................................................................................................................................................................................................8

2. Production Areas, Soils, and Land Preparation – Thomas A. Obreza and Mary E. Collins ...............................................................9

2.1. General information .................................................................................................................................................................................................................................................9

2.2. Characteristics of soil orders important to Florida citrus production ..............................................................................................................9

2.3. Soil series typically found in citrus groves ........................................................................................................................................................................................9

2.4. Soil physical and chemical properties important to citrus nutrient management ............................................................................11

2.5. Vulnerable soils ...........................................................................................................................................................................................................................................................12

2.6. Grove site selection and land preparation .....................................................................................................................................................................................12

2.7. Effects of leveling and bedding on soil fertility in flatwoods citrus groves ...............................................................................................14

2.8. Soil pH and liming ..................................................................................................................................................................................................................................................14

2.9. Organic matter ............................................................................................................................................................................................................................................................15

3. General Soil Fertility and Citrus Tree Nutrition – Thomas A. Obreza, Mongi Zekri, and Stephen H. Futch ...............16

3.1. Essential nutrients ....................................................................................................................................................................................................................................................16

3.2. Macronutrients and citrus production ..............................................................................................................................................................................................16

3.3. Micronutrients and citrus production ..............................................................................................................................................................................................17

3.4. Supplying nutrients to citrus trees .........................................................................................................................................................................................................18

3.5. Nutrient behavior in Florida soils ...........................................................................................................................................................................................................18

3.6. Citrus nutrient requirements ......................................................................................................................................................................................................................20

3.7. Nutrition, irrigation, and fruit quality ..............................................................................................................................................................................................21

3.8. Grove management practices ......................................................................................................................................................................................................................22

3.9. Interactions of nutrition with other grove practices ..........................................................................................................................................................22

4. Soil and Leaf Tissue Testing – Thomas A. Obreza, Mongi Zekri, and Edward A. Hanlon ..................................................................24

4.1. Introduction ...................................................................................................................................................................................................................................................................24

4.2. Benefits of leaf analysis .......................................................................................................................................................................................................................................24

4.3. Steps in leaf sampling ...........................................................................................................................................................................................................................................25

4.4. Benefits of soil analysis ........................................................................................................................................................................................................................................26

4.5. Steps in soil sampling ............................................................................................................................................................................................................................................28

4.6. Traditional vs. alternative sampling strategies ...........................................................................................................................................................................30

4.7. Summary ............................................................................................................................................................................................................................................................................32

4.8. Soil and leaf tissue analysis checklist ...................................................................................................................................................................................................32

5. Precision Agriculture for Citrus Nutrient Management – Arnold W. Schumann and Edward A. Hanlon ....................33

5.1. Common elements of precision agriculture ................................................................................................................................................................................33

5.2. Objectives of precision agriculture for nutrient management ................................................................................................................................33

5.3. Remote sensing ...........................................................................................................................................................................................................................................................33

5.3.1. Field measurements of soil electrical conductivity .............................................................................................................................................33

5.3.2. Ultrasonic canopy measurements ..........................................................................................................................................................................................34

5.3.3. Citrus yield mapping ...........................................................................................................................................................................................................................35

5.4. Precision nutrient application – Variable rate fertilization ........................................................................................................................................35

5.5. Summary ............................................................................................................................................................................................................................................................................37

Table of Contents
4 

6. Fertilizer Sources and Formulations – Thomas A. Obreza and Brian J. Boman .............................................................................................38

6.1. Introduction ...................................................................................................................................................................................................................................................................38

6.2. Solid sources for soil application .............................................................................................................................................................................................................38

6.3. Solid N fertilizer sources and ammonia volatilization .....................................................................................................................................................39

6.4. Solution sources – Fertigation ...................................................................................................................................................................................................................39

6.5. Solution fertilizer salt-out ...............................................................................................................................................................................................................................40

6.6. Solution sources – Foliar sprays ................................................................................................................................................................................................................40

6.7. Suspension sources ..................................................................................................................................................................................................................................................41

6.8. Slow-release sources ...............................................................................................................................................................................................................................................41

6.9. Controlled-release sources ..............................................................................................................................................................................................................................41

6.10. Formulating fertilizer products .................................................................................................................................................................................................................42

6.11. Nutrient sources for organic citrus production .......................................................................................................................................................................42

7. Methods of Fertilizer Application – Thomas A. Obreza, Brian J. Boman, Mongi Zekri, and Stephen H. Futch .........43

7.1. Applying dry fertilizers .......................................................................................................................................................................................................................................43

7.2. Fertigation ........................................................................................................................................................................................................................................................................43

7.3. Applying suspension fertilizers ..................................................................................................................................................................................................................46

7.4. Applying foliar fertilizers .................................................................................................................................................................................................................................46

8. Recommended Fertilizer Rates and Timing – 

Thomas A. Obreza, Kelly T. Morgan, L. Gene Albrigo, and Brian J. Boman ............................................................................................................48

8.1. Fertilizer rates, application frequency, and timing for non-bearing trees (first 3 years in the grove) ...........................48

8.1.1. Solid plantings ............................................................................................................................................................................................................................................48

8.1.2. Resets in established groves ..........................................................................................................................................................................................................50

8.1.3. Example fertilizer program for non-bearing trees ................................................................................................................................................50

8.2. Fertilizer rates for bearing trees (4+ years in grove) ...........................................................................................................................................................50

8.3. Timing and frequency of fertilizer application for bearing trees .........................................................................................................................56

8.4. Slow-release and controlled-release fertilizers ...........................................................................................................................................................................56

8.5. Foliar application of N, P, and K .............................................................................................................................................................................................................57

9. Irrigation Management to Improve Nutrient Uptake – Kelly T. Morgan and Thomas A. Obreza ............................................60

9.1. Water supply ...................................................................................................................................................................................................................................................................60

9.2. Production region characteristics important to irrigation management .....................................................................................................60

9.3. Nutrient uptake efficiency ..............................................................................................................................................................................................................................60

9.4. Allowable soil water depletion ...................................................................................................................................................................................................................60

9.5. Irrigation scheduling .............................................................................................................................................................................................................................................60

9.5.1. Soil moisture measurement ..........................................................................................................................................................................................................60

9.5.2. Water budgeting .......................................................................................................................................................................................................................................61

9.6. Irrigation strategies to improve nutrient uptake and reduce leaching ............................................................................................................61

10. Environmental Issues and Best Management Practices (BMPs) – 

Brian J. Boman, Thomas A. Obreza, and Kelly T. Morgan ..........................................................................................................................................................64

10.1. Environmental nutrient issues related to Florida citrus production ................................................................................................................64

10.2. The BMP era ..................................................................................................................................................................................................................................................................64

10.3. General nutrient BMPs for citrus production ..........................................................................................................................................................................65
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11. Special Situations – Thomas A. Obreza, Brian J. Boman, 

Mongi Zekri, Stephen H. Futch, Lawrence R. Parsons, and James J. Ferguson..........................................................................................................67

11.1. Scions and rootstocks ..........................................................................................................................................................................................................................................67

11.2. Rootstock/nutrition interaction .............................................................................................................................................................................................................67

11.3. Soils high in copper ................................................................................................................................................................................................................................................67

11.4. Calcareous soils ...........................................................................................................................................................................................................................................................68

11.5. Saline soils and water ............................................................................................................................................................................................................................................69

11.6. Using reclaimed water for irrigation ....................................................................................................................................................................................................70

11.7. Fertilization strategies for damaged trees .......................................................................................................................................................................................71

11.7.1. Wind damage .........................................................................................................................................................................................................................................71

11.7.2. Freeze damage ........................................................................................................................................................................................................................................72

11.8. Nutritional deficiencies enhanced by environmental or pathological factors .......................................................................................72

11.9. Organic citrus production ..............................................................................................................................................................................................................................72

11.9.1. General information .......................................................................................................................................................................................................................72

11.9.2. Soil fertility and crop nutrient management guidelines ..........................................................................................................................73

12. References and Further Reading ..........................................................................................................................................................................................................................74

12.1. Refereed journal articles ....................................................................................................................................................................................................................................74

12.2. Non-refereed journal articles .......................................................................................................................................................................................................................75

12.3. Extension publications, articles, and books .................................................................................................................................................................................75

12.4. Historical documents ...........................................................................................................................................................................................................................................75

13. Glossary .............................................................................................................................................................................................................................................................................................76

14. Appendices ....................................................................................................................................................................................................................................................................................79

A. Physical and chemical properties of typical Florida citrus soils ...................................................................................................................................81

B. Nutrient concentrations of fertilizer materials ...............................................................................................................................................................................82

C. Salt index of fertilizer materials .......................................................................................................................................................................................................................84

D. Solubility of fertilizer sources and common fertilizer solutions .................................................................................................................................85

E. Fertilizer mixture formulation example .................................................................................................................................................................................................87

F. Example determination of the fertilizer requirement for bearing citrus trees .............................................................................................88

G. Key to citrus nutrient deficiency symptoms ......................................................................................................................................................................................89

15. Color plates ...................................................................................................................................................................................................................................................................................90
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1.1. Preface

“Nutrition of Florida Citrus Trees, 2nd Edition” is an 

update of Univ. of Florida-IFAS Bulletin SP169, which 

provided guidelines for Florida citrus fertilization since 

1995. The objective of the original edition was to provide 

background information and recommendations to develop 

a sound citrus nutrition program that optimizes financial 

returns while sustaining yields and maintaining soil and 

water quality. The second edition maintains that objective 

as it incorporates the findings of numerous citrus nutrition 

research projects conducted since the mid-1990s.

Updated guidelines presented herein reflect changes in fertilizer recommendations that have occurred as the Florida 

citrus industry has entered the era of Best Management 

Practices (BMPs). In addition to the original chapters, this 

publication adds chapters on 1) production areas and soil 

characteristics, 2) using precision agriculture to manage 

citrus nutrition, 3) irrigation and nutrient management, 

and 4) environmental issues and BMPs.

Supplemental information on subjects related to citrus 

nutrition appear in extensive appendices. Color plates 

depicting nutrient deficiencies and toxicities and a key to 

mineral deficiency symptoms in citrus are included to aid 

in visual analysis of tree nutritional status.

1.2. Nutrition of Florida citrus trees – 

A historical perspective

To maintain a viable citrus industry in Florida, growers 

must be able to economically produce large, high quality 

fruit crops. Prior to the establishment of Univ. of FloridaIFAS and USDA-ARS research programs, high production was not possible because citrus nutritional requirements were poorly understood. For example, early classical 

studies by Michael Peech and T. W. Young showed that 

Florida’s sandy soils had very low capacity to hold nutrients and water.

The first commercial citrus growers had some understanding of the need for the macronutrients nitrogen, phosphorus, and potassium. Nitrogen was generally applied using 

natural organic sources like farm animal manure and bird 

guano. Some mineral nitrogen was mined and imported 

from Chile. Phosphorus was obtained from local mines, 

and potassium was imported from Germany.

Meanwhile, worldwide studies demonstrated that plants 

needed elements in addition to nitrogen, phosphorus, and 

potassium in order to grow properly. In 1939, A. F. Camp 

and B. R. Fudge showed that secondary and micronutrients were needed to grow citrus on Florida soils. Included 

were examples of deficiency symptoms of copper, zinc, 

manganese, magnesium, boron, and iron. They indicated 

how each of the above element deficiencies could be corrected with the exception of iron. At that time, there was 

no known satisfactory fertilizer source of this element.

Other elements were later found to be necessary for 

Florida citrus. The problem of yellow spot disease was first 

reported in 1908. This disease was rather widespread and 

caused extensive defoliation and tree death. In 1951, Ivan 

Stewart and C. D. Leonard reported that this problem was 

due to molybdenum deficiency that could be corrected 

by a spray application of as little as 1 oz of sodium molybdate/acre.

Calcium is commonly thought of as a soil amendment and 

is usually applied as lime. However, when W. F. Spencer 

and R. C. J. Koo planted citrus on new land at the Citrus 

Experiment Station (now the Citrus Research and Education Center) in Lake Alfred, they did not add calcium 

to some of the plots, which resulted in stunted trees that 

showed leaf symptoms specific to calcium deficiency.

Copper deficiency of citrus limited growth and fruit production in many early Florida groves. Following discovery 

of this problem, high rates of copper were applied to trees 

in both foliar sprays and soil applications. Later, I. W. 

Wander and co-investigators found that copper was not 

taken up in abundance by the trees, nor did it leach like 

many other fertilizer elements, which resulted in its accumulation in the surface soil.

Copper accumulation interfered with iron uptake by citrus 

trees, causing leaf chlorosis and defoliation. By 1951, many 

trees were being removed due to this problem. Stewart 

and Leonard found that when organically chelated iron 

(iron-EDTA) was applied to the soil, yellow leaves on irondeficient trees re-greened.

While sulfur is essential for citrus, its deficiency has not 

been reported in Florida because it has been supplied 

through pesticide sprays and dusts, fertilizer components, 

irrigation water, and rainfall.

1. Introduction

Thomas A. Obreza and Kelly T. Morgan 

(Includes contributions by David P. H. Tucker and Larry K. Jackson)
8 

In 1954, the first Florida citrus fertilizer recommendations 

were made by a joint effort of Citrus Experiment Station 

and USDA Horticultural Laboratory scientists. Based on 

data accumulated from many years of experiments, Bulletin 536 was published. This bulletin was revised three 

times and for 41 years was the comprehensive guide for 

citrus tree nutrition. Rates and sources of eleven essential 

fertilizer elements were recommended based on results 

from field experiments.

 In the 1960s, CREC faculty recommended that growers 

change to high analysis fertilizers, thus eliminating much 

of the filler. By so doing, a great deal of the mixing cost 

was eliminated and transportation and application costs 

were reduced. Further reductions in costs were made when 

Spencer and Stewart reported that phosphorus applied to 

established groves had not leached, but had accumulated 

in an available form, resulting in reduced phosphorus 

application rates to established groves. Finally, the use of 

micronutrients was recommended only when deficiency 

symptoms persisted.

Numerous nitrogen fertilizer rate and timing studies were 

conducted by Univ. of Florida and USDA scientists for 

many years, covering a wide range of soil types, tree ages, 

varieties, rootstocks, and cultural conditions. The results 

showed N rates in excess of 200 lbs/acre were justified only 

for very productive groves. In addition, Stewart and Leonard demonstrated that excess nitrogen could reduce yield. 

Maximum production may vary greatly depending on 

other limiting conditions, but nitrogen fertilizer requirements remain similar for a range of production levels and 

conditions. As a result of these findings, Bulletin SP 169 

was published in 1995 by D. P. H. Tucker, A. K. Alva, L. 

K. Jackson, and T. A. Wheaton. This bulletin de-emphasized projected yield or yield goal as the basis to determine 

mature citrus grove nitrogen fertilizer rates in favor of a 

maximum rate of 200 lbs/acre for typical groves and 240 

lbs/acre for “exceptional” groves (defined as groves producing 700 or more boxes/acre annually).

1.3. Florida enters the BMP era

In the late 1980s, the Florida Dept. of Environmental 

Protection (FDEP) surveyed drinking water quality across 

the state and detected the nitrate form of nitrogen in 63% 

of the wells tested. The nitrate-N concentration in 15% of 

the wells was greater than the EPA drinking water standard of 10 mg/L. A large majority of the high-nitrate wells 

were located in Lake, Polk, and Highlands counties, the 

heart of Florida’s central ridge citrus production area.

Although the influence of citrus nitrogen fertilization 

on groundwater nitrate concentration was unknown, the 

combined circumstances of large citrus acreage, relatively 

high annual fertilization rates, high annual rainfall, and 

extremely inert, porous soils led the Florida Dept. of Agriculture and Consumer Services (FDACS) to implement 

a set of voluntary BMPs for nitrogen fertilization of ridge 

citrus trees designed to protect water quality. These were 

the first official nutrient BMPs for Florida citrus production. In exchange for implementing the BMP program, 

FDACS provides citrus producers with a presumption of 

compliance with water quality standards.

Subsequently, citrus production BMP manuals were written for the Indian River, Peace River, and Gulf production 

areas, and grower implementation is now taking place. 

These BMPs go beyond nutrient management to include 

irrigation and drainage management, erosion prevention, 

pesticide use, and aquatic weed control. Essentially the 

entire commercial citrus industry in Florida now has access 

to a voluntary BMP umbrella. Producing citrus under 

BMP implementation allows a grower to farm profitably 

without the threat of administrative penalties if groundwater standards are violated.

This publication provides an understanding of concepts 

and issues of nutrition that can address environmental issues and concerns about profitability of Florida citrus in a 

highly competitive global market.
9

2.1. General information

Florida’s sandy soils are not particularly favorable for management of water and agrichemicals. Most Florida citrus is 

grown on naturally infertile sands that are unable to retain 

more than a minimal amount of soluble plant nutrients 

against leaching by rainfall or excessive irrigation. Florida 

citrus soils range from well-drained Entisols on relatively 

high, rolling landscapes to poorly-drained Alfisols and 

Spodosols on low-lying flatwoods (Fig. 2.1). The root 

zone of these soils are dominated by sand and contain 

only minor quantities of silt, clay, and organic matter. This 

condition makes the management of water and nutrients a 

challenging task for grove managers.

Fig. 2.1. Distribution of soil types planted to citrus in Florida.

Well-drained soils are found through the central part of 

the Florida peninsula as far south as Highlands county on 

the central Florida ridge. Their main advantages are good 

drainage, good aeration, and a deep root zone. The main 

disadvantage is the need for frequent irrigation during 

dry periods. Soil uniformity, lack of a hardpan, and a low 

water table allow for deep, extensive root systems. Such an 

extensive rooting zone enables the tree to overcome the 

low water and nutrient-holding capacity of these soils to 

some extent.

Poorly-drained soils are found in coastal areas and on the 

flatwoods of central and south Florida. The soils on the 

east coast usually are naturally acidic with a subsurface 

hardpan, but some may have marl or shell in the profile 

that makes them alkaline. The water table is close to the 

surface and the soil may pond during the wet season. These 

soils must be drained and bedded before planting citrus. 

Their principal advantages are higher natural fertility and 

water-holding capacity. Disadvantages include poor drainage and increased alkalinity or clay content of the topsoil 

due to deposition of subsurface materials over the natural 

surface during the bedding process.

2.2. Characteristics of soil orders important 

to Florida citrus production

A soil order is the most basic category of soil classification. 

The order provides a general idea about basic physical and 

chemical characteristics of a soil. Characteristics important 

to production on Florida citrus soil orders are as follows:

•	 Entisols are sandy mineral soils low in organic matter, 

natural fertility, and water-holding capacity (Fig. 2.2). 

They have weak or no diagnostic subsurface layers and 

are well to excessively well-drained.

•	 Spodosols are sandy mineral soils low in organic matter 

and natural fertility in the surface layer (Fig. 2.3). They 

contain an acidic subsurface restrictive layer composed 

of aluminum and iron cemented together with organic 

matter.

•	 Alfisols are sandy mineral soils low in organic matter 

in the surface layer but higher in relative natural fertility compared with Spodosols (Fig. 2.4). They contain a 

subsurface layer of loamy material (a mixture of mostly 

clay and sand with little silt) that has a relatively high 

water-holding capacity.

2.3. Soil series typically found in citrus groves

A soil series is the most specific category of soil classification. There are 15 soil series that represent most of the soils 

on which Florida citrus groves have been planted (Table 

2.1). Entisols (other than Basinger) occur on high ridges 

and upland plains at an elevation greater than 100 ft above 

mean sea level (MSL) in the central ridge production area. 

Alfisols, Spodosols, and the Basinger series occur on broad, 

2. Production Areas, Soils, and Land Preparation

Thomas A. Obreza and Mary E. Collins

(Includes contributions by Larry K. Jackson, Ashok K. Alva, David P. H. Tucker, and David V. Calvert)
10 

Fig. 2.2. Candler sand, an Entisol, with surface (A) and subsurface (E) horizons. (Mary Collins)

Fig. 2.3. Myakka sand, a Spodosol, with surface (A), leached 

(E), and restrictive (Bh) horizons. (Mary Collins)

Fig. 2.4. Riviera sand, an Alfisol, with surface (A), leached (E), 

and restrictive (Bt) horizons. (Mary Collins)

Table 2.1. Common soils used for citrus production in Florida.

Series Drainage Typical location (counties)

Entisols

Astatula Excessive Polk, Highlands

Basinger Poor Highlands

Candler Excessive Polk

Tavares Moderate Polk

Spodosols

Immokalee Poor Hendry, Collier, DeSoto

Myakka Poor DeSoto, Hardee, Highlands

Oldsmar Poor Hendry, Collier

Pomona Poor Hardee

Smyrna Poor DeSoto, Hardee

Wabasso Poor St. Lucie, Indian River, Martin

Alfisols

Boca Poor Hendry, Collier

Holopaw Poor Hendry

Pineda Poor St. Lucie, Indian River, Martin, 

Collier

Riviera Poor St. Lucie, Indian River, Martin

Winder Poor St. Lucie, Indian River
11

low flat areas or in sloughs at elevations from 10 to 40 ft 

above MSL in the Gulf and Indian River production areas, 

and 35 to 100 ft above MSL in the Peace River production 

area. Some Alfisols and Spodosols can also occur in depressional areas, even though they are normally located higher 

on the landscape.

2.4. Soil physical and chemical properties 

important to citrus nutrient management

The soil on which citrus is grown greatly influences how 

irrigation water and nutrients should be managed to maximize production, minimize resource use, and protect water 

quality. Soil properties important to nutrient management 

include texture, water-holding capacity, organic matter 

content, soil pH, cation exchange capacity, and coatings 

on sand grains (Appendix A).

•	 Soil texture is the relative proportion of sand, silt, and 

clay in a mineral soil. Texture influences how much water a soil can hold against drainage by gravity and how 

quickly water will drain away. Most citrus soils contain 

94 to 98% sand in the root zone, which makes irrigation water management extremely difficult because sand 

has little capacity to hold water. If too much irrigation 

water is applied at one time, the excess will be lost below 

the root zone, which can induce nutrient leaching.

•	 Soil organic matter includes any organic carbon-based 

material, from freshly deposited plant residues to highly 

decomposed humus or compost. In their native state, 

typical citrus soils may contain as much as 5% organic 

matter, but after a grove is planted organic matter 

decreases, eventually stabilizing around 1 or 2% by the 

time the grove matures. In general, the more chronically wet a citrus soil is, the higher its organic matter 

content tends to be. Soil organic matter is rapidly lost 

by oxidation to carbon dioxide in Florida’s warm and 

humid climate, and it is not replaced in large quantities 

by citrus trees. Use of herbicides beneath tree canopies 

also decreases organic matter accumulation. In a sandy 

soil, organic matter is an extremely valuable component 

because it provides both water and nutrient-holding 

capacity, and its decomposition provides recycled nutrients to plants.

•	 Soil water-holding capacity is directly related to the 

amount of silt, clay and organic matter present. Since 

most Florida citrus soils contain only small amounts 

of these components, water-holding capacity is rarely 

greater than 1 inch per foot of soil depth, and is often 

less than 0.75 inches per foot. Low water-holding capacity soils require light and frequent irrigation to minimize nutrient leaching.

•	 Soil pH affects the availability of plant nutrients including phosphorus (P), calcium (Ca), magnesium (Mg), 

and the micronutrients. Most Florida soils are acidic in 

their native state, so they require lime applications before planting and every few years thereafter depending 

on fertilizer and irrigation water sources. The optimum 

soil pH range for citrus is 6.0 to 6.5. The pH of Florida 

citrus soils can change rapidly as a result of chemical 

reactions caused by lime or fertilizer applications. An 

exception to this principle is a calcareous soil. Some of 

the Alfisols in Table 2.1 can be calcareous due to a substratum of natural calcium carbonate rock or shell that 

dominates their chemistry. The pH of a calcareous soil 

remains relatively constant around 8.2.

•	 Cation exchange capacity (CEC) is a measure of the 

ability of the soil to hold positively charged nutrients 

like Ca, Mg, potassium (K), and ammonium (NH4

+) 

against leaching (Fig. 2.5). Generally speaking, as CEC 

increases, soil fertility increases. Soil CEC is supplied 

by clay and organic matter. Florida citrus soils are low in 

CEC, so nutrient management is difficult. The best fertilizer use efficiency can be obtained by applying mobile 

nutrients like nitrogen (N )and K frequently in small 

doses, similar to irrigation water. Entisols are the least 

fertile citrus soils, followed by Spodosols and Alfisols. 

The increased fertility of Spodosols reflects their slightly 

higher organic matter content, while the fertility of 

Alfisols is greatest because they contain some clay as 

well as organic matter.

Fig. 2.5. Example of the cation exchange reaction that occurs 

when a soil is fertilized with ammonium nitrate.
12 

•	 Coated and non-coated sands relate to the P fixation capacity of Florida soils. Fixation is important because the 

movement of P from agricultural fields to surface water 

bodies has become an environmental concern. Most 

soils nationwide have a moderate to high capacity to adsorb or hold soil P against leaching because they contain 

considerable quantities of silt and clay that provide a 

chemical mechanism to bind P. Florida soils dominated 

by quartz sand lack appreciable amounts of these silts 

and clays. However, in many cases the sand particles are 

coated with iron and/or aluminum compounds that 

also have some capacity to adsorb P.

One way to judge if coated sand grains are present is to 

observe the soil color (Fig. 2.6). Yellow, orange, or brown 

colored sand is more likely to be coated, while beige or 

white sand is not. Therefore, citrus groves on soils containing coated sands have the ability to build a soil P reserve 

following P fertilizer applications. The presence of this P 

reserve can be determined with soil testing, and P fertilization should be curtailed if high soil test P is found. 

Conversely, citrus groves on non-coated sandy soils lack 

the ability to hold P. In this case excessive P fertilization 

may induce P leaching, so P fertilizer should not be used 

indiscriminately because it may be lost to the environment.

Fig. 2.6. Coated and non-coated sand grains.

2.5. Vulnerable soils

The presence of fertilizer and agricultural chemicals in 

groundwater has become an issue in a number of agricultural production areas around the world. More than 

half of the total fresh water used in Florida comes from 

groundwater, and more than 90% of the public rely on 

groundwater supplies for drinking.

Of all fresh water withdrawn in Florida, one-third is 

consumed and two-thirds is returned to the groundwater. 

The quality of this water is important since it may come 

in contact with soluble nutrients, pesticides, or metals 

prior to returning to the surficial aquifer or flowing off 

site. Florida’s unique hydrogeologic features, including a 

thin surface soil layer, high water table, and porous limestone in many areas make the soil susceptible to downward 

movement of nutrients. Soils used for citrus production 

on the central ridge are particularly subject to leaching 

and are referred to as vulnerable soils (Fig. 2.7). These 

soils are well-drained with low organic matter and provide 

ideal conditions for leaching of plant nutrients including 

soil-applied N fertilizer. Removal of N by denitrification is 

minimal in these soils.

Table 2.2. Soil series classified as vulnerable to nutrient leaching.

Adamsville Dade Palm Beach

Archbold Florahome Paola

Astatula Fort Meade Satellite

Bahiahonda Gainesville St. Augustine

Broward Lake St. Lucie

Canaveral Lakewood Tavares

Candler Neilhurst Orsino

Cocoa Orlando

The soils in Table 2.2 have been categorized by the 

Florida Department of Agriculture and Consumer 

Services (based on USDA-Natural Resource Conservation Service information) as highly permeable and well 

drained. Thus, they are at risk in terms of groundwater 

contamination through agrichemical leaching. Although 

such soils dominate throughout the ridge citrus production area, many are scattered throughout other Florida 

citrus producing areas. Producers should consult the 

local NRCS office or county soil survey to determine if a 

grove contains one or more vulnerable soils. County soil 

surveys can be viewed on the NRCS internet web site at: 

http://websoilsurvey.nrcs.usda.gov/.

2.6. Grove site selection and land preparation

When developing a grove site, it is important to determine 

general soil fertility by testing the soil for pH, organic 

matter, and Mehlich 1 (double-acid) extractable P, Ca, and 

Mg. If the site has been previously used to grow citrus or 

vegetables, extractable copper (Cu) concentration should 

also be checked.

If the soil is found to be acidic (pH 5.5 or less), the pH 

should be raised to the 6.0 to 6.5 range by adding lime. 

The higher target pH should be used for soils with Me-
13

Fig. 2.7. Approximate location of vulnerable soils in citrus production regions. (Courtesy of James Turk and Juan Vega, USDANRCS, Gainesville.) See table 2.2 for a list of vulnerable soil series.
14 

hlich 1-extractable Cu greater than 25 mg/kg (ppm). If the 

soil is calcareous (contains free calcium carbonate, with 

pH around 8.2), no attempt to lower the pH should be 

made. If planted to citrus, improving the fertility of highly 

calcareous soils and white sandy soils (“sand soaks”) devoid 

of organic matter will be extremely difficult. Additional 

production costs associated with management of trees on 

such soils can make the difference between profit and loss, 

particularly during periods of low fruit returns.

Renovating old flatwoods groves by rebedding and ditching can have profound effects on fertility, water relations, 

and tree rooting volume. Often the renovation is beneficial, but problems may occur in some cases if high pH 

materials or clay is brought up into the rooting zone.

2.7. Effects of leveling and bedding on soil 

fertility in flatwoods citrus groves

In contrast to central ridge citrus groves that are planted 

along the natural contour of the land, flatwoods grove sites 

must be leveled, slightly sloped, and bedded before planting 

to provide artificial drainage. The topsoil of native Alfisols 

and Spodosols is no more than 6 to 8 inches thick. Below 

this layer is the first subsoil layer, which is usually a white or 

light gray sand that is extremely low in fertility and waterholding capacity. Occasionally, land leveling removes all of 

the topsoil from a higher part of the field and transports it 

to a lower part, leaving the light-colored sandy subsoil as 

the new surface (Fig. 2.8). Citrus tree growth and production in these areas (commonly referred to by flatwoods citrus growers as scraped areas or sand soaks) is usually poor.

After leveling, soil beds are constructed by cutting parallel 

wide and shallow V-shaped furrows about 50 ft apart. The 

soil removed from these furrows is shaped into a convex 

bed between them on which the citrus trees are planted. 

The vertical distance from the bottom of the furrow to 

the top of the bed is usually 2 to 3 feet. When constructing beds, the original soil surface is covered by subsoil 

that may have significantly different physical or chemical 

characteristics than the surface soil. The overburden soil 

can be either coarser or finer-textured than the surface soil, 

but it is almost always lower in organic matter. If the soil 

series has limestone in the profile, the overburden may be 

calcareous. Therefore, the root zone soils in bedded groves 

are often less fertile and lower in water-holding capacity 

compared with the buried original surface layer.

The restrictive subsurface layer in flatwoods soils can affect 

citrus production in two ways. If it is relatively deep, it 

remains intact after bedding and will impede downward 

water percolation. Citrus rooting can be affected by this 

layer due to its influence on shallow water table depth and 

duration. Typically, almost all flatwoods citrus roots reside 

in the top 12 to 18 inches of soil due to the effect of the 

restrictive layer.

Some flatwoods soils have relatively shallow restrictive 

layers that can be excavated during the bedding process, 

so these subsurface materials are sometimes mixed into 

the root zone. The chemical and physical properties of a 

restrictive layer differ substantially from the sandy surface 

layer (Appendix A). Material from a loamy (Bt) layer is 

higher in clay, while a sandy dark red, brown, or black (Bh) 

layer is higher in organic matter. Loamy layers can be either 

acidic or alkaline in pH, while organic-stained layers are 

always highly acidic. In addition, water-holding and cation 

exchange capacities are higher in restrictive layers. The magnitude of influence that soil from these layers might have 

on root zone soil properties is directly related to amount of 

material that was excavated and mixed in during bedding.

2.8. Soil pH and liming

Soil pH measures soil acidity or alkalinity, and is used to 

make liming decisions. Soil pH measurement is quick, 

easy, and inexpensive. Soil pH control is important 

because the availability of most plant nutrients as well as 

those that are toxic to plants are affected by it.

Irrigation water from Florida’s deep aquifers frequently 

contains dissolved limestone that can slowly raise soil pH. 

Higher soil pH is particularly evident in the areas wetted by 

microirrigation emitters. Florida soils vary considerably in 

Ca content. The majority of soil Ca exists as sparingly soluble minerals including Ca-phosphates and Ca-carbonate. 

Fig. 2.8. Effect of leveling a flatwoods citrus site on topsoil (A 

horizon) thickness. Note the thick, dark topsoil on the left and 

lack of same on the right. (Mace Bauer)
15

Calcium must dissolve from these compounds to become 

plant-available. Florida’s coarse-textured soils are low in Ca 

because they are mostly quartz sand. On the other hand, 

calcareous soils are extremely high in Ca. Calcium in soils 

may be classified as non-exchangeable (mineral forms), exchangeable (adsorbed to clay or organic matter colloids), or 

soil solution Ca. Exchangeable Ca is the major Ca reserve 

in soils that is available to plant roots. Calcium availability 

is largely a factor of the supply in the soil.

The target pH of 6.0 to 6.5 for Florida citrus production 

is based on a study of pH and Ca interactions conducted 

on a ridge soil (Candler fine sand). A clear advantage of 

pH 6.0 over pH 5.0 was evident, and pH 7.0 was no better 

than pH 6.0 at all Ca levels. Therefore, if a soil test does 

not show excessive copper accumulation, a soil pH of 6.0 

is sufficient for citrus production. Soil pH should be raised 

to 6.5 when soil tests show a build-up of copper because 

this element becomes toxic at low soil pH.

Soil pH can be increased by applying either calcitic or 

dolomitic lime. In addition to affecting soil pH, calcite is 

an effective source of Ca, whereas dolomite supplies both 

Ca and Mg. Therefore, although either calcite or dolomite 

could be effectively used for citrus production, the choice 

of dolomite would be more appropriate for soils that also 

require Mg.

In groves with favorable soil pH but low soil Ca, gypsum 

or a more soluble fertilizer like calcium nitrate can be used 

as a source of available Ca. In most bearing groves, soil pH 

is generally above 6.0, so liming is not required. Gypsum is 

a source of moderately-available Ca with no effect on soil 

pH, while calcium nitrate is a rapidly-available Ca and N 

fertilizer with a slightly alkaline reaction in the soil.

Although the application of dolomite can alleviate Mg 

deficiency, tree response is usually slow. Application of 

dolomite as a source of Mg is not recommended if the soil 

pH is in the desired range. Under this condition, applying 

MgSO4

 or MgO to the soil or Mg(NO3)2 as a foliar spray 

can correct Mg deficiency.

The current soil pH recommendation for non-bearing 

and bearing citrus takes into account: 1) higher pH soils 

now in production, 2) the high pH of groundwater used 

for irrigation, and 3) greater use of rootstocks like Swingle 

citrumelo that grow poorly in high pH soils.

2.9. Organic matter

As discussed earlier, organic matter is an extremely valuable 

component of sandy soils because it provides both water 

and nutrient-holding capacity, and its decomposition provides recycled nutrients to plants. The opportunity to add 

imported organic matter to a citrus grove is greatest prior 

to planting because it can be more readily applied and incorporated into the soil where the tree rows will be located.

Florida citrus has been successfully grown for decades 

without adding organic matter from outside sources, so it 

is not a necessary practice. However, because of its benefits 

to soil fertility and its increased availability since the mid 

1990s, organic matter addition has become more practical. Florida landfills no longer accept horticultural waste, 

so some county waste disposal operations have turned 

to mulching or composting for disposal. These materials 

are usually provided to consumers at no cost other than 

transportation. Materials intended as mulches are not recommended for application to citrus groves as soil amendments because they may rob N from trees as they decompose. Finished compost is appropriate for immediate soil 

application, but mulch would need to be composted on 

site before it would be safe to incorporate it.

There is no particular target rate for composted organic 

matter application. A general rule is, some is better than 

none, and more is better than less. A grower’s decision to 

apply organic matter should be based on the proximity 

of a suitable supply plus transportation, spreading, and 

incorporation costs. Because of the large volumes required 

for meaningful application rates (e.g. 10 to 50 tons/treated 

acre), uniform application of a lower rate across an entire 

grove is not recommended. Rather, the grower should 

identify the weaker soils in the grove and concentrate 

higher rates of organic matter application in those areas.

Ozores-Hampton et al. (1998) suggested optimum physical and chemical properties for compost applied to agricultural land:

•	 35 to 55% moisture by weight.

•	 50% or more organic carbon.

•	 pH between 5.0 and 8.0.

•	 20 to 60% water-holding capacity by weight.

•	 Less than 6.0 dS/m soluble salts.

•	 500 to 1000 lbs/yd3

 fresh bulk density.

•	 Particle size passes 1-inch screen.

•	 15:1 to 25:1 C-to-N ratio.

•	 No viable weed seeds.

Of these characteristics, the two most important are Cto-N ratio and soluble salts. Lower values of each indicate 

compost more favorable for application to a Florida citrus 

grove.
16 

3.1. Essential nutrients

Seventeen elements are essential for the growth and functioning of green plants. Carbon (C), hydrogen (H), and 

oxygen (O), which make up about 95% of tree biomass, 

are provided by nature. C and O are taken up by leaves as 

carbon dioxide (CO2

) from the air. They combine with H, 

taken up as water by the roots, to produce carbohydrates. 

Photosynthesis takes place in chlorophyll-bearing cells, using light as an energy source. Carbohydrates, together with 

proteins, fats, and other organic compounds derived from 

them, are the true plant foods. They are used to make new 

plant tissues and provide energy for growth and fruiting.

The other 14 mineral elements are nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), 

sulfur (S), iron (Fe), zinc (Zn), manganese (Mn), boron 

(B), copper (Cu), molybdenum (Mo), chlorine (Cl), and 

nickel (Ni). Florida’s sandy soils often do not contain a sufficient supply of many of these nutrients, so growers may 

need to provide some of them through fertilizer application.

When any essential element is in short supply, tree function is restricted. A severe shortage of an element typically 

produces a characteristic deficiency symptom exhibited 

by the leaves, which usually persists until the deficiency is 

corrected. Twigs and fruits may also exhibit characteristic 

symptoms. Sometimes two or three elements are deficient 

in varying degrees, resulting in confusing visual symptoms. 

Conversely, excessive amounts of some elements may be 

present in the soil and may prevent the tree from functioning properly. Visual symptoms and leaf and soil analysis are 

all useful to evaluate nutritional status.

Mineral nutrients are divided into macronutrients, which 

are elements that plants require in large amounts (N, P, K, 

Ca, Mg, S), and micronutrients, which are needed only 

in small amounts (Fe, Zn, Mn, B, Cu, Mo, Ni, Cl) (Table 

3.1). The macronutrients are divided into two groups: 

primary elements (N, P, and K) and secondary elements 

(Ca, Mg, and S). Micronutrients are sometimes referred to 

as “minor” or “trace” elements, but these terms are misleading. For example, the role of Fe in plant metabolism 

should not be considered less important than the role of 

K. Iron deficiency can result in total crop loss, so its role is 

not a “minor” one, and it is not of minor importance. The 

difference between Fe and K is in the amount required by 

plants, so the use of the terms micro and macronutrients is 

more appropriate.

Table 3.1. Relative essential mineral element composition of a 

6-year-old ‘Hamlin’ orange tree (excluding Cl and Ni). (Derived from Mattos et al., 2003).

Element No. of atoms 

relative to Mo

% of total tree 

dry weight

Mo 1 0.00003

Cu 100 0.002

Mn 200 0.003

Zn 300 0.006

Fe 600 0.010

B 800 0.002

S 11,000 0.096

P 13,000 0.116

Mg 18,000 0.120

K 66,000 0.728

Ca 98,000 1.096

N 237,000 0.932

3.2. Macronutrients and citrus production

Nitrogen (N) is of primary importance in citrus production. It has more influence on tree growth, appearance, 

fruit production, and fruit quality than any other element. 

When N is in short supply, growth is limited and the foliage becomes pale green or yellow. When N is supplied to 

bearing trees at sub-optimal rates for a long period of time, 

the trees adjust by recycling N from the oldest leaves into 

the new ones and the old leaves are shed prematurely, leading to a thin canopy. Inside leaves that should function for 

up to 2 years or more are reduced to a life of 1 year or less. 

The green color of the remaining leaves may be nearly normal, but the canopy is hollow inside. Yield can be reduced 

somewhat, but the typical yield response curve (Fig 3.1) 

shows that a rather large decrease in N supply is required 

before yield is greatly decreased.

In cases of persistent N shortage, defoliation, fruit drop, 

and shoot death can occur. When N is present above 

visible deficiency, shoot growth and yield increase with 

increasing N supply up to the optimum N rate. A sufficient N concentration in the tree is required for maxi3. General Soil Fertility and Citrus Tree Nutrition

Thomas A. Obreza, Mongi Zekri, and Stephen H. Futch

(Includes contributions by Larry K. Jackson, Ashok K. Alva, David P. H. Tucker, and David V. Calvert)
17

mum vegetative growth, flowering, and fruit yield. A high 

N concentration increases tree growth and may require 

increased applications of other elements, particularly K. 

Luxury (excessive) consumption of N can lead to excessive 

vegetative growth at the expense of fruit yield.

Phosphorus (P) is listed on a fertilizer label as P2O5 and is 

referred to as available phosphoric acid. Phosphorus does 

not readily leach if the soil pH is 6.0 or higher, and removal by a citrus crop is only around 2 lbs of elemental P/100 

boxes of fruit. Most mature Florida citrus groves contain 

sufficient residual P that accumulated from previous 

fertilizer applications, so regular P fertilizer application 

is usually not necessary. Most previously non-cultivated 

soils used for new citrus plantings are naturally low in P, so 

fertilizer application may be needed for the first few years 

until P accumulates in the root zone.

Potassium (K) (also called potash) is listed on a fertilizer 

label as K2O, and is important to yield, fruit size, and juice 

quality. Potassium does not accumulate to a great extent 

in sandy soils used to produce citrus, even with repeated 

fertilizer applications. Potassium deficiency is not common when a grove is fertilized normally, but it can develop 

on high pH soils or when high N rates stimulate high 

fruit production. Too little K can slow vegetative growth 

and result in thinning of the topmost foliage. Potassium 

deficiency reduces fruit number and size, increases fruit 

creasing, plugging and drop, and decreases juice soluble 

solids, acid, and vitamin C content. High K fertilizer rates 

do not increase cold hardiness of citrus trees.

Calcium (Ca) is the most abundant mineral element by 

weight in citrus trees, residing mainly in the leaves. Ca is 

rarely deficient since occasional applications of CaCO3

(lime) are used to control soil acidity, and because Ca is 

present in irrigation water. Florida’s alkaline soils have an 

abundance of Ca because they contain dissolved calcium 

carbonate (limestone).

Magnesium (Mg) is an essential constituent of chlorophyll. A deficiency produces a characteristic chlorotic 

pattern and may cause premature defoliation. Seedy citrus 

varieties may need more Mg than seedless ones because 

seeds store a large amount of Mg. Dolomitic limestone is 

often used to correct acidity and supplies slowly-available 

Mg. Calcium is abundant in alkaline soils, which can be 

antagonistic to Mg uptake.

Sulfur (S) is utilized by citrus trees in an amount similar to 

P. It is supplied with fertilizers like ammonium sulfate and 

sulfates of micronutrient metals. Sulfur is a major component of the soil organic fraction and becomes available to 

plants as organic matter decomposes. Sulfur is also present 

in some irrigation water sources. When S is deficient in a 

citrus tree, the symptom looks like N deficiency.

3.3. Micronutrients and citrus production

Iron (Fe) deficiency causes a chlorotic pattern that first 

appears on young shoots because mobility of Fe within 

the plant is low. It occurs in trees growing in alkaline soil, 

waterlogged soil, or soils very low in organic matter soil. 

Other Fe deficiency problems have occurred where Cu is 

high in the soil.

Copper (Cu) deficiency causes fruit corking, ammoniation (curved branching), inter-nodal stem gumming, and 

twig dieback. The deficiency can be corrected by applying 

Cu fertilizer to the soil. Copper should not be included in 

fertilizer if foliar Cu sprays are used, or if a grove soil test 

shows sufficient Cu (Chapter 4). For new plantings on 

previously non-cultivated flatwoods soils, Cu should be 

included in the fertilizer for the first 2 or 3 years.

Zinc (Zn) deficiency symptoms are expressed in citrus 

trees as severe chlorosis where leaf tissue becomes nearly 

white, except for green veins. New leaves grow progressively smaller as the deficiency becomes more severe, and 

shoot internodes become shorter causing a rosette effect. 

Severe Zn deficiency restricts growth and reduces fruit 

yield.

Manganese (Mn) deficiency produces a mild form of 

interveinal chlorosis on acidic, sandy soils. The “marl 

chlorosis” found on calcareous soils is the result of comFig. 3.1. Generic response of citrus yield to N fertilizer rate.
18 

bined deficiencies of Mn and Zn, and sometimes Fe. 

Temporary mild deficiency patterns on new shoots are not 

detrimental to growth or fruiting of citrus trees. Corrective measures should only be taken in the case of persistent 

deficiency symptoms.

Boron (B) deficiency causes fruit drop, gum pockets in 

the peel, gumming around the core, and brown areas in the 

albedo and central axis. It sometimes occurs when growers use only high-analysis fertilizers (without micronutrients), or following a prolonged drought. Boron should 

be applied every year either as a soil or foliar application, 

but not both. Foliar application to correct a deficiency is 

advantageous due to rapid tree uptake. Boron fertilization may not be needed at all if trees are irrigated with 

reclaimed water.

Molybdenum (Mo) deficiency produces a symptom 

described as “yellow spot.” Unlike other nutrients, Mo is 

less available in acidic than in slightly alkaline soils. Mo 

deficiency is rare in Florida. If it occurs, the soil usually has 

become too acidic with time. Liming the soil effectively 

relieves the deficiency, but more rapid response is obtained 

if sodium molybdate is applied as a foliar spray.

The exact roles of chlorine (Cl) and nickel (Ni) in citrus 

tree metabolism are not clear.

3.4. Supplying nutrients to citrus trees

A sufficient supply of essential nutrients is critical to nutrient management and sustainability. If a single element is 

below the critical availability level, crop growth and yield 

will fall even if the other elements are in sufficient supply. 

A balance of available nutrients is a key component to 

profitability because it allows for positive nutrient interaction. For example, in the case of N fertilization a shortage 

of another nutrient could decrease N uptake, reduce N use 

efficiency, and increase the potential for N loss.

Soil application of macronutrients is favored over foliar 

application due to the high uptake demand by citrus trees. 

However, fertilizer applied to the soil is subject to various 

fates including leaching, runoff, and fixation to forms not 

available to plants. Solution fertilizers applied to the tree 

foliage are less prone to these losses, but only small quantities of nutrients can penetrate leaves. Foliar fertilizer application may be considered for the nutrients N, P, K, Mg, 

Zn, Mn, and B. It is especially useful when soil properties 

like high pH inhibit nutrient availability.

Foliar fertilizer application can reduce or eliminate soil 

applications of micronutrients since they are taken up in 

low amounts (Table 3.1). Foliar application is the fastest 

method of promoting micronutrient uptake by plants in 

the short term when a nutritional deficiency is diagnosed, 

but should not be relied upon for long-term tree nutrition 

unless the soil is calcareous (Chapter 11).

Fertilization represents about 20% of total citrus production costs, but it can have a large effect on potential 

profitability at extreme highs or lows. Visual evaluation 

of nutritional status, soil and plant analysis, field history, 

production experience and economics are all important 

guidelines for making fertilizer rate and source decisions.

3.5. Nutrient behavior in Florida soils

Plant nutrients exist in both organic and inorganic forms 

in soil. Organic forms are found in fresh plant residue, soil 

organic matter (humus), living soil organisms (e.g. bacteria 

and fungi), soil amendments (e.g. biosolids or compost), 

and synthetic organic materials (e.g. some N fertilizers). 

Organic materials are the key component of nutrient 

recycling. They are a stable storehouse of plant nutrients 

because they are not rapidly lost from the soil.

Nutrients associated with organic matter are not immediately plant-available, but are slowly released as the material 

is decomposed by soil microbes. The decomposition rate 

depends on the material’s physical and chemical characteristics and the climate. Florida’s warm and humid conditions are ideal for decomposition of almost any organic 

material, so organic matter does not accumulate in citrus 

grove soils over the long term. Nutrients are continuously 

released in inorganic form as decomposition proceeds. 

The recycling process is complete once these nutrients are 

taken up by growing plants. Many of the nutrients in citrus 

tree residues (dropped leaves, twigs, and fruit; dead roots) 

are returned to the tree in this manner.

Inorganic plant nutrients exist in solid form (minerals or 

precipitates), in adsorbed form (bonded to a solid phase 

material), on the cation exchange complex (Fig. 2.5), or in 

the soil solution. The ionic nutrient forms that plants use 

(Table 3.2) must dissolve, desorb, or exchange into the soil 

solution before they can be taken up. If the soil solution is 

not replenished with nutrients rapidly enough to satisfy 

plant demand, plant nutrition will be less than optimum.

In an intensive crop production system, fertilizers added to 

the soil supplement the natural nutrient supply and prevent nutrient deficiencies. Most fertilizers applied to citrus 

groves are inorganic minerals or soluble salts that quickly 

dissolve into plant-available (ionic) form. The soil can react 

with some of these ionic forms, rendering them unavailable 

to plants. In the absence of these reactions, nutrients may 
19

leach with water that percolates through the root zone. The 

general characteristics and behavior of nutrients in sandy 

Florida soils planted to citrus are outlined below.

Nitrogen

•	 95% of the natural N that resides in the soil is associated with organic matter. Soil humus contains about 5% 

N. The N release from organic matter depends on how 

much is there and how fast the material decomposes. 

This release rate is fast enough to support plant growth 

in a natural landscape, but is too slow for intensive agricultural production on sandy soils.

•	 Biological ammonification converts organic N to 

mineral N (ammonium, NH4

+). Ammonium is also 

a component of some mineral N fertilizers. Nitrification, which also depends on microbial activity, converts 

NH4

+ to nitrate (NO3-

) in days to weeks. Thus, soil 

solution N is dominated by negatively-charged nitrate. 

There is no mechanism to hold nitrate in the soil, so it 

leaches easily.

•	 Most of the N lost from soils is a result of N loading of 

the soil from fertilizer or animal waste application, followed by N leaching from the soil with excessive rainfall 

or irrigation.

Phosphorus

•	 P occurs naturally in some Florida soils as calcium phosphate minerals. These minerals can also slowly form 

following P fertilizer application. Soil phosphates are 

relatively insoluble, which can affect plant availability.

•	 If a soil has the capacity to adsorb, or “fix” P, then 

added P will accumulate in the root zone. Phosphorus 

fixation occurs when soluble P forms nearly insoluble 

compounds with Fe or aluminum (Al) at low soil pH or 

Ca at high soil pH. The best P availability in these soils 

occurs around pH 6.5.

•	 Florida’s sandy soils may or may not have the capacity to 

hold applied P fertilizer depending on the type of sand 

present. Sand coated with Fe or Al compounds can fix 

P in the root zone, while non-coated sand cannot (Fig. 

2.6). If a soil is dominated by non-coated sand, P may 

leach.

•	 Adsorbed P can be transported via surface runoff (erosion) while soluble P can be transported via leaching. 

Phosphorus loss from the soil results from long-term 

loading of the soil with P from animal wastes or fertilizers, followed by erosion of soil and organic matter 

particles or leaching, depending on the soil.

Potassium

•	 Florida soils are naturally low in K, so intensive agricultural production requires the use of K fertilizer.

•	 The ionic form of K can be held by the soil cation exchange complex (Fig. 2.5), which delays leaching. However, Florida soils planted to citrus have naturally low 

cation exchange capacity in the root zone (Appendix A, 

Table A.1), so K+ leaches almost as readily as NO3

-

.

•	 K is not fixed in sandy soils and does not form insoluble 

compounds, so it is easily lost from the root zone. Thus, 

K fertilizer application is required every year in Florida 

citrus groves.

Calcium and Magnesium

•	 Ca and Mg exist as solid compounds in the soil (mostly 

in combination with carbonate or phosphate) and in 

ionic forms held by the cation exchange complex.

•	 Solid forms of Ca and Mg are sparingly soluble and can 

reside in the soil for many years if the pH is not too 

acidic. Dissolution is more rapid at low pH, which is the 

basis of the liming reaction.

•	 Because they are divalent cations, Ca and Mg dominate 

on the cation exchange complex, limiting their mobility 

in soil.

Sulfur

•	 90% of the S that occurs naturally in soils is associated 

with organic matter. Soil humus contains about 0.5% 

S. Like N release, S release depends on organic matter 

quantity and decomposition rate. Organic S release 

combined with S from other sources like rain or irrigation water usually provides this nutrient to plants at a 

sufficient rate even in intensive agricultural production.

•	 The plant-available form of sulfur (sulfate) is a negative 

ion, which makes it prone to leaching. Sulfate can be 

adsorbed by soils but adsorption usually occurs much 

deeper in the soil profile than the majority of plant 

roots.

•	 Calcium sulfate (gypsum) is a sparingly-soluble compound that is applied as a long-term source of available 

Ca, but it also supplies S to plants.

Copper, Iron, Manganese, and Zinc

•	 These micronutrients form compounds that are only 

slightly soluble in sandy soils, thus they are not mobile 

nutrients. Solubility increases somewhat as pH decreases (Fig. 3.2), so it is important to not over-lime a soil. At 
20 

alkaline pH, some plants suffer micronutrient deficiencies due to almost total insolubility.

•	 If applied to the soil as soluble fertilizer, these micronutrients will precipitate near the soil surface.

Boron

•	 The plant-available form of B is negatively charged (borate), so it can easily leach from sandy soil.

•	 B fertilizer should be regularly applied to Florida citrus 

groves, but there is a narrow range between deficiency 

and toxicity.

Molybdenum

•	 Mo is the only micronutrient that increases in solubility as soil pH increases. Thus, it is an immobile nutrient 

within the soil pH range that is favored for citrus tree 

growth.

Nutrient movement in Florida’s sandy soils is summarized 

in Table 3.2.

3.6. Citrus nutrient requirements

This section describes the typical citrus yield response 

curve and discusses nutrient requirements in relation to 

anticipated yield. Fertilizer rate guidelines for non-bearing 

and bearing trees are provided in Chapter 8.

The relationship between nutrient supply and yield of a 

wide variety of annual and perennial crops has been studied for decades. The relationship between plant response 

(yield) and fertilizer rate is called the yield response curve. 

The shape of this curve is similar for a range of crops and 

conditions.

The curves in Fig. 3.1 illustrate how citrus yield increases 

with increasing N rate for two conditions. Fertilizer N 

is used in this example, but the nature of the response 

curve is similar for other limiting nutrients. At very low 

N rates there is a large yield response to each added unit 

of N. As yield increases, each additional unit of N results 

in a smaller increment in yield. This smaller response to 

increasing input is also referred to as the law of diminishing returns. The two response curves in Fig. 3.1 compare 

the effect of N rate when other factors are not limiting and 

the response when yield is limited to one-half by a second 

factor. The shapes of the curves are similar, and the rate of 

N where the slope levels off is only slightly higher for the 

more productive grove.

The yield response curves shown never completely flatten 

out, indicating that ever higher N rates theoretically will 

Fig. 3.2. Effect of soil pH on nutrient availability as expressed 

by relative bar width.

Table 3.2. Qualitative assessment of nutrient movement in 

Florida’s sandy soils based on ionic form in the soil solution 

and ability to precipitate or be adsorbed by the soil.

Nutrient

Ionic form 

taken up by 

plants

Nutrient subject 

to precipitation or 

adsorption?

Nutrient 

mobile in 

sandy soil?

N NH4

+, NO3- No Yes

P PO4

3- Yes Yes/No*

K K+ No Yes

Ca, Mg Ca2+, Mg2+ Yes No

S SO4

2- Yes Yes/No*

Cu, Mn, 

Fe, Zn

Cu2+, Mn2+, Fe3+, Zn2+ Yes No

B H3

BO3, No Yes

Mo MoO4

2- Yes No

* Depends on soil properties (see discussion above).
21

produce small additional yield increases. Because fertilizer 

cost was a small portion of total production cost in the 

past, high N rates were commonly applied to produce the 

highest possible yield. However, experiments with citrus 

have rarely demonstrated a benefit of N fertilizer rates 

higher than about 200 lbs/acre, regardless of the production potential. Instead, yield declined in several experiments when N rate was increased beyond the optimum 

range.

Nutrients removed with the harvested crop must be 

replaced. The amount the crop removes varies from a fraction of a lb/acre for some of the micronutrients to as much 

as 100 lbs/acre of N or K from a high-producing grove 

(Table 3.3). For oranges, approximately 0.12 lbs N/box is 

removed with the harvest. Therefore, crop removal ranges 

from 12 lbs N/acre for a 100 box/acre yield to around 100 

lbs N/acre for a grove producing 800 boxes/acre. 

Nutrient uptake from applied fertilizers is not 100% efficient, so more nutrients must be applied than the minimum required by the tree. N use efficiency, expressed as lbs 

N removed by the crop divided by lbs N applied, ranges 

from 0.2 to 0.4 in groves with low to moderate yield. 

However, N efficiencies around 0.5 have been observed in 

groves with a good production record. Application of 200 

lbs N/acre supplies sufficient N for an 800 box/acre orange yield when N use efficiency is 0.5. A grower using the 

latest fertilization technology (e.g. fertigation, controlledrelease fertilizers) with good irrigation management may 

be able to exceed an N use efficiency of 0.5.

3.7. Nutrition, irrigation, and fruit quality

Florida has the highest citrus fruit quality standards in the 

world. The most important quality factors for Florida citrus growers, production managers, processors, and packers 

include fruit juice content, soluble solids and acid concentrations, soluble solids/acid ratio, fruit size, and color. 

Florida citrus growers discern between quality factors for 

the fresh and processing markets. For example, fruit size, 

shape, color, and maturity date are most important for 

fresh fruit, but high juice content and soluble solids are desired for processed fruit. Fruit quality is affected by factors 

including cultivar, rootstock, climate, soil, pests, irrigation, 

and nutrition.

The effects of nutrition and irrigation on fruit quality 

should be understood and taken into consideration by 

citrus growers to increase profitability, enhance sustainability, and improve worldwide competitiveness. Excessive irrigation and fertilization reduce fruit quality, so 

supplying sufficient nutrition and using sound irrigation 

scheduling techniques should be high priorities of every 

grower. Citrus trees require a properly designed, operated, 

and maintained water management system and a balanced 

nutrition program formulated to provide specific needs for 

tree maintenance and expected yield and fruit quality.

Irrigation is a major component of fertilizer program efficiency. Citrus trees with sufficient water and nutrients 

grow stronger, tolerate pests and stresses better, yield more 

consistently, and produce high quality fruit. On the other 

hand, excessive or deficient irrigation or fertilization may 

result in reduced fruit quality.

Table 3.3. Total amounts of various nutrients in 100 boxes1 of orange fruits.

Nutrient

Hamlin2 Hamlin3 Hamlin4 Parson 

Brown3 Valencia3 Sunburst3 Average

lbs nutrient/100 boxes of fruit

N 12.5 10.6 10.8 11.3 13.5 13.6 12.1

P 1.4 1.5 1.7 1.5 2.0 1.8 1.7

K 17.6 13.6 13.9 13.3 14.4 14.0 14.5

Ca 4.5 4.0 5.2 4.9 4.3 3.4 4.4

Mg 1.9 1.1 1.0 1.2 1.2 1.0 1.2

S 1.1 --- 0.8 --- --- --- 1.0

Fe 0.024 0.020 0.036 0.030 0.072 0.036 0.036

B 0.020 --- 0.025 --- --- --- 0.023

Zn 0.020 0.032 0.008 0.032 0.029 0.041 0.027

Mn 0.011 0.020 0.004 0.023 0.023 0.023 0.017

Cu 0.006 0.005 0.006 0.006 0.007 0.007 0.006

1

1 box of fruit = 90 lbs. 2A. K. Alva, unpublished data. 3Paramasivam et al. (2000). 4Mattos et al. (2003).
22 

The most important management practices influencing 

fruit quality are irrigation and N, P, and K nutrition. Some 

micronutrients like B and Cu can also affect fruit quality, 

but only if they are deficient. In general, when any nutrient 

element is severely deficient, fruit yield and fruit quality 

may be negatively affected.

Trends in fruit quality response to increasing nutrient and 

water availability are described and summarized below:

Nitrogen

•	 Increases juice volume and color, total soluble solids 

(TSS), and acid concentration.

•	 Increases TSS/box and TSS/per acre. However, excessive N, particularly with inadequate irrigation, can 

result in lower yields with lower TSS/acre.

•	 Decreases fruit size and weight.

•	 Increases green fruit at harvest. High N may delay color 

break and increase re-greening of Valencia oranges.

•	 Increases creasing and scab on susceptible varieties, but 

decreases peel blemishes like wind scar, mite russeting, 

and rind plugging.

•	 Reduces stem-end rot and green mold of fruit in storage.

Phosphorus

•	 Reduces acid concentration, which increases TSS/acid 

ratio.

•	 Increases number of green fruit.

•	 Reduces peel thickness.

•	 Increases expression of wind scar but reduces that of 

russeted fruit.

Potassium

•	 Decreases juice content, TSS, TSS/acid ratio, and juice 

color.

•	 Increases acid content.

•	 Increases fruit size, weight, green fruit and peel thickness.

•	 Reduces splitting, creasing, and fruit plugging.

•	 Reduces stem-end rot of fruit in storage.

Magnesium

•	 Slightly increases TSS/box and TSS/acid ratio.

•	 Slightly increases fruit size and weight.

•	 Decreases rind thickness.

Irrigation

•	 Increases juice content and TSS/acid ratio.

•	 Reduces TSS and acid concentration.

•	 Increases fruit size and weight, and green fruit at harvest.

•	 Decreases peel thickness.

•	 Increases blemish from wind scar, scab and Alternaria

brown spot (overhead irrigation only), but reduces rind 

plugging.

•	 Reduces stem-end rot, but increases green mold of fruit 

in storage.

Specific effects on juice and external fruit qualities summarized in Table 3.4 are based on numerous field experiments 

conducted for many years that evaluated the response 

of oranges to irrigation and fertilization practices. Most 

of these effects were consistently observed, but some of 

them appeared to depend on local conditions and growing regions. These observations are useful to help develop 

a strategy aimed at improving fruit quality for a particular 

variety or location.

3.8. Grove management practices

Management practices that improve fertilizer nutrient-use 

efficiency include:

•	 Using a leaf and soil testing program (Chapter 4).

•	 Using precision fertilizer application (Chapter 5).

•	 Selecting fertilizer materials and rates that match nutrient requirements (Chapters 6 and 8).

•	 Carefully placing fertilizer over the root zone (Chapter 7).

•	 Timing to avoid the rainy season (Chapters 8 and 10).

•	 Split applications (Chapter 8).

•	 Applying N fertilizer at a rate consistent with historical 

or expected production (Chapter 8).

•	 Irrigation management to maximize production and 

minimize leaching (Chapter 9).

Groves with non-nutritional limiting factors do not 

produce more fruit if the grower exceeds basic fertilizer requirements in an attempt to boost yield. Instead, 

excess fertilizer is not used by the tree, efficiency declines, 

and potential for leaching loss increases.

3.9. Interactions of nutrition with 

other grove practices

Nutrition management interacts with irrigation, pest 

control, weed management, and vegetative growth control 

(hedging and topping). Nutrition and irrigation are linked 

through fertigation and the need to provide maximum 

nutrient uptake while minimizing nutrient leaching. Water 

and nutrient uptake efficiency increases as trees mature 

due to greater interception by closely interwoven root systems. Fertilization and irrigation outside the root zone is 
23

economically and environmentally unsound and promotes 

weed growth.

Nutrient considerations for pest control

Luxuriant growth caused by excessive fertilization or irrigation may increase incidence of foliar diseases like citrus 

canker and fungal diseases like scab, Alternaria brown 

spot, and post-bloom fruit drop (PFD). Excessive vegetative growth may also increase insect pest problems including the citrus leafminer and the Asian citrus psyllid, which 

vectors Huanglongbing (citrus greening) disease. Controlling tree growth at containment size through pruning 

is more difficult when vegetative growth is promoted by 

excessive inputs. Such excess vegetative growth competes 

with fruit production and may suppress it.

Until specific fertilization recommendations for groves 

infected with citrus greening disease are developed, groves 

should be provided with sufficient nutrition to maintain 

current fruit production (Chapter 8). However, if a grove 

is being visually monitored for greening symptoms, it is 

important to minimize signs of micronutrient (Zn, Mn, 

Fe) deficiencies so the disease can be more easily detected. 

Likewise, tree growth (particularly of young trees) during 

the fall and winter makes it difficult to control psyllids, so 

fertilizer application during this period should be minimized.

Table 3.4. Increasing levels of nutrients within recommended ranges result in the responses shown, whereas excess nutrition can 

reduce fruit yield and quality (Koo, 1988). Key to symbols: Increase (+), Decrease (–), No change (o), No information (?).

Measurement

 Macronutrient element Micronutrient element

Irrigation N P K Ca Mg Mn Zn Cu Fe B

Juice quality

Juice content + + o o o o o o o o +

Soluble solids (SS) + o – o + o o o + o –

Acid (A) + – + o o o o o o o –

SS/A ratio – + – o + o o o o o –

Juice color (red) + o – ? ? ? ? ? ? ? o

Juice color (yellow) + o – ? ? ? ? ? ? ? +

Solids/box + o – o + o o o + o –

Solids/acre + + + o + o o o o o +

External fruit quality

Size – o + o + o o o o o +

Weight – o + o + o o o o o +

Green fruit + + + o o o o o o +

Peel thickness + – + o – o o o o o –

Peel blemishes

Wind scar – + o ? ? ? ? ? ? ? +

Russet – – o ? o o o o o o o

Creasing + + – ? ? ? ? ? ? ? o

Plugging – o – ? ? ? ? ? ? ? –

Scab + o o ? ? ? ? ? ? ? +

Storage decay

Stem-end rot – o – ? ? ? ? ? ? ? –

Green mold – o o ? ? ? ? ? ? ? +

Sour rot o o o ? ? ? ? ? ? ? o
24 

4.1. Introduction

Nutrient deficiency or excess will cause citrus trees to grow 

poorly and produce sub-optimal yield and/or fruit quality. 

Diagnosis of potential nutritional problems should be a 

routine citrus-growing practice. Quantifying nutrients in 

soils and trees eliminates guesswork in adjusting a fertilizer 

program (Fig. 4.1).

Fig. 4.1. Proper soil and leaf tissue sampling and analysis can 

accurately gauge citrus grove nutrition and help improve fertilizer programs. (Mongi Zekri)

This chapter explains the value of leaf and soil analysis 

in determining fertilizer programs that increase fertilizer 

efficiency while maintaining maximum yield and desirable fruit quality. Soil testing and leaf tissue testing have 

different uses or purposes depending on the property or 

nutrient, so care must be taken to use the correct test when 

diagnosing citrus nutrition (Table 4.1).

4.2. Benefits of leaf analysis

Leaf tissue analysis is the quantitative determination of the 

total mineral nutrient concentrations in the leaf. Tissue 

testing includes analysis for N, P, K, Ca, Mg, S, Mn, Zn, 

Cu, Fe, and B. Chlorine concentration is usually sufficient 

given most field conditions, but Cl may become excessive where soil or irrigation water is saline. Similarly, Mo 

deficiency or toxicity is rare. The goal in tissue analysis is to 

adjust fertilization programs such that nutritional problems and their costly consequences are prevented.

Table 4.1. Summary of the usefulness of soil testing and leaf 

tissue testing as citrus nutrient management tools.

Property or nutrient Soil testing Leaf testing

pH √

Organic matter √

N √

P √ √

K √

Ca √ √

Mg √ √

Cu √ √

Zn, Mn, Fe, B √

Leaf analysis is a useful tool to detect problems and adjust 

fertilizer programs for citrus trees because leaf nutrient 

concentrations are the most accurate indicator of fruit 

crop nutritional status. Because citrus is a perennial plant, 

it is its own best indicator of appropriate fertilization. 

Leaves reflect nutrient accumulation and redistribution 

throughout the plant, so the deficiency or excess of an 

element in the soil is often reflected in the leaf. Considerable research involving citrus leaf testing has established 

its reliability as a management tool, but sampling guidelines should be followed precisely to insure that analytical 

results are meaningful.

Tissue analysis:

•	 Determines if the tree has had a sufficient supply of 

essential nutrients.

•	 Confirms nutritional deficiencies, toxicities or 

imbalances.

•	 Identifies hidden toxicities and deficiencies when visible 

symptoms do not appear.

•	 Evaluates the effectiveness of fertilizer programs.

•	 Provides a way to compare several fertilizer treatments.

•	 Determines the availability of elements not tested for by 

other methods.

Leaf analysis integrates all the factors that might influence 

nutrient availability and uptake. Tissue analysis shows the 

relationship of nutrients to each other. For example, K 

deficiency may result from a lack of K in the soil or from 

excessive Ca, Mg, and/or Na. Similarly, adding N when 

K is low may result in K deficiency because the increased 

growth requires more K. 

4. Soil and Leaf Tissue Testing

Thomas A. Obreza, Mongi Zekri, and Edward A. Hanlon
25

4.3. Steps in leaf sampling

Procedures for proper sampling, preparation and analysis 

of leaves have been standardized to achieve meaningful 

comparisons and interpretations. If done correctly, the 

reliability of the chemical analysis, data interpretation, 

fertilization recommendations, and adjustment of fertilizer programs will be sound. Therefore, considerable 

care should be taken from the time leaves are selected for 

sampling to the time they are received at the laboratory for 

analysis.

Leaf sample timing

•	 Leaf samples must be taken at the correct time of year 

because nutrient concentrations within leaves continuously change. As leaves age from spring through fall, N, 

P, and K concentrations decrease, Ca increases, and Mg 

first increases and then decreases (Fig. 4.2). However, 

leaf mineral concentrations are relatively stable from 4 

to 6 months after emergence in the spring.

•	 The best time to collect 4 to 6-month-old spring flush 

leaves is July and August (Fig. 4.3). If leaves are sampled 

later in the season, summer leaf growth can easily be 

confused with spring growth.

Leaf sampling technique

•	 A sampled citrus grove block or management unit 

should be no larger than 20 acres. The sampler should 

make sure that the selected leaves represent the block 

being sampled.

•	 Each leaf sample should consist of about 100 leaves 

taken from non-fruiting twigs of 15 to 20 uniform trees 

of the same variety and rootstock that have received the 

same fertilizer program.

•	 Use clean paper bags to store the sample. Label the bags 

with an identification number that can be referenced 

when the analytical results are received.

•	 Avoid immature leaves due to their rapidly changing 

composition.

•	 Do not sample abnormal-appearing trees, trees at the 

edge of the block, or trees at the end of rows because 

they may be coated with soil particles and dust.

•	 Do not include diseased, insect-damaged, or dead leaves 

in a sample.

•	 Select only one leaf from a shoot and remove it with its 

petiole (leaf stem).

Fig. 4.2. Changes in concentration of N, P, K, Ca, and Mg in 

citrus leaves with age. The shaded areas denote the recommended sampling period and the optimum concentration 

range for each element.

Fig. 4.3. Sample 4 to 6-month-old spring flush leaves from 

non-fruiting twigs. (Thomas Obreza)
26 

Special Case: Diagnosing growth disorders

•	 Collect samples from both affected trees as well as normal trees.

•	 Trees selected for comparison sampling should be of the 

same age, scion type, and rootstock.

•	 If possible, confine the sampling area to trees that are 

close to each other.

Handling of leaf samples

•	 Protect leaves from heat and keep them dry. Place them 

in a refrigerator for overnight storage if they cannot be 

washed and oven dried the day of collection.

•	 For macronutrient analysis, leaves do not need to be 

washed.

•	 If accurate micronutrient analysis is desired, the leaves 

will need to be washed (see below).

•	 Dry the leaves in a ventilated oven at about 140o F.

Preparation for analysis

•	 Leaves that have been sprayed with micronutrients 

for fungicidal (Cu) or nutritional (Mn, Zn) purposes 

should not be analyzed for those elements because it is 

almost impossible to remove all surface contamination 

from sprayed leaves.

•	 For accurate Fe and B or other micronutrient determinations, leaf samples require hand washing that is best 

done shortly after collection before they dehydrate.

•	 For micronutrient determinations, rub the leaves between the thumb and forefinger while soaking them in 

a mild detergent solution, then thoroughly rinse with 

pure water. It is difficult to remove all surface residues, 

but this procedure removes most of them.

Analysis and interpretation

•	 The laboratory determines the total concentration of 

each nutrient in the leaf sample. Since total concentration is determined, there should be no difference in leaf 

analysis results between different laboratories.

•	 To interpret laboratory results, compare the values with 

the leaf analysis standards in Table 4.2. These standards 

are based on long-term field observations and experiments conducted in different countries with different 

citrus varieties, rootstocks, and management practices, 

and are used to gauge citrus tree nutrition throughout 

the world.

•	 The goal in nutrition management is to maintain leaf 

nutrient concentrations within the optimum range 

(Table 4.2) every year. If the interpretation for a particular nutrient is not optimum, various strategies can 

be used to address the situation (Table 4.3).

4.4. Benefits of soil analysis

Soil analysis measures organic matter content, pH, and 

extractable nutrients, which are useful in formulating and 

improving a fertilization program. Soil analysis is particularly useful when conducted for several consecutive years 

so that trends can be observed. However, a citrus grower 

cannot rely on soil analysis alone to formulate a fertilizer 

program or diagnose a nutritional problem in a grove.

Similar to leaf analysis, methods to determine organic matter and soil pH are universal, so results should not differ 

between laboratories. However, soil nutrient extraction 

procedures vary from lab to lab. Several accepted chemical 

procedures exist that remove different amounts of nutrients from the soil because the extractants vary in strength. 

To draw useful information from soil tests, consistency in 

use of a single extraction procedure from year to year is important to avoid confusion when interpreting the amount 

of nutrients extracted.

A soil extraction procedure does not measure the total 

amount of nutrients present, nor does it measure the quantity actually available to citrus trees. A perfect extractant 

would remove nutrients from the soil in amounts that are 

exactly correlated with the amount available to the plant. 

Therefore, the utility of a soil testing procedure is how well 

the extractable values correlate with the amount of nutrient a plant can take up. The process of relating these two 

quantities is called calibration.

A soil test is only useful if it is calibrated with plant response. Calibration means that as a soil test value increases, nutrient availability to plants increases in a predictable 

way (Fig. 4.4). Low soil test values imply that a crop will 

respond to fertilization with the particular nutrient in 

question. High soil test values indicate the soil can supply 

all the plant needs, so no fertilization is required. The soil 

test value that separates predicted fertilizer response from 

non-response is called the critical or sufficiency soil test 

value (Fig. 4.5).

In Florida, soil testing for mobile, readily leached elements 

like N and K has no practical value. In addition to organic 

matter and pH, soil testing is important for P, Mg, Ca, and 

Cu. The Univ. of Florida Extension Soil Testing Laboratory (ESTL) has used the Mehlich 1 (double acid) extrac-
27

Table 4.2. Guidelines for interpretation of orange tree leaf analysis based on 4 to 6-month-old spring flush leaves from 

non-fruiting twigs (Koo et al., 1984).

Element Unit of measure Deficient Low Optimum High Excess

N % < 2.2 2.2 – 2.4 2.5 – 2.7 2.8 – 3.0 > 3.0

P % < 0.09 0.09 – 0.11 0.12 – 0.16 0.17 – 0.30 > 0.30

K % < 0.7 0.7 – 1.1 1.2 – 1.7 1.8 – 2.4 > 2.4

Ca % < 1.5 1.5 – 2.9 3.0 – 4.9 5.0 – 7.0 > 7.0

Mg % < 0.20 0.20 – 0.29 0.30 – 0.49 0.50 – 0.70 > 0.70

Cl % --- --- < 0.2 0.20 – 0.70 > 0.701

Na % --- --- --- 0.15 – 0.25 > 0.25

Mn mg/kg or ppm2 < 18 18 – 24 25 – 100 101 – 300 > 300

Zn mg/kg or ppm < 18 18 – 24 25 – 100 101 – 300 > 300

Cu mg/kg or ppm < 3 3 – 4 5 – 16 17 – 20 > 20

Fe mg/kg or ppm < 35 35 – 59 60 – 120 121 – 200 > 200

B mg/kg or ppm < 20 20 – 35 36 – 100 101 – 200 > 200

Mo mg/kg or ppm < 0.05 0.06 – 0.09 0.10 – 2.0 2.0 – 5.0 > 5.0

1

Leaf burn and defoliation can occur at Cl concentration >1.0%.

2

ppm = parts per million.

Table 4.3. Adjusting a citrus fertilization program based on leaf tissue analysis.

Nutrient What if it is less than optimum in the leaf ? 

Options:

What if it is greater than optimum in the leaf ? 

Options:

N

1. Check yield.

2. Check tree health.

3. Review water management.

4. Review N fertilizer rate.

1. Check soil organic matter.

2. Review N fertilizer rate.

P 1. Apply P fertilizer (see Chapter 8). 1. Do nothing.

K 1. Increase K fertilizer rate (see Chapter 8).

2. Apply foliar K fertilizer.

1. Decrease K fertilizer rate.

Ca

1. Check soil pH.

2. Check soil test Ca status.

3. Consider applying lime or soluble Ca fertilizer 

depending on soil pH.

1. Do nothing.

Mg

1. Check soil test Mg status.

2. Check soil pH.

3. Consider applying dolomitic lime or soluble Mg 

fertilizer depending on pH.

1. Do nothing.

Micronutrients

1. Check soil pH and adjust if needed.

2. Apply foliar micronutrients.

3. Include micronutrients in soil-applied fertilizer.

1. Check for spray residue on tested leaves.

2. Do nothing.
28 

tion procedure since 1977. The Mehlich 1 test was developed for sandy soils with pH < 6.5, CEC < 10 meq/100 g, 

and organic matter < 5%. Most of the soils used to produce citrus in Florida meet these criteria. The exceptions 

are the calcareous soils of the Indian River production area 

that do not meet the pH requirement.

Univ. of Florida soil test interpretations for P, K, and Mg 

(Table 4.4) were established from experiments with annual field and vegetable crops conducted for many years. 

Limited soil test calibration work with Florida citrus trees 

suggests that the interpretations in Table 4.4 are suitable 

for citrus.

Some commercial agricultural laboratories use the Mehlich 1 extraction procedure, but others use procedures 

different from Mehlich 1 as their preferred soil test 

method. Additional extractants used to determine P 

include Mehlich 3, ammonium acetate buffered at pH 4.8, 

and Bray P1. Other extractants for Ca and Mg include 

Mehlich 3 and ammonium acetate buffered at either pH 

4.8 or pH 7.0. Some interpretations for these extractants 

were developed by Koo et al. (1984) through experimentation, field observation, and best professional judgment 

(Table 4.5). Others were derived from correlations with 

the Mehlich 1 extractant (Alva, 1993; Sartain, 1978).

The single most useful soil test in a citrus grove is for pH. 

Soil pH greatly influences nutrient availability (Fig. 3.2). 

Some nutrient deficiencies can be avoided by maintaining soil pH between 6.0 and 6.5. Deficiencies or toxicities 

are more likely when the pH is outside this range. If soil 

pH is too low, the soil test laboratory runs a buffer test to 

determine the rate of lime need to raise the top 6 inches of 

soil to pH 6.5.

In some cases, soil tests can determine the best way to correct a deficiency identified by leaf analysis. For example, 

Mg deficiency may result from low soil pH or excessively 

high soil Ca. Dolomitic lime applications are advised if the 

pH is too low, but magnesium sulfate is preferred if soil Ca 

is very high and the soil pH is in the desirable range. If soil 

Ca is excessive and soil pH is relatively high, then foliar 

application of magnesium nitrate is recommended.

A poor relationship may exist between soil test values and 

leaf nutrient concentrations in perennial crops like citrus. 

Often fruit trees contain sufficient levels of a nutrient even 

though the soil test is low. On the other hand, a high soil 

test does not assure a sufficient supply to the trees. Tree 

nutrient uptake can be hindered by problems like drought 

or flooding stress, root damage, and cool weather. Tissue 

analysis combined with soil tests can help identify the 

problem.

4.5. Steps in soil sampling

Standard procedures for sampling, preparing, and analyzing soil should be followed for meaningful interpretations 

of the test results and accurate recommendations.

Soil sample timing

•	 In Florida, soil samples should be collected once per 

year at the end of the summer rainy season and before 

fall fertilization (August to October).

•	 It is convenient to take annual soil samples when collecting leaf samples to save time and reduce cost.

•	 The accuracy of soil test interpretations depends on 

how well the soil sample represents the grove block or 

management unit in question.

Fig. 4.4. Ideal soil test calibration curve.

Fig. 4.5. Soil test interpretation categories and their relationship to expected fertilizer response.
29

Table 4.4. Interpretation of soil analysis data for citrus using the Mehlich 1 (double-acid) extractant.

Soil test interpretation

Element Very Low Low Medium High Very High

mg/kg (ppm)1

P < 10 10 – 15 16 – 30 31 – 60 > 60

Mg2 --- < 15 15 – 30 > 30 ---

Ca2 2503 > 250

Cu < 254 25 – 505 > 506

1

parts per million (ppm) x 2 = lbs/acre.

2

A Ca-to-Mg ratio greater than 10 may induce Mg deficiency.

3

The Univ. of Florida Extension Soil Testing Laboratory does not interpret extractable Ca. 

Work with Florida citrus trees suggests that a Mehlich 1 soil test Ca of 250 mg/kg or greater is sufficient.

4

Cu toxicity is unlikely even if soil pH is less than 5.5.

5

Cu toxicity is possible if soil pH is less than 5.5.

6

Cu toxicity is likely unless soil pH is raised to 6.5.

Table 4.5. Soil test interpretations for other extraction methods compared with Mehlich 1.

Extractant Nutrient

Soil test interpretation

Very Low Low Medium High Very High

(Less than sufficient) (Sufficient)

Mehlich 1

P

mg/kg 

(ppm)1

< 10 10 – 15 16 – 30 31 – 60 > 60

Mehlich 32 < 11 11 – 16 17 – 29 30 – 56 > 56

Ammonium acetate pH 4.83 ≤ 11 > 11

Bray P13 ≤ 40 > 40

Bray P23 ≤ 65 > 65

Mg

mg/kg (ppm)

Low Medium High

Mehlich 1 < 15 15 – 30 > 30

Mehlich 34 < 25 25 – 33 > 33

Ammonium acetate pH 4.85 < 14 14 – 26 > 26

Less than sufficient Sufficient

Ammonium acetate pH 7.03 ≤ 50 > 50

Ca

mg/kg (ppm)

Less than sufficient Sufficient

Mehlich 1 ≤ 250 > 250

Mehlich 34 ≤ 200 > 200

Ammonium acetate pH 4.85 ≤ 270 > 270

Ammonium acetate pH 7.03 ≤ 250 > 250

1

parts per million (ppm) x 2 = lbs/acre.

2

Estimated from unpublished correlation data (T. A. Obreza, 2006).

3

From Koo et al. (1984).

4

Estimated from correlation data (Alva, 1993).

5

Estimated from correlation data (Sartain, 1978).
30 

Soil sampling technique

•	 Each soil sample should consist of one soil core taken 

about 8 inches deep at the dripline of 15 to 20 trees 

within the area wetted by the irrigation system in the 

zone of maximum root activity (Fig. 4.6).

•	 Sampled areas should correspond with grove blocks 

where leaf samples were collected. The area should contain similar soil types with trees of roughly uniform size 

and vigor.

•	 Thoroughly mix the cores in a non-metal bucket to 

form a composite sample. Take a subsample from this 

mixture and place it into a labeled paper bag.

Fig. 4.6. Sample soil near the dripline of the trees, not in the 

row middle. (Thomas Obreza)

Special case: Diagnosing growth disorders

•	 Collect soil samples from beneath affected trees as well 

as normal trees and analyze them separately.

•	 If possible, confine the sampling area to trees that are 

close to each other.

Preparation for analysis

•	 Soil samples should be air-dried before shipping to the 

laboratory for analysis.

Analysis and interpretation

•	 The basic soil analysis package run by most agricultural 

laboratories includes soil pH and extractable P, K, Ca, 

and Mg. Organic matter is sometimes part of the basic 

package or it may be a separate analysis. Extractable Cu 

is normally determined upon request.

•	 Since extractable nutrients are measured, the magnitude of soil test values may differ between different 

laboratories, but this difference is not a concern as long 

as the extraction method is calibrated for citrus.

•	 The laboratory interprets each soil test result as very 

low, low, medium, high, or very high, and may also 

provide fertilizer recommendations accordingly. Citrus growers can independently interpret the numerical 

results according to UF-IFAS guidelines based on the 

extractant used (Tables 4.4 and 4.5).

•	 The interpretations should be used to make management decisions regarding soil pH adjustment or fertilizer application (Table 4.6).

4.6. Traditional vs. alternative sampling strategies

A practical nutrient management strategy uses tissue and 

soil analysis results as tools to help determine nutrient 

requirements for large grove blocks, followed by uniform 

fertilizer application across the entire area. An inherent 

problem with this approach is that some trees may be overfertilized and others may be under-fertilized. Citrus grove 

variability is common, especially on flatwoods soils. It is 

important to take this variability into consideration so the 

grove can be managed more efficiently.

A basic principle of traditional sampling is to return to 

roughly the same sampling locations from year to year. 

This technique assumes that the selected area is less variable but also representative of the entire grove or major 

portion of the block. Representative sites are selected 

based on tree observation, past experience, crop yield, soil 

type, and/or remotely-sensed images. Traditional sampling 

minimizes sampling errors, the number of samples taken, 

cost, and time required, but it does not fully indicate field 

variability.

With technological advances, the popularity of grid sampling for precision agriculture has increased in Florida’s 

citrus industry. The first step in this strategy is to place a 

1 to 5-acre grid over a grove map. The second step is to 

take soil and/or leaf samples either at the center of each 

grid section or at the point where the grid lines intersect 

(Fig. 4.7). The individual taking the samples records the 

geographic location of each point with a Global Positioning System (GPS) instrument. The third step is to match 

the analysis results with the geographic data and construct 

variability maps using Geographic Information System 

(GIS) software. If appropriate, fertilizer or lime may be 

custom-applied using an applicator equipped with variable 

rate technology (VRT).
31

Fig. 4.7. Example of the grid sampling strategy for selecting soil 

and leaf sampling locations. The red dots show pre-determined 

sampling locations that will be recorded with GPS equipment 

and used to construct variability maps.

Nutrient management using grid sampling information is 

still in development and more research is needed before 

VRT becomes widely used to manage Florida citrus tree 

nutrition. Dense grid sampling can be quite expensive and 

has limited practicality. Growers should carefully compare 

the potential for a positive return with the cost of the program before employing this method.

Between traditional and grid sampling strategies lies 

the “management zone” method (Fig. 4.8). Knowledge 

of grove characteristics such as soil types, high and low 

yielding areas, soil water and nutrient holding capacities, 

and depth to the water table allows a grower to delineate 

management zones. The zone concept requires less sampling than the grid method, but it is more targeted than 

the traditional strategy. With this technique, different 

fertilizer rates can be applied to a smaller number of zones 

without VRT equipment.

Growers should remain flexible and prepared to adjust 

sampling and management strategies. Emerging technology will continue to refine sampling systems and integrate 

information such as yield, tree age, tree size, and soil maps, 

aerial photographs, and satellite images into nutrient management decision-making.

Table 4.6. Adjusting a citrus fertilization program based on soil analysis.

Property or nutrient What if it is below the sufficiency value in the soil? 

Options:

What if it is above the sufficiency value in the soil? 

Options:

Soil pH1 1. Lime to pH 6.0.

1. Do nothing.

2. Use acid-forming N fertilizer.

3. Apply elemental sulfur.

4. Change rootstocks.

Organic matter2 1. Do nothing (live with it).

2. Apply organic material. 1. Do nothing.

P

1. Check leaf P status.

2. Apply P fertilizer if leaf P is below optimum (see 

Chapter 8).

1. Do nothing.

K 1. Apply K fertilizer (see Chapter 8). 1. Lower K fertilizer rate.

Ca 1. Check soil pH and adjust if needed.

2. Check leaf Ca status.

1. Do nothing.

2. Check leaf K and Mg status.

Mg

1. Check soil pH and adjust with dolomitic lime if 

needed.

2. Check leaf Mg status.

1. Do nothing.

Cu 1. Do nothing. 1. Lime to pH 6.5.

1

The sufficiency value for soil pH is 6.0.

2

There is no established sufficiency value for soil organic matter.
32 

Fig. 4.8. Example of soil and leaf tissue sampling locations 

using the management zone method. The grove zone area 

delineated by the blue rectangle is a productive area, while 

that delineated by the red rectangle is a weak area. The yellow 

zigzag line denotes the sampling pattern within each management zone.

4.7. Summary

Tissue and soil analysis are powerful tools to confirm nutrient deficiencies and toxicities, identify “hidden hunger,” 

evaluate fertilizer programs, study nutrient interactions, 

and determine fertilizer rates. However, if any steps in site 

selection, sampling, or analysis are faulty, the results may 

be misleading. 

Experience interpreting sample results is essential due to 

the many interacting factors that influence the concentrations of elements in soil and leaf tissue. Tree age, cropping 

history, sampling techniques, soil test interpretations, 

and leaf analysis standards all must be considered before 

making a final diagnosis. If done properly, tissue and soil 

analysis will lead to more economical and efficient use of 

fertilizers because excessive or insufficient application rates 

will be avoided.

4.8. Soil and leaf tissue analysis checklist

Use this checklist as a guide for starting a soil and leaf tissue testing program:

	A sampling program is most effective if it is done annually.

	Leaf tissue testing is valuable for all elements.

	Soil testing is most useful for pH, P, Ca, Mg, and Cu.

	Use the standard sampling procedures for soil and leaves 

described in this chapter.

	Be aware that spray residues or dust on leaf surfaces affect sample results; wash leaves for accurate micronutrient analysis. Avoid sampling recently sprayed trees.

	Be aware that a number of different soil extracting solutions exist, and they can differ in their ability to extract 

plant nutrients, especially P.

	Interpretation of leaf and soil tests should be used to 

make fertilizer or liming decisions. Wise use of the 

results allows optimal citrus production and minimizes 

fertilizer loss.
33

5.1. Common elements of precision agriculture

The term precision agriculture includes equipment that 

improves citrus management for high quality fruit production. Proper water management is a key component for 

efficient nutrient management, and precision agricultural 

tools may improve nutrient and water-use efficiencies. 

In turn, producers can anticipate lower production costs 

through effective nutrient and water resource management with the expectation of sustained high yields and 

improved chance for profit.

•	 Remote sensing applies to non-destructive measurements. Remote sensing typically involves acquiring and 

processing satellite or aerial images photographed in the 

visible or near-infrared portions of the spectrum. Useful 

information derived from remote sensing includes grove 

variability, tree canopy size and health, soil type, and 

water stress in trees. Today, many more tools are available including ultrasonic and laser scanners for canopy 

volume or electro-magnetic soil sensors for detecting 

profile properties below ground.

•	 A geographic information system (GIS) is a computerized “graphic database” allowing storage, retrieval, 

display and processing of digital images or drawings 

with known positions on the earth’s surface. Several 

technologies are involved with GIS. Maps of an area 

can be digitized and used to plot positional information 

captured from global positioning system receivers. Differences in spectral reflectance of groves from aerial or 

satellite images can be used by grove managers to locate 

high yielding or low yielding areas within a grove.

•	 By obtaining signals from several satellites, a global positioning system (GPS) can be used to precisely locate a 

position on the earth’s surface, within a grove, or specific 

trees within a grove. This system works equally well to 

locate the path of a vehicle through the grove. Position 

location (geo-referencing of data, observations, objects, 

maps and images with GPS) is an essential prerequisite 

for meaningful processing and display on a GIS.

•	 Mobile computing and data storage: Portable computers collect and analyze sensor data, GIS information, and 

GPS data streams. The integration of these technologies 

allows for decision management on the fly as a vehicle 

moves through the grove. Handheld computers are valuable for making and recording field observations during 

scouting, leaf sampling or soil surveying. When used in 

conjunction with a GPS and GIS software, a handheld 

computer can be used to navigate through the field, 

which allows location and marking of trees, plots, soil 

types or other information already contained in the GIS.

•	 Soil mapping: Once the purview of the USDA-Natural 

Resource Conservation Service (NRCS), most county 

soil surveys and related maps have been digitized. These 

maps can be used as one of the layers in a GIS decision 

management system. Soil mapping with geophysical 

survey or electromagnetic induction instruments such 

as the EM38 (Geonics, LTD), or the Veris 3000 (Veris 

Technologies) allows rapid collection of digital geo-referenced map data correlated with chemical and physical 

properties of soil profiles.

•	 Variable-rate inputs: Using the technologies described 

above, grove managers can use additional controllers 

on traditional agricultural equipment to precisely apply 

fertilizers and other chemicals at spatially-varying rates 

as needed.

5.2. Objectives of precision agriculture 

for nutrient management

Many possible objectives exist for different precision 

agriculture programs. Four important objectives related to 

citrus nutrient management are:

•	 Reduce fertilizer amounts per unit land area to lower 

citrus production costs.

•	 Increase fertilizer nutrient use efficiency to lower environmental impacts.

•	 Increase fruit yield and quality.

•	 Automate and digitally record grove operations to 

speed data analysis and increase efficiency and convenience.

5.3.	 Remote sensing

5.3.1. Field measurements of soil electrical conductivity

Soil profile properties can be measured remotely using 

geophysical survey or electrical conductivity (EC) sensors. The use of EC sensors for nutrient management is 

an indirect measurement, reflecting the dissolved salt and 

hence also the ion concentrations in the soil. Since fertil5. Precision Agriculture for Citrus Nutrient Management

Arnold W. Schumann and Edward A. Hanlon
34 

izers are salts, changes in dissolved and adsorbed fertilizer 

can be sensed as changes in electrical conductivity. Since 

the measurement involves all salts in the soil, the portion 

attributed to fertilizer can only be estimated, but has been 

attempted for some crops in uniform soils with relatively 

small changes in salinity. Both nutrients and salts from 

other sources (e.g. saline well water) are measured by this 

remote sensing equipment. Distinguishing salty water 

from fertilizer salts can be difficult, especially if the soil 

varies naturally throughout the grove.

Another use of this type of sensor is to detect limiting 

layers in soil profiles. A soil must provide sufficient soil 

volume for proper tree root growth and nutrient uptake. 

Improper soil volume for tree root growth may lead to 

tree stunting with resulting loss of yield and quality, or 

tree mortality. Common soil profile limitations in citrus 

production are described below.

•	 Along the central Florida ridge production area, 

soils are deep and well-drained, but are infertile and 

droughty.

•	 In citrus flatwoods production areas, root damage may 

result from shallow (perched) water tables and associated capillary rise.

•	 Flatwoods production areas exhibit shallow clay layers and/or cemented spodic horizons that restrict or 

prevent root growth.

•	 In most citrus production areas, soils are sandy with low 

organic matter content and fertility, especially in the 

E-horizon of the subsoil.

•	 Citrus grove soils are often prone to soil compaction 

due to vehicular traffic.

Since field EC can be measured with mobile sensors, these 

data can be linked with GPS information to create relevant 

maps of each grove.

5.3.2. Ultrasonic canopy measurements

Citrus tree canopy height and volume can be measured 

remotely using a vertical sensor array that sends out ultrasonic pulses and detects the distance to the tree canopy 

(Fig. 5.1). The sensor array has a differential global positioning satellite system (DGPS) instrument that records 

its position within the grove. Both the DGPS and ultrasound readings are recorded and processed by a computer 

program to create a map of the canopy volumes within the 

grove (Fig. 5.2).

Calculated canopy volumes (light green) are superimposed 

on an aerial photograph of the grove (Fig 5.2), which has a 

considerable number of resets. From this map, tree canopy 

sizes can be displayed on a frequency diagram (Fig 5.3). 

When shown in this manner, new resets, resets planted in 

1989 after a freeze, and original grove trees can be identified.

Since citrus yield is directly related to canopy volume, this 

type of diagram can help growers make decisions concerning long-term operations within their groves. For example, 

this grove has a wide range of canopy volumes and can 

be expected to have a considerable range in yield as well. 

Managing for somewhat less canopy volume variability in 

this grove could improve yields and reduce environmental 

impacts of agrichemicals.

Canopy volume measurements can also be used to aid 

other management strategies like changes in irrigation 

for drought or delineation of wet zones within the grove, 

soil textural changes that influence tree growth in production, etc.

Fig. 5.1. Sensing the height and volume of the tree canopy.

Fig. 5.2. Map of citrus tree canopy volume.
35

Fig. 5.3. Frequency histogram of tree size within a citrus grove.

5.3.3. Citrus yield mapping

An automatic tub position logger can record the grove 

position where fruit was harvested using GPS technology 

(Fig. 5.4). One tub (red dot) is equivalent to 10 boxes of 

fruit. Yield maps produced from the positioning data and 

the number of tubs per unit grove area can be used to identify both high and low production sites within the grove. 

In the example shown in Fig. 5.5, the high yield locations 

are dark green (maximum yield of 543 boxes/acre). Low 

yielding locations (light yellow) produced only 181 boxes 

per acre. The average for the entire grove was 336 boxes 

per acre. Fertilizer savings can be realized by applying the 

rate of fertilizer needed by the trees based on their yield 

(Section 8.2, Fig. 8.3) using yield maps like the one in Fig. 

5.5 with a variable rate fertilizer applicator (Section 5.4).

5.4. Precision nutrient application – 

Variable rate fertilization

After determining canopy volume and yield throughout 

the grove, the next step is to apply fertilizers only where 

supplemental nutrients are needed. Variable rate fertilizer 

spreaders can reduce fertilizer use in citrus groves by as 

much as 40%. In addition to reducing production costs, 

risk of nutrient movement from the grove is minimized. In 

variable rate fertilization of citrus groves, granular fertilizer 

is accurately placed in independent left and right ‘bands’ 

under the trees. The amount of fertilizer is regulated according to either a GPS-guided prescription map, or by 

the number of sensors that detect a tree canopy in left- or 

right-hand rows.

Roots are the primary target for fertilizer applications, and 

their growth pattern in the soil follows tree canopy volume 

growth above the surface. Fertilizer should not be placed 

Fig. 5.4. Locations of harvested tubs of fruit are marked by the 

red dots.

Fig. 5.5. Citrus yield map identifying areas of high (dark green) 

and low (yellow) production.
36 

where roots are not present. Tree spacing and bedding 

also affects root growth patterns. Immature trees or resets 

should not receive the same fertilization as mature trees 

(see Chapter 8). If dry fertilizer is uniformly band-applied 

to a grove with varying tree sizes, fertilizer will be wasted 

(Fig. 5.6). This problem can be solved with variable rate 

application equipment that uses canopy sensors in a lookahead mode, rapidly positioning fertilizer dispenser valves 

on each side of the spreader to:

•	 Avoid dispensing fertilizer where there are no trees.

•	 Adjust the applied fertilizer rate based on tree size 

(canopy volume).

•	 Make these measurements and valve adjustments in a 

synchronized fashion as the spreader is moving through 

the grove.

Fig. 5.6. Areas in the tree row where fertilizer can be wasted 

when using simple band placement. The fertilizer bands cover 

considerably more area than the area in which citrus roots are 

located. Fertilizer applied in areas without roots is wasted.

Sensing and valve adjustment must be automatically synchronized with the speed of the application equipment to 

apply fertilizer properly to the correct trees. Variable rate 

fertilization is most effective in groves with high spatial 

variability because the technology is designed to exploit 

variability. Perfectly uniform groves with no gaps between 

canopies would not benefit from variable rate technology. 

A grove containing a mixture of mature trees, young trees, 

and/or resets (such as that in Fig 5.3) will benefit the most 

from using this technology to apply fertilizers.

Not all variable-rate applicator controllers perform well on 

the fly. Some commercial controllers do not support the 

look-ahead feature to allow accurate fertilizer placement 

under trees while avoiding other locations where fertilizer is not needed. Other controllers and their valves have 

response times that are too slow to cope with the rapidly 

changing fertilizer requirements of a variable grove.

Fig. 5.7 shows the use of photoelectric diffuse reflectance 

sensors with look-ahead and tree height sensing capabilities. Sensor angle increases with vertical height allowing 

sensors to be placed lower than the height of the tallest 

trees. For smaller trees, the angle of the sensors is decreased. Trigonometry calculations allow the estimate 

of sensor height to be calculated accurately as a function 

of the sensor angle. Compare the benefits of this sensor 

arrangement with that in Fig. 5.1, and note the reduced 

height of the sensor array in Fig. 5.7.

Fig. 5.7. Variable rate fertilization is linked to measurements of 

tree height.

Since canopy volume is related to yield and tree height, fertilizer rates can be adjusted based on tree height. The proportion of fertilizer to be applied is shown as a function of 

tree height in Fig. 5.7. The look-ahead sensing capability 

must be coupled with the mechanical parts of the fertilizer 

application equipment. Calculations must be based on the 

ground speed of the vehicle, the height of the tree, and the 

time delay for the equipment to respond to the demand 

for changing fertilizer rates.

Slow reaction time for a variable rate application system 

means that fertilizer will be applied at the wrong rates 

(Fig. 5.8). To avoid this bias in fertilizer application, the 

variable rate application system must be responsive and 

properly tuned. When purchasing a variable rate spreader, 

insist on rapid response times and look-ahead sensing using well-matched components. The target rate is the UF/

IFAS recommended fertilizer rate proportioned using the 

percentages shown in Fig. 5.7 for applicable tree heights in 

this particular example. A rule of thumb is not to reduce 
37

variable fertilizer rates based on tree height to less than 

50% of the maximum rate used. The smallest trees have a 

less efficient root system and a smaller horizontal canopy 

profile that further reduces the applied rates.

Fig. 5.8. Bias in actual applied fertilizer rate compared with 

the required rate when using a variable rate spreader with slow 

response times.

5.5. Summary

•	 Precision agriculture tools can add considerable

strength to grove management decisions.

•	 Sensing devices can be used to determine water table

depth and other soil properties that aid in irrigation, 

drainage, and fertilizer management decisions.

•	 Tree sensing equipment can be used to estimate canopy

volume and canopy height.

•	 Canopy volume and tree height can be used when planning tree replacement strategies, fertilizer management, 

and zone irrigation decisions.

•	 In combination with variable-rate applicators and

appropriate look-ahead technology, fertilizers can be applied accurately based on tree need and production.

•	 Grove managers should consider use of precision agriculture tools to keep production costs low and improve 

citrus yields while avoiding potential environmental 

concerns.
38 

6.1. Introduction

Nitrogen fertilizers are classified as inorganic (e.g. ammonium nitrate), synthetic organic (e.g. urea), or natural 

organic (e.g. animal manure). Inorganic and synthetic 

organic high-analysis N fertilizers are most economical 

to use in citrus groves. They are rapidly-available to plants 

unless coated as a component of controlled-release fertilizer. Natural organic materials are more slowly available 

and lower in analysis, so higher application rates of these 

products are needed to supply equal amounts of available 

N compared with high-analysis fertilizer. For this reason, 

they are usually more expensive per unit of N.

The other macronutrients (P, K, Ca, Mg, and S) are usually applied as inorganic fertilizers. An exception is when 

a grower applies a natural organic material like animal 

manure that contains a wide range of nutrients. Major P 

and K fertilizers are manufactured from mined products. 

Calcium, Mg, and S fertilizers are derived from mined 

sources like limestone and gypsum.

Micronutrients applied to citrus are usually inorganic 

or synthetic organic fertilizers. Common micronutrient 

fertilizers and their analyses are listed in Appendix B. Salt 

index values (Section 11.5) for typical fertilizer sources 

and examples of how to calculate the salt index of a fertilizer blend are shown in Appendix C.

6.2. Solid sources for soil application

Solid sources are typically bulk-blended into N-K2O or 

complete N-P2O5-K2O fertilizers, often including micronutrients, for spreading in citrus groves. Uniform particle 

size is required to prevent bulk blends from separating 

during transport to the grove or transfer from delivery 

trailer to spreader (Fig. 6.1). 

Most solid nutrient sources are readily water-soluble and 

rapidly available for tree uptake (Fig. 6.2; Appendix D, 

Table D.1). Solid fertilizers are applied with conventional 

spreading equipment and are sometimes applied by hand 

to young trees. Common solid sources applied to citrus 

grove soils include:

Nitrogen

•	 Ammonium nitrate

•	 Ammonium sulfate

•	 Urea

•	 Calcium nitrate

•	 Potassium nitrate

•	 Diammonium phosphate

Phosphorus

•	 Ordinary superphosphate

•	 Concentrated superphosphate

•	 Monoammonium phosphate

•	 Diammonium phosphate

Potassium

•	 Potassium chloride (muriate of potash)

•	 Potassium sulfate

•	 Potassium-magnesium sulfate

•	 Potassium nitrate

Calcium

•	 Calcium carbonate (calcitic lime)

•	 Calcium sulfate (gypsum)

•	 Calcium nitrate

Magnesium

•	 Magnesium carbonate (dolomitic lime)

•	 Potassium-magnesium sulfate (sul-po-mag)

•	 Magnesium sulfate

•	 Magnesium oxide

Sulfur

•	 Ammonium sulfate

•	 Potassium sulfate

•	 Potassium-magnesium sulfate

•	 Ordinary superphosphate

•	 Calcium sulfate (gypsum)

•	 Elemental sulfur

6. Fertilizer Sources and Formulations

Thomas A. Obreza and Brian J. Boman

Fig. 6.1. Unloading dry solid fertilizer from a delivery trailer to 

a grove spreader. (Stephen Futch)
39

Iron

•	 Iron sulfate

•	 Iron oxy-sulfate

•	 Iron EDTA and HEDTA

•	 Iron DTPA

•	 Iron EDDHA

•	 Iron sucrate

•	 Iron humate

Manganese

•	 Manganese sulfate

•	 Manganese oxy-sulfate

•	 Manganese oxide

Zinc

•	 Zinc sulfate

•	 Zinc oxide

•	 Zinc EDTA and HEDTA

Copper

•	 Copper sulfate

Boron

•	 Borax (Sodium tetraborate)

Molybdenum

•	 Ammonium molybdate

•	 Sodium molybdate

6.3. Solid N fertilizer sources and 

ammonia volatilization

Loss of N fertilizer through ammonia volatilization is a 

concern in citrus groves because solid N sources applied 

to the soil surface are rarely incorporated. Up to 50% of 

the N in solid urea or ammonium-containing fertilizer

sources can volatilize to the atmosphere when applied to 

citrus under two circumstances: 1) surface-applied ammonium fertilizer sources on calcareous or freshly-limed soils; 

and 2) surface-applied urea on acidic or alkaline soils.

Ammonia is easily lost from urea because it rapidly converts to ammonium carbonate following surface application. If not incorporated or watered in, ammonium 

carbonate readily decomposes to produce ammonia and 

carbon dioxide gases. An ammonium carbonate solution 

has a pH of about 8.6, which causes large ammonia losses 

whenever the gas is free to escape to the atmosphere as 

with surface application. Thus, urea volatilizes readily 

because it creates its own alkaline environment around 

each granule. If solid urea is dissolved and moved into the 

soil by irrigation or rainfall immediately after application, 

volatilization is insignificant.

When ammonium-containing fertilizers are surfaceapplied to soils containing free calcium carbonate (e.g. 

calcareous or freshly-limed soils), an alkaline environment 

is maintained that allows conversion of ammonium ions to 

ammonia gas. The degree to which this reaction proceeds 

depends on the anion associated with the ammonium 

fertilizer. Those N fertilizers that react to form Ca-reaction 

products of low solubility will lose considerably more 

ammonia than fertilizers producing reaction products of 

relatively higher solubility. For example, ammonium sulfate will produce low-solubility gypsum (CaSO4

) in combination with soil calcium, while ammonium nitrate will 

produce highly-soluble calcium nitrate. Thus, if ammonium sulfate and ammonium nitrate are surface-applied 

to calcareous soil and are not immediately irrigated into 

the soil, more ammonium will volatilize from ammonium 

sulfate. Note that N fertilizer in the nitrate form is not 

subject to volatilization.

6.4. Solution sources – Fertigation

Fertigation is the application of solution fertilizer with 

irrigation water, typically through a microsprinkler or 

drip system (Fig. 6.3; Chapter 7). The two most common 

nutrients applied to citrus through fertigation are N and 

K. Fertilizer injected into a microirrigation system should 

Fig. 6.2. Cold water solubility of macronutrient fertilizer be a true solution with no solid contaminants. Solutions 

sources.
40 

are made by dissolving readily-soluble sources of plant 

nutrients in water. Nutrient sources used to manufacture 

true solutions include:

Nitrogen

•	 Ammonium nitrate

•	 Urea

•	 Urea-sulfuric acid

•	 Ammonium sulfate

•	 Ammonium thiosulfate

•	 Calcium nitrate

•	 Potassium nitrate

Phosphorus

•	 Ammonium polyphosphate

•	 Phosphoric acid

Potassium

•	 Potassium chloride

•	 Potassium nitrate

•	 Potassium sulfate

•	 Potassium thiosulfate

Micronutrients

•	 Borax

•	 Copper sulfate (acidified)

•	 Magnesium sulfate

•	 Manganese sulfate (acidified)

•	 Zinc sulfate

•	 Zn, Mn, Cu, and Fe chelates (EDTA, DTPA, and/or

EDDHA)

Growers should be cautious when applying solutions 

containing P through a microirrigation system. If the pH 

of the fertilizer-water mixture is not kept acidic, solution 

P can combine with dissolved Ca in the irrigation water to 

form an insoluble precipitate that will clog irrigation emitters. Similarly, when using a Ca-based source like calcium 

nitrate in solution, avoid any form of phosphate or sulfate, 

and use chelated, complexed or nitrate forms of micronutrients instead of the sulfate form.

Some commonly used liquid formulations, their analyses, 

and weights per gallon are listed in Appendix D.

6.5. Solution fertilizer salt-out

Solution fertilizer salt-out (crystallization) in storage tanks 

can be a problem during the winter. As a rule of thumb, 

the more complex the formulation, the greater the tendency for salt-out. The most important factor affecting 

salt-out temperature of a fertilizer solution is its concentration of N and K. The higher the analysis of a solution, 

the higher the crystallization temperature. For example, a 

10-0-10 solution fertilizer made from ammonium nitrate 

and potassium chloride will salt-out at about 60° F, while 

8-0-8 and 6-0-6 solutions made from the same sources will 

salt out at about 41° F and 27° F, respectively. Solutions 

made with potassium nitrate will salt-out a few degrees 

lower than solutions made with potassium chloride. The 

addition of micronutrients to the solution does not significantly change the salt-out temperature.

Solution fertilizer suppliers can provide salt-out temperatures for specific mixtures. If prolonged temperatures below the salt-out temperature are expected, crystallization 

should be prevented by diluting the solution with water. 

Adding enough water to lower the N and K2

O analyses 

to less than 5% will prevent salt-out most of the time. If 

there is no room in the storage tank for dilution, adding 

polyphosphate can lower the salt-out temperature several 

degrees. However, polyphosphates are most effective when 

used with lower analysis solutions.

6.6. Solution sources – Foliar sprays

Both foliar-applied urea and potassium phosphite have 

been shown to increase flowering, fruit yield, and total 

soluble solids yield of Valencia orange trees in Florida. Foliar N has also been used as a substitute for part of a soilapplied N fertilization program in an effort to decrease N 

leaching potential.

If urea is to be foliar-applied, only spray-grade, low-biuret 

(less than 0.25%) material should be used to avoid biuret 

toxicity. Use caution when tank mixing urea with pesticides, 

crop oil, and other products because it can be phytotoxic 

when applied at higher rates, particularly in combination 

with oil. Nutrient sources applied in foliar sprays include:

Fig. 6.3. Nutrient solution sources for fertigation are stored 

in large plastic tanks at the irrigation pump station. (Thomas 

Obreza)
41

Nitrogen

•	 Low-biuret urea

Phosphorus

•	 Potassium phosphite

•	 Ammonium phosphite

•	 Phosphorous acid

Potassium

•	 Potassium nitrate

•	 Potassium thiosulfate

•	 Monopotassium phosphate

•	 Dipotassium phosphate

Micronutrients

•	 Manganese/zinc nitrates

•	 Manganese/zinc sulfates

•	 Copper sulfate

•	 Synthetic and natural organic chelates (many forms)

•	 Borax

6.7. Suspension sources

Suspensions are fluid fertilizers in which solids are prevented from settling by a suspension agent, usually a swelling-type clay like attapulgite or bentonite. Suspensions 

are higher in concentration than true solution fertilizers 

because they contain both dissolved and non-dissolved 

fertilizer. They can be uniformly applied to the soil surface 

as part of fertilizer-herbicide mixtures that sometimes 

include micronutrients. Mechanical agitation may be 

necessary to maintain fertilizer concentration uniformity 

in the tank. Suspensions are made from the same fertilizers 

sources used to make true solutions (see Section 6.4).

6.8. Slow-release sources

Slow-release sources are materials of limited water solubility that release plant-available nutrients as they decompose 

or degrade in the soil following application. Except for a 

few slow-release K sources, almost all slow-release fertilizers are N sources. The release process is either biological or 

chemical, and slow-release fertilizers are grouped accordingly. For example, release of N from urea formaldehyde 

requires both dissolution of the fertilizer and microbial 

decomposition, while release of N from isobutylidene 

diurea (IBDU) involves slow dissolution only. Processed 

waste products release N through microbial degradation. 

Examples include biosolids, composts, and tankages.

Slow-release nutrient sources applied to Florida citrus 

include:

•	 Sulfur-coated urea

•	 Urea formaldehyde

o Ureaform

o Methylene urea

•	 IBDU

•	 Organiform (tankage)

•	 Animal manures

•	 Municipal biosolids

•	 Municipal composts

6.9. Controlled-release sources

Controlled release fertilizers (CRFs) contain one or more 

plant nutrients in a form that either delays their availability for plant uptake after application or extends their availability significantly longer than rapidly-available fertilizers 

like ammonium nitrate, urea, or potassium chloride. CRFs 

were initially developed for their horticultural benefits, 

but they have also attracted attention in the BMP era 

because they can reduce potential negative environmental 

effects of crop fertilization.

Controlling the release of nutrients is accomplished by 

surrounding conventional water-soluble fertilizer sources 

with a coating (Fig. 6.4). Substances used as coatings 

include polymers, plastics, waxes, and sulfur, either alone 

or in combination. The standard (reference) release rate of 

a particular material is controlled by varying the coating 

thickness or physical characteristics during manufacture, 

but nutrient release is also typically influenced by soil temperature or water content.

CRFs are substantially more expensive than conventional 

sources so their use may be limited to special situations 

such as young tree fertilization or nutrient management in 

environmentally-sensitive areas. Many diverse CRF products have been developed and the technology continues to 

advance. Most CRFs contain a complete N-P-K fertilizer 

Fig. 6.4. Coated fertilizer sources control the release of watersoluble nutrients to citrus trees and can last up to 12 months. 

(Thomas Obreza)
42 

combination with small concentrations of micronutrients. 

Growers are advised to consult the label of such products 

to determine release characteristics and recommended application strategy.

6.10. Formulating fertilizer products

More then 80% of the fertilizer sold in Florida is bulkblended at the request of the customer. It is possible to 

obtain almost any requested combination of nutrients by 

blending the various base sources listed in Appendix B. In 

most fertilizer plants, blends are prepared by mixing the 

base sources and conveying the mixture to a bulk trailer or 

into bags. An example of the procedure used to formulate 

a fertilizer mixture is described in Appendix E.

6.11. Nutrient sources for organic citrus production

Organic citrus production relies on animal manures (Table 

6.1), plant materials, and some mined raw minerals as nutrient sources as opposed to inorganic chemicals and compounds. Essentially all manufactured or synthetic fertilizers are prohibited in certified organic groves. In particular, 

materials containing chloride, nitrate, and highly-soluble 

phosphate cannot be used. Although biosolids (processed 

domestic wastewater residuals) are organic materials, they 

are prohibited from organic farming.

Examples of nutrient sources that can potentially be used 

for organic citrus production are listed below. Some are 

allowed with no restrictions, and others are restricted to 

special cases. Organic growers should consult their certifying organization for specific rules and guidelines.

•	 Legumes

•	 Composted food and forestry by-products

•	 Wood ash

•	 Crop residues

o Green manures

o Peat moss

o Straw

o Seaweed

•	 Animal manures

o Beef

o Dairy

o Poultry

o Horse

•	 Meals

o Alfalfa

o Bone

o Blood

o Cottonseed

o Fish

o Feather

o Hoof and horn

o Leather

o Soybean

•	 Minerals and salts

o Agricultural limestone

o Basalt

o Borax

o Bordeaux mixtures

o Chilean sodium nitrate

o Colloidal phosphate

o Greensand (glauconite)

o Granite (ground)

o Gypsum (mined raw material only)

o Kiln dust

o Langbeinite (K-Mg sulfate – mined raw material only)

o Micronutrient-sulfate salts

o Natural rock phosphate

o Potassium sulfate (mined only)

o Sodium nitrate (mined only) – limited use

o Sodium molybdate

o Sulfur (mined only)

o Zinc sulfate

Section 11.9 presents additional information about soil 

fertility and nutrient management guidelines for organic 

citrus production.

Table 6.1. Manure nutrients.

Manure

lbs of nutrient 

per ton of manure C:N 

ratio N P2

O5 K2O

Beef 24:1

scraped from 

paved surface 11-14 7-9 10-13

scraped from 

dirt feedlot 21-26 14-16 20-23

Dairy 11-30:1

scraped from 

paved surface 10 3-6 6-9

with bedding 9 3 6

Poultry

broiler house litter 72 78 46 10-14:1

stockpiled litter 36 80 34

layer-deep pit 38 56 30 3-10:1

layer-undercage 26 31 20

Horse (fresh) 12 6 12 22-50:1

Note: Nutrient contents listed are general averages. Samples of the 

manure to be applied should be analyzed before calculating application rates.
43

7.1. Applying dry fertilizers

The choice of fertilizer application method becomes important as growers seek to improve nutrient use efficiency 

and reduce losses to leaching and runoff. Dry solid fertilizer 

spreaders should apply materials directly over the root zone, 

avoiding the row middle. It may be necessary to prune tree 

skirts to improve the spreading pattern and uniformity. 

When applying fertilizers to young trees, managers should 

take advantage of manual or electronic spreader adaptations 

that deliver fertilizer rates accurately to small tree root zones 

while leaving out the area between trees where roots are not 

present. Effort should be made to reduce surface movement 

of applied fertilizers by rainfall or wind.

Aerial application of micronutrients and other sprays is an 

accepted practice in Florida’s citrus industry, but this application method is not recommended for dry fertilizers. 

For economical and efficient fruit production, it is essential that spreaders be calibrated to apply exact amounts 

of fertilizers per acre. Plant nutrients should be applied 

according to individual crop requirements. Reduced yield 

may result from insufficient nutrient application, while 

excess nutrient application can lead to accumulation in 

soils and adjacent surface or groundwater.

Equipment needed to calibrate a typical grove spreader 

such as that shown in Fig. 7.1:

•	 A tray (catch pan) that slides under the opening where 

the chain pulls fertilizer from the hopper. This tray fits 

just above the fans.

•	 A spring scale or balance to weigh the fertilizer.

Calibration steps:

1. Position the empty tray so it will catch the fertilizer 

distributed from the hopper during calibration.

2. Drive the tractor and spreader along a row middle or 

edge of a block as if fertilizing normally for a distance 

of exactly five trees. Record the speed, RPM and gear of 

the tractor.

3. Collect the fertilizer discharged into the tray and weigh it.

4. Repeat the above procedure several times until a reliable 

average weight is obtained.

5. Use this equation to convert the weight of material to 

application rate in lbs/acre or lbs/tree, assuming the 

spreader is fertilizing two rows at a time, i.e. running 

every other row middle:

Fertilizer material application rate (lbs/acre) =

Weight collected (lbs)

[10 trees ÷ tree density (trees/acre)]

For example, if an average of 28 lbs of fertilizer were 

collected and the grove density was 145 trees/acre, the 

application rate would be:

28 lbs ÷ (10 ÷ 145) = 406 lbs/acre.

7.2. Fertigation

Fertigation is the application of soluble fertilizers with irrigation water. Advantages of fertigation:

•	 Fertilizer is placed in the wetted area where the most 

active roots are located.

•	 Fertilizer may be applied more frequently in small 

amounts so that it is available when the tree needs it.

•	 Increased fertilizer application frequency can increase 

fertilizer efficiency and reduce leaching.

•	 Compared with conventional ground application, fertigation can produce similar or better tree growth, yield, 

and fruit quality with less fertilizer.

7. Methods of Fertilizer Application

Thomas A. Obreza, Brian J. Boman, Mongi Zekri, and Stephen H. Futch

(Includes contributions by David P. H. Tucker)

Fig. 7.1. Conventional fertilizer spreader equipped with a split 

chain and rear deflector plates to apply dry, solid fertilizer 

beneath the citrus tree canopy. (Mongi Zekri)
44 

•	 Application cost is lower than that of dry or foliar fertilizer application since fertilization is incorporated into 

the normal irrigation schedule.

Disadvantages of fertigation:

•	 Fertilizer application uniformity and coverage depend 

on proper design, installation, and maintenance of the 

irrigation system.

•	 Extra equipment (injection device, tank, backflow prevention system) must be added to the irrigation system 

(Fig. 7.2).

•	 Soluble fertilizers are more expensive than granular 

fertilizers on an equal nutrient basis.

•	 Fertilizers injected into an irrigation system may contribute to emitter plugging.

•	 Fertigation does not allow for variable rate fertilization 

based on tree size.

To effectively fertigate citrus trees, growers must properly 

maintain their microirrigation systems to apply water and 

fertilizer uniformly (Obreza, 2004). In addition, growers 

must determine:

•	 The most suitable fertilizer formulations for injection;

•	 The most appropriate fertilizer analysis for different age 

trees and specific stages of growth;

•	 The fertilizer amount to apply during a given fertigation 

cycle or event;

•	 The timing and frequency of applications per season.

Fig. 7.2. Fertigation equipment, including fertilizer tank (left), 

filters (center), and injector (right). (Mongi Zekri)

Properly managed applications of plant nutrients through 

irrigation systems significantly enhance fertilizer efficiency 

while maintaining or improving yield and fruit quality. On 

the other hand, poorly managed fertigation may result in 

substantial yield loss. Keep in mind that if a very wet soil 

is fertigated when following a pre-determined fertigation 

schedule, fertilizer and water will be wasted because water 

and nutrient uptake are drastically reduced if the soil is 

saturated.

Nutrient solutions for fertigation are available in varying 

forms and concentrations. Formulations usually contain 

two or more nutrients and the solubility of various formulations vary significantly. Fertilizer solubility is critical 

when preparing stock solutions for fertigation, especially 

when dissolving dry materials in water. Preparing handmade nutrient stock solutions from dry fertilizers may 

require considerable time and effort and can generate 

sediments. Therefore, commercially-prepared true liquid 

fertilizer solutions are preferred.

Solutions are convenient because they can be directly 

injected into the irrigation water stream with a variable 

rate injection pump. Thus, they are less spill-prone than 

dry fertilizer. Although transportation cost makes liquids 

more expensive, they save time and labor and prevent 

problems associated with hand-made mixes. They also 

eliminate problems caused by insoluble materials found in 

some dry fertilizers.

Be careful when injecting fertilizers containing P or S into 

microirrigation systems. These elements may react with 

dissolved calcium and magnesium in the irrigation water 

to form insoluble precipitates that can clog irrigation lines 

and emitters. However, phosphoric acid can be safely 

injected into most water sources since it acidifies the solution, preventing precipitation. Most N sources have low 

clogging potential except for ammonium phosphate. This 

material increases water pH, which may cause Ca and Mg 

to precipitate.

Injected fertilizers must remain in solution throughout 

the entire time the irrigation system is running. To help 

avoid plugging, a properly designed microirrigation system 

should include:

•	 A method of filtering irrigation water;

•	 A means of injecting chemicals into the water;

•	 Equipment for flushing the system;

•	 In some cases, a settling basin to allow aeration and the 

removal of solids.

Most fertilizers are highly corrosive and are a potential 

health threat if they contact the skin. When fertigating, 

take these safety precautions:

•	 Wear appropriate protective clothing and eye wear 

when handling liquid fertilizers.

•	 Inspect all system components including pumps, injection devices, lines, filters, and tanks prior to use.

•	 Establish a routine fertigation monitoring program 

that emphasizes the start-up and shut-down periods in 

particular.
45

•	 Calibrate and frequently re-check injection rates and 

times to ensure proper system operation.

•	 Prevent leaks, runoff, excess applications, and application to areas near surface water.

•	 Flush all system components with clean water following 

each fertigation.

Fertilizer salts in irrigation water can burn leaves even if 

relatively low salinity water is used. When injecting fertilizers, check the electrical conductivity (EC) of the irrigation water-fertilizer mixture and try to maintain it below 

1.5 dS/m (mmhos/cm), which is equivalent to about 1000 

ppm total dissolved solids (TDS). It is preferable to inject 

small doses of fertilizer more frequently rather than less 

frequent injections of larger doses.

It is essential to have legal backflow prevention devices 

installed in the irrigation system to keep fertilizer from 

siphoning back into the water supply (Fig. 7.3). Managers should consult state and local regulations that address 

their equipment needs based on the type of water supply 

and selected injection device.

The injection system should be made of corrosion resistant 

materials like reinforced ethyl vinyl acetate (EVA) hoses, 

nylon or polypropylene fittings, and polypropylene or 

fiberglass tanks. The injection device should have its own 

check valve plus a screen to prevent non-dissolved particles 

from entering the system. Fertilizer injection should take 

place upstream of filters so that any contaminants or precipitates can be filtered out. Injection should stop during 

filter back-washing. If the system has a filter that uses part 

of the supply water to continuously back-wash, fertilizer 

must be injected downstream of it. Injection of highly 

acidic or corrosive materials should take place downstream 

of filters that may be subject to corrosion.

Fig. 7.3. Schematic diagram of fertigation equipment with 

backflow prevention devices circled.

Fertigation rates and timing should be calibrated for each 

irrigation zone. A single injection should last at least 

as long as the time it takes for water to travel from the 

injection point to the furthest emitter when the system is 

operating at normal pressure and flow rate.

There is a large increase in EC when fertilizer is present in 

irrigation water, so fertilizer movement through an irrigation system can be traced by following changes in EC with 

a portable EC meter. The time from the beginning of an 

injection to a sudden increase in EC at the farthest emitter 

from the injection point indicates the travel time of the 

fertilizer. For many systems, this time is 20 to 30 minutes. Flush time should be longer than travel time so that 

nutrients do not remain in the tubing, otherwise microbial 

growth will be enhanced. An alternate method to determine travel time is to inject liquid soap and observe the 

time it takes for bubbles to appear at the farthest emitters.

Maximum injection time depends on soil type and water 

requirement of the trees. Flush time should not be excessive to avoid leaching freshly-applied fertilizer below the 

root zone.

Fertilizer injection rates can be measured with a chemical 

flow meter or volumetrically. If a chemical flow meter is 

used on the high-pressure side of an injector, the flow meter should be rated for the pressure used. Volumetric measurements can be made by determining the time needed to 

inject a known volume of fertilizer under normal operating conditions.

For all fertigation methods, the required fertilizer injection volume can be calculated with the equation:

Volume = (Acres x N) ÷ (F x Density)

where

Volume = volume of fertilizer to be injected (gal)

Acres = grove area to be fertigated (acres)

N = amount of N to be applied per acre (lbs N/acre)

F = percentage of N in the fertilizer expressed as a decimal 

(e.g. F = 0.08 for an 8-0-8 analysis)

Density = fertilizer solution density (lbs/gal)

The weight per gallon of the liquid fertilizer solution is 

usually shown on the fertilizer label. Alternately, a known 

volume of solution can be weighed and converted to a 

weight per gallon. Appendix D (Table D.2) lists the density of many common fertilizer solutions.

Example

•	 The desired N rate is 150 lbs/acre/year.

•	 Fertilizer is to be applied in 20 equal doses.
46 

•	 The fertilizer solution is an 8-0-8 analysis made from 

ammonium nitrate and potassium chloride.

•	 The fertilizer solution density is 9.7 lbs/gal.

•	 The grove to be fertilized is 80 acres.

Calculations:

Dividing the annual rate of 150 lbs N/acre into 20 fertigations results in a single-dose application rate of 7.5 lbs N/

acre. The volume of fertilizer to be injected is then calculated from the equation:

Volume = (80 acres x 7.5 lbs N/ac) ÷ (0.08 N x 9.7 lbs/gal) 

= 773 gal

Injecting the fertilizer during a 60-minute period would 

require 773 gal/60 min = 12.9 gal/min. Alternately, 10 

gal/min of fertilizer could be injected for 77 minutes, resulting in 770 gallons injected. Injection rates may have to 

be adjusted to compensate for equipment capacities. If 10 

gal/min is above the range of the injection system, increase 

the injection time and/or fertigate more frequently.

In mature groves irrigated with typical micro-sprinklers 

that apply water between 0.10 and 0.15 inches/hr within 

the irrigation pattern, fertigation and flush cycles should 

be completed in 2 to 3 hours. Injection intervals that are 

too short result in tree exposure to high rates of salinity, 

and salt burn may result if the irrigation pattern contacts 

lower leaves and fruit. Injection times that are too long 

may result in leaching if the water-holding capacity of the 

root zone soil is exceeded before irrigation is completed.

Special attention is needed when fertigating young trees 

equipped with downspray micro-sprinklers that confine 

the irrigation pattern to a 3 to 4-ft diameter circle. The water application rate of these emitters can result in excessive 

irrigation and nutrient leaching. For example, a 15 gal/hr 

emitter with a 4-foot diameter wetted area has an effective application rate of 1.9 inches/hr. This rate may result 

in leaching even if a fertigation-flush cycle requires only 1 

hour to complete. To minimize leaching while downsprays 

are attached to emitters, post-injection irrigation must be 

the minimum required to flush the lines.

As trees mature and the root system expands to a much 

larger soil volume, the wetted area must be increased to 

achieve successful fertigation. The wetted area for a mature 

tree should cover most of the area under the canopy or at 

least 50% of the total land area. Patterns that irrigate less 

than 30% of the total land area may be unable to supply 

nutrients to enough of the root zone, and may cause leaching. Knowledge of the application rate of the irrigation 

system and the soil water content prior to irrigating is critical to fertigation management. The use of soil moisture 

sensors may be beneficial, particularly those that measure 

water content at multiple depths.

7.3. Applying suspension fertilizers

Suspension fertilizers are applied with a standard herbicide 

boom that places the fertilizer directly over the root zone 

(Fig. 7.4). Nozzles used to apply suspension fertilizers are 

larger than those typically used to apply herbicides (e.g. 

flooding or flood-jet nozzles). It is important to continuously agitate fertilizer in the tank through air sparging or 

mechanical recirculation to assure application uniformity.

Boom application equipment should be modified with 

manifolds and nozzles made of a salt-resistant material 

like stainless steel. Applying fertilizer with a boom provides the opportunity to co-apply other agrichemicals like 

herbicides, insecticides, and fungicides. The salt effect of 

the liquid fertilizer can complement residual herbicides by 

burning existing weeds. However, care should be taken to 

avoid incompatibility when mixing materials.

Fig. 7.4. Boom applicator used to apply suspension fertilizers. 

(Stephen Futch)

7.4. Applying foliar fertilizers

Foliar nutrient application to citrus trees is common for 

economic and environmental reasons. Under specific conditions, it can improve nutrient uptake efficiency because 

nutrients are directly absorbed by the leaves. Foliar spraying can provide specific nutrients to the tree on a timely 

basis during critical stages of growth, flowering, and fruit 

development.

A well-planned foliar nutrition program can supplement 

soil fertilizer applications especially when the citrus root 

system is unable to keep up with crop demand or when soil 

nutrients are unavailable. In some cases, a significant por-
47

tion of nutritional needs can be met with a foliar program. 

Foliar application is not intended to replace a soil-applied 

N-P-K fertilization program. However, some macronutrients can be foliarly applied at rates sufficient to influence 

young tree growth, yield, and fruit quality.

Foliar application is an excellent means to supply plant requirements of secondary and micronutrients like Mg, Zn, 

Mn, Cu, B, and Mo. Foliar application of micronutrients is 

more effective than soil application in the short-term with 

the exception of Fe. Foliar sprays are taken up more rapidly 

by the tree but their effect typically lasts only as long as it 

takes for the targeted growth flush to mature.

Foliar application can be integrated into an annual citrus 

nutrition program. It can be used to help trees through 

short but critical periods of nutrient demand, such as bud 

differentiation, flowering, fruit set, and fruit development. 

It is also useful when soil or environmental conditions 

are unfavorable for nutrient uptake by roots, such as cold 

weather, prolonged wet or dry soils, calcareous soil, or any 

other condition that decreases the ability of the tree to 

take up nutrients when there is a demand. Foliar spraying 

is particularly useful when a nutritional deficiency is diagnosed because it is the most rapid way to effect nutrient 

uptake by citrus trees.

Foliar fertilizers are usually applied to citrus trees with a 

conventional grove airblast sprayer (Fig. 7.5), typically in 

100 to 250 gal of water per acre. The goal of airblast spraying is to replace the air contained within the tree canopy 

with spray-laden air. Sprayer travel speed must be slow 

enough to create air momentum to penetrate the canopy. 

However, unlike fungicide or miticide applications, it is 

not necessary to achieve highly-uniform spray coverage 

of leaves or fruit. In many cases, nutrient sprays can be 

applied exclusively from the bed tops in 2-row bedded 

flatwoods groves and from alternate row middles in ridge 

groves.

Nutrient absorption is most rapid during the first several 

hours after application. Environmental conditions that 

cause stomata (leaf openings) to close reduce nutrient 

uptake efficiency. It is best to apply foliar sprays when air 

temperature is cool and humidity is high, such as early 

morning or late evening. Applying fertilizer during these 

times also decreases the chance of leaf burn. Maximum 

benefit of Zn, Mn, and Cu is obtained when spraying 

young leaves that are two-thirds to nearly fully expanded, 

but before hardening off. Treating the spring flush is preferable to later growth flushes.

Fig. 7.5. Airblast sprayer used to apply soluble nutrients to 

citrus tree foliage. (P. Chris Wilson)

Micronutrient and other nutrient sources including chelates and nitrate-based materials are often applied together 

with pesticides, spray oils, surfactants and other products. 

Sometimes the chemistry of these mixes combined with 

the environmental and tree conditions at the time of application cause phytotoxicity and occasionally abscission of 

foliage and fruit. Reduced product efficacy may also occur. 

Information on compatibility of various product mixtures 

in the spray tank and the interaction of the components 

following deposition on the foliage and fruit surface is 

scarce. Poor quality water, particularly due to salinity and/

or high pH, can also contribute to the problem. Additives 

with strong penetrant activity should not be included in 

foliar spray tank mixes. Reducing the number of components in tank mixes and spraying when trees are under 

minimal stress should reduce the potential for damage.

When applying foliar nutrients, it is important to ensure 

that the pH of the spray solution is between 6 and 7. Solution pH control is particularly important when applying 

urea. If the pH of a urea spray solution exceeds 7, free 

ammonia may be generated that dramatically increases 

the potential for leaf burn. Take this precaution especially 

when growing fruit for the fresh market, where fruit blemishes (burn) can substantially reduce marketable yield.
48 

8.1. Fertilizer rates, application frequency, 

and timing for non-bearing trees 

(first 3 years in the grove)

8.1.1. Solid plantings

Nitrogen

Young citrus tree care requires managing irrigation, nutrition, weeds, diseases, pests, and cold protection to stimulate rapid canopy growth. Water and N availability are the 

most important factors affecting growth of young trees. 

Obtaining optimum growth requires substantial irrigation 

and N inputs, but excesses of either are non-productive, 

costly, and may result in loss of N by leaching or runoff.

Nitrogen fertilizer recommendations for non-bearing 

trees were derived from numerous young-tree fertilization studies conducted throughout Florida (Table 8.1 and 

Fig. 8.1). These guidelines include a range of rates by tree 

age because a number of factors influence the N fertilizer 

requirement. Criteria for selecting a rate within the recommended range include:

•	 Soil type – Trees planted in soils high in organic matter 

(e.g. 2% or greater) or with a loamy texture require less 

fertilizer than trees on low-organic matter sandy soils.

•	 Land history – New plantings on land previously used 

for pasture or vegetable production require less fertilizer during the first 1 to 2 years compared with trees 

replanted in established groves due to mineralization of 

accumulated organic matter.

•	 Fertilizer source – Use of controlled-release formulations may allow a reduction in fertilizer rate.

The annual N rate applied to solid-set 2 to 3 year old 

trees should not exceed 200 lbs/acre (Fig. 8.1).

Phosphorus

Before deciding to apply P fertilizer to young trees, test the 

soil for P and compare the results with the ranges in Tables 

4.4 or 4.5.

•	 If soil test P is in the high or very high range, do not apply P fertilizer.

•	 If soil test P is in the medium range, apply P fertilizer at 

a P2

O5 rate equal to 50% of the N rate.

•	 If soil test P is in the low range, apply P fertilizer at a 

P2O5 rate equal to 75% of the N rate.

•	 If soil test P is in the very low range, apply P fertilizer at 

a P2

O5 rate equal to the N rate.

If soil testing justifies P fertilizer application, test the soil 

again the following year and compare with Tables 4.4 

or 4.5 to determine if P fertilization can be decreased or 

omitted. Initiate a leaf tissue testing program for P in year 

3, and compare the results with the standards in Table 4.2.

Potassium

Apply K fertilizer at a K2

O rate equal to the N rate.

Calcium

Liming the soil to pH 6.0 to 6.5 and maintaining it in that 

range will supply needed Ca. If soil pH is favorable, there 

is no need to apply Ca unless soil test Ca is less than sufficient. In this case, an application of gypsum or soluble Ca 

fertilizer may be considered. If soil pH is above 6.5, the soil 

will likely contain sufficient Ca for tree growth.

Magnesium

If soil test Mg is medium or lower (Tables 4.4 and 4.5), 

apply Mg fertilizer at a rate equal to 20% of the N rate. 

Curtail Mg fertilizer application if a subsequent soil test 

shows Mg in the high range.

Micronutrients

If trees are planted on previously cultivated land (e.g 

complete grove renovation or land converted from other 

agricultural uses where fertilizer was applied), do not apply 

micronutrients unless leaf analysis indicates they are below 

optimum or leaf/twig/fruit deficiency symptoms appear.

If trees are planted on previously non-cultivated land, apply Mn, Cu, and B at 5%, 2.5%, and 0.33% of the N rate, 

respectively, until soil and leaf analysis and/or tree appearance indicate that one or more may be omitted. Boron 

may need to be applied every year because it leaches readily. Do not routinely apply Zn, Fe, or Mo unless prompted 

by visual symptoms.

8. Recommended Fertilizer Rates and Timing

Thomas A. Obreza, Kelly T. Morgan, L. Gene Albrigo, and Brian J. Boman

(Includes contributions by James J. Ferguson, Frederick S. Davies, David P. H. Tucker, Ashok K. Alva, and T. Adair Wheaton)
49

Table 8.1. Recommended N rates and minimum number of annual applications for non-bearing citrus trees.

Year in grove lbs N/tree/year

(range)

Lower limit of annual application frequency

Controlled-release fertilizer Dry soluble fertilizer Fertigation

1 0.15 – 0.30 1 6 10

2 0.30 – 0.60 1 5 10

3 0.45 – 0.90 1 4 10

Fig. 8.1. Recommended N rates for non-bearing citrus trees on a per-acre basis as a function of planting density. To determine the 

per-acre rate, find the planting density on the x-axis, move up into the colored band, and find the recommended N rate range on 

the y-axis.
50 

Nutrient management

Applying fertilizer in several small doses increases fertilizer 

efficiency because it maintains more constant N availability and reduces leaching potential. A minimum of 4 to 6 

annual applications of dry fertilizer is recommended for 

young trees. Splitting fertigation into 10 or more yearly 

applications is common. The cost of liquid injection during irrigation is relatively small particularly if the injection 

can be automated. One or two applications of controlledrelease fertilizer is satisfactory because nutrients are 

protected from leaching. Controlled-release formulations 

may be applied pre-plant, incorporated after planting, 

or broadcast to insure uniform distribution of nutrients 

throughout the enlarging root zone of young trees.

Non-bearing trees fertilized after September may be 

slightly less cold-hardy. However, citrus tree growth is triggered by favorable temperatures and soil moisture, not by 

fertilization. Omitting fertilizer in the fall will not prevent 

growth. Nutrient uptake is reduced at lower soil temperatures, particularly for trees on Swingle citrumelo rootstock. Trees on Swingle can become quite chlorotic during 

the winter months even with fall fertilization.

Irrigation management of young trees is critical because 

water stress can occur rapidly as the soil surrounding the 

limited root system dries, and because young tree growth 

is particularly sensitive to water stress. Instances where 

young tree growth improved after a grove was converted to 

fertigation may have been due more to improved soil water 

regime than nutrient delivery method.

Excessive irrigation is often a problem when managing 

young trees. Small microsprinkler wetted patterns used to 

irrigate young trees apply water at high rates. Short irrigation durations of 30 minutes or less may be required 

to avoid nutrient loss below the root zone. Irrigation line 

flushing times after fertigation must also be minimized to 

avoid nutrient leaching.

8.1.2. Resets in established groves

Resets in established groves should be fertilized like 

solid-set non-bearing trees. Resets may not grow well if 

they only receive fertilizer during mature tree application 

because only a small amount of material may be deposited 

in the young tree root zone. Resets will most likely not 

require P fertilizer, but this can be checked with a soil test. 

Controlled release materials can be applied 1 to 2 times 

per year without compromising tree growth in reset situations. In closely spaced groves, reset growth may be restricted due to competition from the adjacent older trees.

8.1.3. Example fertilizer program for non-bearing trees

Fertilizer rates for trees during the first 3 years in the grove 

are calculated on a per tree basis. For example, if a 2-year 

old tree is scheduled to receive 0.4 lbs N per tree per year 

in four equal applications, then 0.1 lbs N will be applied 

each time. Using a fertilizer containing 10% N, the tree 

will receive 1.0 lb of fertilizer per application.

8.2. Fertilizer rates for bearing trees 

(4+ years in grove)

Nutrient management for bearing trees requires many of 

the same considerations important for non-bearing trees. 

Nitrogen continues to be the most important element for 

tree growth, fruit yield, and fruit quality, but others also 

have substantial effects on production and fruit quality. Harvesting the crop removes a significant amount of 

nutrients from the grove (Table 3.3), but fruit production 

accounts for only part of the fertilizer requirement.

Nitrogen

Continued strong vegetative growth is an objective for 

several years after fruit production begins, so N fertilizer 

application supports both canopy expansion and fruit 

production. In addition, fruit quality becomes important 

for both processed and fresh fruit. Orange and grapefruit 

groves tend to receive higher N fertilizer rates if the fruit 

is grown for processing, since returns are based on lbs 

solids/acre (total sugar) production. If the fruit is grown 

for the fresh market where fruit size, shape, peel thickness, 

texture, and color are important, the N fertilizer rate is 

usually lower, perhaps two-thirds or three quarters of the 

processed fruit rate.

Young bearing trees (years 4 through 7 in the grove). 

Recommended N fertilizer rates (Table 8.2) provide 

enough N for canopy expansion towards containment 

size while producing maximum economic yields of high 

quality fruit. The N rate selected should be based on soil 

characteristics, yield potential, and tree needs as indicated 

by leaf analysis interpretation (Table 4.2).

•	 For grapefruit, the recommended annual N rate is 120 

to 160 lbs/acre.

•	 For oranges and other varieties, the recommended annual N rate is 120 to 200 lbs/acre.

•	 If justified by an exceptionally high-producing young 

grove (>650 boxes/acre), the yield-based N recommendation for trees 8+ years of age (Table 8.2) may be used.

Mature bearing trees (years 8+ in the grove). Once trees 

reach containment size, further canopy growth is not 
51

desired so nutrition inputs can be stabilized and possibly 

reduced. Nitrogen fertilizer management should focus on 

1) maintaining tree biomass, 2) generating sufficient vegetative growth to replenish fruiting wood, and 3) replacing 

N exported with the harvested crop. The guidelines for 

annual N fertilizer rates accounts for the needs of both 

vegetative growth and crop removal (Table 8.2).

Grapefruit:

•	 The recommended annual N rate is 120 to 160 lbs/

acre. For groves producing more than 800 boxes/acre, 

up to 180 lbs/acre may be considered. The N rate applied should be based on tree needs as indicated by leaf 

analysis interpretation, soil characteristics, desired fresh 

fruit quality characteristics, and yield potential.

•	 The optimum leaf N concentration associated with 

best grapefruit quality is around 2.2% (Fig. 8.2), 

which is lower than the optimum leaf N range for orange production (2.5 to 2.7%). To achieve high yields 

of large fruit, growers should adjust N fertilizer rates to 

maintain grapefruit leaf N around 2.2%. 

Oranges:

•	 The annual N rate should fall within the range of 140 

to 250 lbs/acre. The recommended rate for a specific 

grove depends on either expected yield potential (for 8 

to 11-year-old trees) or 4-year running average production history (for 12+ year-old-trees) expressed as 

either fruit yield or soluble solids production (Fig. 8.3).

•	 When basing N fertilization on expected yield potential, the rate should be selected considering 1) how well 

the young, bearing trees have produced, and 2) leaf 

tissue analysis.

•	 If leaf N is consistently maintained in the optimum 

range, additional fertilizer will not increase yield (Fig. 

8.4), and may reduce some aspects of fruit quality.

Table 8.2. Recommended N rates and minimum number of annual applications for bearing citrus trees.

Year in grove

Oranges Grapefruit Other varieties Lower limit of annual application frequency

lbs N/acre/year (range) Controlled-release 

fertilizer

Dry soluble 

fertilizer Fertigation

4 through 7 120 – 200 120 – 160 120 – 200 1 3 10

8 and up 140 – 250

Yield-based1 120 – 1602 120 – 3003 1 3 10

1

See Fig. 8.3 for specific production-based N fertilizer rate recommendations.

2

For grapefruit groves producing more than 800 boxes/acre, the maximum recommended N rate is 180 lbs/acre.

3

For Orlando tangelos, the maximum recommended N rate is 250 lbs/acre. For Honey tangerines (Murcotts), the maximum recommended 

N rate is 300 lbs/acre.

Fig. 8.2. As grapefruit leaf N increases from 2.0 to 2.3%, yield 

increases and Brix/acid ratio decreases slightly. Fruit size increases as leaf N approaches 2.2%, then decreases substantially 

(He et al., 2003).
52 

Fig. 8.3. Production-based N fertilizer rate recommendations for Florida oranges. Find the projected yield potential (8 to 11 

year-old-trees) or 4-year running average production (12+ year-old trees) on the x-axis, move up or down to the straight line, and 

find the recommended N rate range on the y-axis.
53

•	 The base N rate recommendation (140 lbs/acre) is for 

groves producing 200 boxes/acre or 1300 lbs solids/

acre.

•	 The high end of the N rate range (250 lbs/acre) is for 

groves producing 1000 boxes/acre or 5800 lbs solids/

acre.

•	 Beginning at the base N rate, the recommended N rate 

increases:

o 14 lbs N/acre for every 100 box/acre increase in 

expected yield potential or 4-year running average 

yield; or

o 2.5 lbs N/acre for every 100 lbs solids/acre increase 

in expected yield potential or 4-year running average 

yield.

Fig. 8.4. As orange leaf N increases from 2.1% to 3.0%, yield 

increases to a maximum around 2.6%, then decreases (Alva et 

al., 2006).

Applying nutrients, particularly N, using variable rate 

application should be considered (see Section 5.4). Yield 

maps (Section 5.3.3) indicate how yield varies across a 

block or grove. The use of variable rate technology is encouraged if the necessary equipment or custom applicator 

is available.

Other varieties:

•	 For other varieties, the recommended annual N rate is 

140 to 250 lbs/acre using the method above to calculate a production-based rate for specific groves.

•	 For the special case of Orlando tangelos, the maximum 

recommended annual N rate is 250 lbs N/acre.

•	 For the special case of Honey tangerines (Murcotts), the 

maximum recommended annual N rate is 300 lbs N/

acre.

Example N fertilizer rate calculation for mature, 

bearing orange trees using box/acre yield:

Recommended N rate = Base N rate + 

{[(Avg. yield – 200 boxes/acre)/100] x 14 lbs/acre}

If a grove has produced an average of 500 boxes/acre 

during the past 4 years, then

Recommended N rate = 140 lbs N/acre + 

{[(500 – 200)/100] x 14 lbs N/acre} 

= 140 + 42 = 182 lbs N/acre

Example N fertilizer rate calculation for mature, 

bearing orange trees using lbs solids/acre yield:

Recommended N rate = Base N rate + 

{[(Avg. yield – 1300 lbs sol./acre)/100] x 2.5 lbs/acre}

If a grove has produced an average of 3500 lbs solids/

acre during the past 4 years, then

Recommended N rate = 140 lbs N/acre + 

{[(3500 – 1300)/100] x 2.5 lbs N/acre} 

= 140 + 55 = 195 lbs N/acre

Leaching rain rule: If 3 or more inches of rain falls within 

72 hours after an N fertilization, “replacement” fertilizer 

may be applied up to one-half of the N rate affected by the 

rain (not to exceed 30 lbs/acre). If the affected N fertilizer 

source was 100% slow-release or controlled-release, this 

rule does not apply. If the source was a mixture of watersoluble and slow or controlled-release N, this rule applies 

only to the soluble N fraction.

Phosphorus

Determine the need for P fertilization using leaf tissue and 

soil test results.

•	 Sample leaves and soil using the procedures described in 

Chapter 4.

•	 Compare the analytical results with the interpretations 

provided in Tables 4.2, 4.4, and 4.5.

•	 Follow the P fertilization guidelines in Table 8.3.

Potassium

Apply K fertilizer at a K2

O rate equal to the N rate. If 

leaf K is consistently below optimum from year to year, 

increase the K2

O rate by 25%, especially if the grove soil is 

calcareous.
54 

Calcium

Liming the soil to pH 6.0 to 6.5 and maintaining it in that 

range will supply needed Ca. If soil pH is favorable, there 

is no need to apply Ca unless soil test Ca is less than sufficient or leaf tissue Ca is below optimum. In these cases, 

application of gypsum or soluble calcium fertilizer should 

be considered. If soil pH is above 6.5, the soil will likely 

contain sufficient Ca for tree growth and fruit production.

Magnesium

If soil test Mg is medium or lower (Tables 4.4 and 4.5) or if 

leaf Mg is below optimum (Table 4.2), apply Mg fertilizer 

at a rate equal to 20% of the N rate. Alternatively, Mg may 

be applied in a foliar spray. Curtail Mg fertilizer application if a subsequent soil test shows Mg in the high range or 

if leaf Mg improves to optimum or greater.

Micronutrients

The quantities of micronutrients in 100 boxes of fruit are 

extremely small (Table 3.3). Removal of micronutrients 

by the harvested fruit from even a high-producing grove is 

negligible compared with the amount present in the soil. 

In high pH (>7) soils, micronutrient availability (except 

Mo) decreases considerably. Micronutrients (with the 

exception of B and Cu) should be applied as needed based 

on visual, persistent foliar deficiency symptoms or low leaf 

analysis values. Recommended methods, timing, and rates 

for micronutrient application to citrus groves are shown in 

Table 8.4.

Factors influencing the effectiveness of foliar sprays 

include the formulation used, metallic rate per acre, and 

timing of the spray with respect to leaf age. Many commercially-available micronutrient formulations applied at their 

recommended label rates will maintain sufficient leaf concentrations but usually will not correct moderate to severe 

deficiencies. Water-soluble micronutrient fertilizers may 

be included with post-bloom or summer sprays, preferably 

timed when leaves of new growth flushes are two-thirds to 

fully expanded.

Copper. Some central ridge and east coast grove soils 

contain 300 to 400 lbs Cu/acre in the top 6 inches of soil. 

A moderate routine disease control spray program contributes an additional 3 to 4 lbs Cu/acre/year so no soil 

application of Cu fertilizer is needed in this situation. Soil 

pH below 5.5 can solubilize soil Cu that is toxic to plants. 

As little as 1 ppm Cu in the soil solution can kill roots. 

Table 8.3. Recommendations for P fertilization of bearing citrus trees based on leaf tissue and soil tests taken according to the 

guidelines described in Chapter 4 (leaf and soil samples taken in July or August of each year).1

If leaf tissue P is... ...and soil test P is... ...the recommendation for P fertilization is:

Excessive

or High

Soil test P value is 

not applicable. Do not apply P fertilizer to the soil for 12 months following leaf and soil sampling, 

then sample again and re-evaluate. Optimum Sufficient

Optimum Less than sufficient Apply 8 lbs P2

O5/acre to the soil for every 100 boxes/acre of fruit produced during 

the current year. Sample leaves and soil again in 12 months and re-evaluate.

Low Less than sufficient Apply 12 lbs P2

O5/acre to the soil for every 100 boxes/acre of fruit produced during the current year. Sample leaves and soil again in 12 months and re-evaluate.

Deficient Less than sufficient Apply 16 lbs P2

O5/acre to the soil for every 100 boxes/acre of fruit produced during the current year. Sample leaves and soil again in 12 months and re-evaluate.

1

These recommendations do not pertain to foliar-applied P.

Table 8.4. Recommended methods, timing, and rates for micronutrient application to citrus groves.

Mn Zn Cu B Fe

Method

Foliar Yes Yes Yes Yes No

Soil Yes1 No Yes Yes Yes

Timing

Foliar When spring flush leaves reach full expansion

Soil Anytime as needed

Rates

lbs metallic equivalent/acre

Foliar 3 to 5 5 3 to 5 1⁄4 ---

Soil 7 to 10 --- 5 1 See below2

1

Soil applications of Mn are not recommended on calcareous soils.

2

Acid soil: Fe-EDTA, 2⁄3 oz elemental Fe/tree; Calcareous soil: Fe-EDDHA, 13⁄4 oz elemental Fe/tree.
55

Maintaining soil pH close to neutral is recommended to 

reduce the potential for Cu toxicity. Some rootstocks (e.g. 

Swingle citrumelo) that perform poorly in high pH soils 

are also sensitive to high soil Cu. When replanting on old 

grove sites with low soil pH, the potential for toxicity is 

high. While the emphasis with old grove soils is on managing high soil Cu, most previously non-cultivated Florida 

sandy soils are deficient in Cu. If a grove is developed on 

a virgin sandy soil, Cu fertilizer should be included in the 

fertilizer blend (see Section 8.1.1).

Iron. Soil applications of non-chelated, inorganic Fe fertilizers usually cannot correct visible Fe deficiency symptoms. Because these Fe fertilizers readily precipitate, they 

are unavailable to the tree. In neutral to slightly acidic soils 

Fe deficiency can also be a problem if Cu is present at high 

concentration.

Calcareous soils may contain high total Fe but it is extremely insoluble. Visible Fe deficiency is common on 

these soils. The primary factor causing Fe chlorosis in an 

alkaline soil is the effect of the bicarbonate ion (HCO3

-

) 

on Fe uptake and/or translocation in the plant, resulting 

in an inactivation or immobilization of Fe in plant tissue. 

Citrus rootstocks vary widely in their ability to overcome 

low Fe stress (Table 8.5). The easiest way to avoid alkalineinduced Fe chlorosis in citrus trees that are planted on 

calcareous soils is to use tolerant rootstocks.

Iron chlorosis should be corrected by soil application of 

chelated Fe fertilizer. Chelates are superior sources of Fe 

for plants because sufficient Fe can be supplied at lower 

rates compared with inorganic Fe sources. The effectiveness of an Fe chelate depends greatly on soil pH (Table 

8.6). Fe-EDTA and Fe-HEDTA, which are relatively inexpensive, will correct Fe deficiency if soil pH is less than 6.5. 

Do not apply these chelates to alkaline soil because they 

will readily break down resulting in loss of available Fe by 

precipitation.

Iron chlorosis of citrus trees on susceptible rootstocks 

growing on calcareous soil is not easily remedied. Effective 

Fe chelates for these soils are available, but the treatments 

can be expensive and leaf greening is usually transient. 

Choose Fe-DTPA for mildly-alkaline soils (pH 7.5 or 

less), and choose Fe-EDDHA for highly-calcareous soils 

(pH greater than 7.5).

Organically-complexed Fe exists in by-products like 

wastewater residuals (biosolids) or certain drinking water 

treatment residuals (Fe-humates). Biosolids are potentially 

useful because they contain a high concentration of Fe in a 

complexed form that does not readily precipitate. Research with Fe-humate applied to citrus trees showed that 

moderate Fe deficiency could be corrected for relatively 

low cost.

Zinc. Soil pH is the most important factor regulating 

plant-available Zn. Zinc precipitates at alkaline pH markedly decreasing availability so soil pH less than 7 is the preferred situation. Although there are natural mechanisms in 

the soil-plant system that increase the availability of Zn in 

alkaline soils, Zn deficiencies are common.

If Zn deficiency symptoms are transient and not widespread in a citrus grove, applying Zn fertilizer is not recommended. However, special consideration should be given to 

groves being visually monitored for citrus greening disease 

symptoms (see Section 3.9). Trees on Carrizo citrange rootstock tend to show Zn deficiency symptoms more readily 

even though the tree is not likely Zn deficient.

Application of foliar Zn fertilizer is usually combined with 

pesticide sprays scheduled in April or May at 3 to 5 lbs of 

metallic Zn/acre using either ZnO or ZnSO4

. A number 

of other formulations are available for foliar application, 

including nitrates and organically-chelated forms using 

lignin sulfonate, glucoheptonate, or alpha-keto acids. 

Practically speaking, inorganic and organic Zn fertilizer 

sources are about equally effective with respect to foliar 

absorption.

Table 8.5. Citrus rootstocks ranked according to Fe-chlorosis 

susceptibility.

Sour orange (C. aurantium)

Lowest 

susceptibility

Rough lemon (C. jambhiri)

Cleopatra mandarin (C. reticulata)

C. macrophylla

C. volkameriana

Sweet orange (C. sinensis)

Moderate 

susceptibility

Carrizo citrange (C. sinensis x P. trifoliata)

Trifoliate orange (P. trifoliata)

Highest 

susceptibility

Swingle citrumelo (C. paradise x P. trifoliata)

Table 8.6. Effective pH range of various Fe chelates.

Iron chelate Effective soil pH range

Fe-EDTA 4.0 to 6.5

Fe-HEDTA 5.0 to 6.5

Fe-DTPA 4.0 to 7.5

Fe-EDDHA 4.0 to 9.0

Fe-citric acid Not suitable for soil application
56 

Application of Zn directly to acidic soils is rarely economically practical due to the massive rates required to correct 

a deficiency. Zinc should not be soil-applied to groves on 

calcareous soils because the alkaline pH renders the Zn 

unavailable almost immediately.

Manganese. The behavior of Mn in the soil is similar to 

that of Zn especially with respect to relative availability in 

acidic and alkaline soils. Either sulfate, oxysulfate, or some 

oxide forms of Mn can be used to correct Mn deficiency 

with the degree of effectiveness decreasing in that order. 

Soil application of Mn is not recommended on calcareous 

soils where Mn deficiency is commonly encountered.

For groves on acidic soils that show persistent Mn deficiency symptoms on young foliage, soil application of 7 

to 10 lbs of Mn as MnSO4

 per acre is recommended. On 

calcareous or heavily limed acid soils, foliar application of 

3 to 5 lbs of Mn per acre is recommended. A special effort 

to prevent Mn deficiency symptoms should be made in 

groves being visually monitored for citrus greening disease 

symptoms (see Section 3.9).

Boron. Boron is required in miniscule amounts and there 

is only a small range between deficient and toxic fertilizer 

rates. It should be applied annually as a foliar spray or in 

a dry fertilizer mixture at approximately 1/300 of the N 

fertilizer rate. If trees are irrigated with reclaimed water, B 

fertilization may not be necessary.

Molybdenum. Molybdenum is also required in very small 

amounts. If Mo deficiency occurs, it usually means that the 

soil is highly acidic. The deficiency is corrected by a foliar 

spray that may last for several years. Soil applications are 

not satisfactory.

8.3. Timing and frequency of fertilizer 

application for bearing trees

Bearing tree nutrition management must support both 

vegetative growth and the current fruit crop. Spring vegetative growth is particularly important because it forms 

the fruiting wood for the following year’s crop. The period 

of highest nutrient requirement begins in late winter and 

extends through early summer. During this time, flowering 

and fruit development competes with spring vegetative 

growth flushes. Flowers and fruitlets take up accumulated 

nutrients but some of these are temporarily lost during 

the flower-fruitlet shedding process. The tree is then left 

with the fruit it can sufficiently support to maturity. This 

process continues until the May-June drop of fruitlets is 

completed, after which nutritional requirements for fruit 

development decrease. Best fruit quality is obtained when 

fall and early winter nutritional status, particularly N, is 

moderately low.

Based on the nutritional demands during a typical year, a 

basic fertilizer application schedule divides the total annual requirement into three equal increments:

•	 The first increment should be made available between 

early February and the initiation of flowering.

•	 The second increment should be made available between flowering and late May.

•	 Third increment should be made available between September and mid-October, avoiding the summer rainy 

season as much as possible.

Fertilizer may be applied during the fall and winter particularly in the southern portion of the citrus-growing region 

where trees often grow throughout the year. Any time 

growth is induced by warm weather, sufficient nutrients 

should be available. However, fertilizer applied in January may not be readily taken up if soils are cold. If a freeze 

occurs shortly after a winter fertilizer application, much of 

the fertilizer will be lost by leaching if micro-sprinklers are 

used for freeze protection.

Be aware that fall or winter fertilizer applications can make 

psyllid control more difficult (see Section 3.9), delay fruit 

color development, and increase the susceptibility of trees 

to freeze injury.

Much of Florida’s citrus industry exists on shallow flatwoods soils with intensive irrigation so the danger of root 

damage from high salinity is greater than it is on deep 

sandy soils. Split fertilizer applications minimize salt damage potential, decrease leaching during the summer rainy 

season, and help maintain a continuous nutrient supply 

during south Florida’s long growing season.

8.4. Slow-release and controlled-release fertilizers

Slow-release N sources

Citrus growers can reduce the number of fertilizer applications per year when slow-release N sources like sulfurcoated urea, urea formaldehyde, methylene urea, or IBDU 

make up 40 to 60% of the total N in the fertilizer. For example, a grower applying 100% water-soluble N fertilizer 

(e.g. ammonium nitrate) four times per year could cut the 

application frequency in half if the N source was changed 

to a 50-50 blend of ammonium nitrate and IBDU. However, the total N rate applied per year should be the same 

in both cases.

If a natural organic material like manure or biosolids is 

included as part of the N applied to citrus, the mineraliza-
57

tion rate must be considered when determining the rate 

to apply. Organic matter decomposes relatively quickly in 

Florida’s warm and humid environment so N mineralizes 

much faster than it would in a cooler northern climate. 

Roughly half of the N in biosolids and two-thirds of the N 

in poultry (layer) manure becomes plant-available during 

the first year after application. For example, if a poultry 

manure application supplied 100 lbs/acre of total N to a 

grove, about 66 lbs N/acre would become available to the 

trees during the next 12 months.

Research conducted in several Florida locations showed 

that N mineralization is front-loaded in the case of both 

poultry manure and biosolids (Table 8.7). Most of the N 

becomes plant-available in the first month after application, followed by a gradual release of the remainder during 

the subsequent 11 months. Research has suggested that 

the amount of plant-available N released from either material after 12 months is negligible. The N not accounted 

for by the plant-available fraction most likely volatilizes, 

denitrifies, is used by soil microbes, or remains in the soil 

as recalcitrant organic matter.

Table 8.7. Approximate rate of N availability from poultry 

(layer) manure and biosolids following application to the soil 

(Hanselman et al., 2004).

Time after 

application 

(months)

Poultry manure Biosolids

Available N as a percentage of total N applied

0 – 1 50 35

1 – 3 6 8

3 – 6 4 6

6 – 12 4 7

Total 64 56

Controlled-release fertilizers

Commercial fertilizer companies have blended together 

individual coated fertilizer materials, each with a different 

release rate, to create controlled-release mixtures suitable for single annual applications to citrus. For example, 

a fertilizer made to match the nutrient requirements of 

citrus trees might be composed of a suite of water-soluble, 

3-month, 6-month, and 9-month materials. A blend like 

this would be applied once per year in February. Ideally, it 

would release two-thirds of its nutrients from February to 

June, and the remaining one-third would gradually release 

between June and October.

Coated fertilizers provide the highest nutrient uptake 

efficiency potential of any fertilizer class. Recent research 

suggests they can sustain equal or increased fruit production when applied at lower N rates compared with a standard water-soluble N fertilization program. Since they are 

considerably more expensive than water-soluble materials, 

applying coated fertilizers at lower rates may be necessary 

to maintain an economically-competitive fertilization program. Since the marketing of coated fertilizers for mature 

citrus groves is relatively new, growers are encouraged to 

consult the fertilizer manufacturer or blender for release 

rates and efficiencies of specific products.

8.5. Foliar application of N, P, and K

The amount of plant nutrients that can be taken up through 

the leaves of a citrus tree is miniscule compared with the 

amount that can enter through the roots. Micronutrients 

can be successfully applied with foliar sprays because the 

tree does not require them in large amounts (Table 3.1) but 

leaves are not usually thought of as a major uptake site for 

macronutrients. However, there are special cases where foliar application of N, P, and/or K is justified. Citrus growers 

should realize that a positive response to foliar sprays of N 

or P may be due to additional effects of the materials on tree 

physiology beyond simple enhancement of tree nutrition.

Nitrogen

Commercial forms of urea are available that can be readily absorbed by citrus leaves particularly if applied with a 

non-ionic surfactant. Foliar urea sprays applied during the 

winter have enhanced the number of flowers and yield of 

Valencia oranges in both research plots and commercial 

trials. These sprays presumably work only if some induction has taken place from natural cold or drought stress. 

Therefore, for effective use in the winter, urea sprays 

should be applied after some natural flower induction has 

occurred but before most bud differentiation starts. In 

Florida, significant induction may not start until late December some years, while in other years some buds may be 

differentiating by early January. After some cool temperature induction or 30 days of drought stress, foliar sprays 

of 50 to 60 lbs of spray grade urea per acre can enhance 

flower bud induction and may increase fruit yield. Care 

must be taken to apply the correct rate, because leaf 

damage can occur if urea application is excessive.

Maximum penetration of urea into citrus leaves occurs 

within 24 to 48 hours after spray application. Optimum 

conditions for foliar uptake include:

•	 Air temperature between 77 and 88° F.

•	 High relative humidity.

•	 Spray solution with a pH between 7 and 8 to prevent 

urea breakdown.
58 

Under favorable environmental conditions, roughly half 

of foliar-applied urea penetrates the leaves, while most of 

the other half is lost through volatilization. The rate of 

foliar-applied N should be considered as part of the 

total annual N rate applied to the grove. For example, a 

foliar spray of 50 lbs urea/acre applies 23 lbs N/acre. If the 

fertilization plan calls for a total of 180 lbs N/acre/year, 

only 157 lbs N/acre (180 – 23) should be included in the 

soil-applied fertilizer program.

In Florida citrus production areas where groundwater 

nitrate contamination exists or is seen as a potential problem, urea sprays should be evaluated to provide a portion 

of the tree N requirements, especially during the summer 

months when leaching potential is the greatest.

Phosphorus

Citrus leaves are extremely impervious to the phosphate 

(PO4

3-) form of P so foliar application of a liquid P material like ammonium polyphosphate is not recommended. 

Conversely, the phosphite (PO3

3-

) form of P is more readily absorbed into plant tissue and once inside the plant 

it remains stable. Phosphite does not readily convert to 

phosphate in the plant so the nutritional value of absorbed 

PO3

3- is uncertain. However, phosphite is officially recognized by FDACS as a source of P for crops.

In California, research showed that foliar applications of 

phosphite were able to replace standard P fertilization in 

citrus crops suffering from P deficiency. The conversion 

of phosphite to phosphate likely occurred prior to plant 

absorption, resulting from slow chemical oxidation or by 

oxidizing bacteria and fungi found living on citrus leaves. 

Phosphite also showed fungicidal activity and increased 

citrus floral intensity, yield, fruit size, total soluble solids, 

and anthocyanin concentration, usually in response to a 

single foliar application.

In Florida, a pre-bloom foliar application of 2.6 quarts 

of 28% P2

O5 as potassium phosphite per acre to Valencia 

oranges significantly increased flower number, fruit yield, 

and total soluble solids yield compared with an untreated 

control. These results suggest that the effect of phosphite 

was not due to the molecule’s fungicidal attributes, but to 

other growth-stimulating properties.

Citrus growers should identify their production goal for 

the year (e.g. increased yield, increased fruit size, or improved fruit quality) to determine if a phosphite application is justified. Be aware that phosphite materials, if not 

formulated correctly, have significant phytotoxicity potential and may induce adverse reactions with other materials 

in the spray tank such as micronutrients or pesticides.

Potassium

Many factors contribute to the size of fruit in a particular 

year such as fruit load, rainfall pattern, fertilization program, hedging and topping, and rootstock/scion combination. However, it is difficult to predict how these factors 

combine to affect final fruit size at harvest. The easiest factor to manipulate is nutrient management. Among other 

fruit qualities, increased K fertilization is associated with 

larger fruit size (Table 3.4).

Effects of low K on fruit yield and quality generally precede appearance of leaf deficiency symptoms. Decreased 

yield and small fruit have been observed on trees with leaf 

K in the range of 0.5 to 0.8%, while K concentrations of 

1.2% or more have been associated with maximum yield of 

high quality fruit.

Applying foliar sprays of K cannot entirely substitute for 

soil-applied fertilizer, but they can serve as a supplement, 

and their ability to increase fruit size has been demonstrated. Foliar-applied K has also corrected K deficiency 

of citrus on calcareous soil. Applying potassium nitrate 

(KNO3

) in this manner increases leaf K more rapidly 

compared with soil-applied fertilizers because plant uptake 

is much faster, but the positive effect is shorter-lasting.

Salt index. The salt index of a fertilizer (Section 11.5) 

measures its tendency to increase the osmotic pressure 

of the soil solution compared with an equal amount of 

sodium nitrate. High soluble salt concentrations in the 

soil may develop an osmotic pressure exceeding that of the 

plant sap, possibly resulting in dehydration and permanent 

injury. When salt solutions are sprayed on leaf surfaces 

similar results may occur. Typically, the higher the salt 

index, the greater the potential to burn to leaves or fruit. 

Materials. Potassium sources used for foliar K application include KNO3

 (13-0-44), monopotassium phosphate 

(MKP, 0-52-34), and dipotassium phosphate (DKP, 

0-18-20). DKP is made by combining MKP and potassium hydroxide. When applying MKP or DKP, a small 

amount (3 to 5%) of low-biuret urea should be included to 

enhance uptake. KNO3 is usually applied without surfactants or urea.

Application rates and timing. Research in the Indian 

River production area showed that about 8 lbs of K2O per 

acre were needed per foliar application to achieve satisfactory results. Higher rates did not show additional benefit, 

and lower rates resulted in less fruit enlargement.
59

If foliar spraying with 100 or more gal of water/acre, the 

potential for burn is relatively low, so the least expensive 

K source may be chosen. When applying foliar K with 

low-volume equipment, MKP or DKP should be used to 

minimize the burn potential. MKP has been applied to 

grapefruit at rates as high as 106 lb MKP in 125 gal water 

per acre under hot, dry conditions with no adverse effects. 

The low salt index of MKP (only 1/6 that of KNO3

 per 

unit of K2O; Appendix C, Table 1) makes it safe to use.

Caution: Be careful if tank mixing MKP with other 

materials. The MKP-water solution has a pH of about 4.5, 

which may not be suitable for some tank mix combinations. DKP makes a pH-neutral solution that would be a 

better choice if tank-mixing with other materials.

Timing is important for K applications to enhance fruit 

size. Potassium is a primary component of cell walls, 

accounting for more than 40% of fruit mineral content. 

About 70% of final fruit size is related to the number 

of cells in the fruit, so more cells usually means larger 

fruit. Cell division typically stops by late April and size 

change throughout the rest of the year comes from cell 

enlargement. Therefore, the maximum effect of foliar K is 

achieved from applications that make it available during 

bloom and post-bloom when it can be used during both 

cell division and rapid cell enlargement phases. An additional application with the summer spray (normally in 

July) is also recommended to ensure sufficient K through 

the summer growing season.

Grapefruit size enhancement occurred in about half the 

fall field trials in the Indian River area, suggesting that late 

summer or fall K applications may be effective some years. 

Fall applications were most effective in years with wet summers and falls. Shorter day lengths and cooler weather results in a dramatically decreased fruit expansion rate after 

mid-October in most years. Thus, if foliar K applications 

for fruit enlargement are considered during late summer 

or fall, they should be made in August or September to be 

most effective.

Expected results. Studies on Sunburst and Valencia 

showed that foliar-applied K produced 25 to 33% more 

larger-sized fruit compared with non-treated plots. In addition, there was a corresponding increase in soluble solids 

yield in the Valencia experiments. Combining pre-bloom, 

post bloom, and summer K sprays may increase average 

fruit diameter 0.16 to 0.24 inches, which can equal 1 or 

more pack sizes. When fall application was successful, 

grapefruit diameter increased 0.08 to 0.16 inches, or about 

1⁄2 to 1 size category. Foliar K will not produce large fruit 

from small fruit, but it can move a significant portion of 

the fruit into a larger size class. The following observations 

and recommendations are based on the Indian River area 

experiments:

•	 The recommend program for most citrus varieties is 

8 lbs K2

O/acre per application, applied pre-bloom 

(typically February), post-bloom (typically April), and 

summer ( July).

•	 If the summer and fall are wetter than usual, later K 

applications may be considered. When exercising this 

option, schedule the applications during August and 

September.

•	 Foliar K application has had little or no effect on juice 

volume, acid, Brix, or Brix:acid ratio.

•	 Diameters of smaller fruit tended to increase more than 

larger fruit when foliar K applications were made.

•	 Fruit burn was not observed at the following spray concentrations:

o 25 lbs KNO3/acre applied in 125 gal of water/acre.

o 15 lbs MKP/acre applied in 32.5 gal of water/acre.

o 15 lbs MKP/acre applied in 10 gal of water/acre by 

airplane.
60 

9.1. Water supply

Competition for water is increasing in all citrus production areas. Florida’s expanding population increases water 

demand in the urban sector, which reduces water availability for agriculture. Growing a high-quality citrus crop 

is water-intensive, but growers do have options to remain 

competitive. By increasing water uptake efficiency (the 

amount of water taken up by the trees compared with the 

amount of water applied to the grove), growers can continue to achieve normal production while reducing water 

withdrawals.

If more water is applied than the soil can hold, it drains 

below the root zone and is wasted. Nutrients, especially N, 

move with water as it passes through the soil (leaching) either downward to groundwater or laterally toward ditches 

and canals. When nutrients are leached, they are no longer 

available to the trees and may become an environmental 

concern. Understanding how water and nutrients move 

through the soil is important in improving their use.

9.2. Production region characteristics 

important to irrigation management

The central Florida ridge (Fig. 2.7) features, well-drained, 

sandy Entisols (Appendix A). These soils permit rapid 

infiltration of rain and irrigation water, making them 

vulnerable to nutrient leaching. Nitrate leaching is a major 

concern to citrus producers on the ridge. The maximum 

irrigation depth to wet the majority of the root zone in 

these soils is 3 to 4 ft.

The Gulf, Peace River, and Indian River citrus production 

areas are dominated by poorly-drained flatwoods soils 

(Fig. 2.7) that require artificial drainage to produce high 

quality citrus. Nitrate leaching is greatly reduced in flatwoods soils compared with the ridge. The citrus root zone 

in these soils is typically 18 inches or less.

9.3. Nutrient uptake efficiency

Improving water uptake efficiency will also improve nutrient uptake efficiency (the amount of nutrients taken up by 

the plant compared with the amount of nutrients applied 

as fertilizer). In Florida’s sandy soils, nutrient and water 

uptake efficiencies are linked. Management methods that 

improve irrigation water uptake efficiency will increase the 

proportion of applied nutrients that are taken up by the 

tree, potentially leading to increased growth and yield.

9.4. Allowable soil water depletion

As soil dries out, water becomes increasingly more difficult for trees to remove, which can eventually cause water 

stress. Tree health and yield will suffer if the soil is allowed 

to get too dry. To provide adequate water for flowering, 

fruit set, and vegetative growth, maximum soil water 

depletion should be no more than 25 to 33% of available 

water from February to June. Once the rainy season starts, 

the maximum depletion level can be increased to 50 to 

66% of available water. This additional allowable depletion 

increases the capacity of the soil to hold rainfall without 

leaching nutrients. The same depletion levels in the fall 

and winter months will save water without reducing yield.

9.5. Irrigation scheduling

Successful irrigation management maintains sufficient 

water and nutrients in the root zone to maximize plant 

growth and health. Growers who focus on improving 

water and nutrient uptake efficiency will reduce N and 

P losses and decrease environmental impacts at the same 

time. While some nutrient loss is unavoidable due to 

excess rainfall, loss due to management decisions can be 

minimized.

Proper irrigation scheduling applies an appropriate volume 

of water to a citrus grove at the appropriate time based on 

tree need, soil properties and weather conditions. Scheduling methods include:

•	 Experience

•	 Calendar method (e.g. 0.8 inches every 4th day during 

the dry season)

•	 Monitoring soil water status

•	 Calculating a water budget

9.5.1. Soil moisture measurement

Experience or the calendar method can provide a reasonably good irrigation schedule, but they are not accurate 

enough to maximize water uptake efficiency and prevent 

nutrient leaching. Using soil moisture sensors improves 

accuracy because they quantify changes in soil water status. 

These devices may be fixed in one location, portable, or 

hand-held. They measure soil moisture at one depth or at 

multiple depths. General categories include:

9. Irrigation Management to Improve Nutrient Uptake

Kelly T. Morgan and Thomas A. Obreza
61

•	 Tensiometers

•	 Electrical resistance blocks

•	 Time domain reflectometry (TDR) probes

•	 Capacitance probes

Considerations when using soil moisture sensors to schedule irrigation include:

•	 Knowing the soil water-holding capacity and tree root 

zone depth (Appendix A, Table A.1).

•	 Placing sensors where the majority of roots are located 

such as at the dripline of the tree.

•	 Using multiple sensors both across the grove and with 

depth to fully characterize the tree root zone.

•	 Moving sensors to follow root growth as the tree canopy 

expands in developing groves.

•	 Basing irrigation on the soil depth containing the greatest root density.

•	 Managing root zone soil moisture between field capacity and the maximum allowable available water depletion (one-fourth to two-thirds depletion, depending on 

time of year).

Fig. 9.1 shows an example of how a multi-level capacitance 

probe could be used to adjust an irrigation schedule to 

maximize water uptake efficiency and minimize nutrient 

leaching. The four graphed lines represent soil moisture 

content at 4-inch (red), 8-inch (blue), 12-inch (purple), 

and 20-inch (green) depths in the soil and the x-axis shows 

a 16-day time period separated into 2-day increments.

Fig. 9.1. Continuous monitoring of soil moisture at 4, 8, 12, 

and 20-inch depths in the soil by a multi-level capacitance 

probe installed in the root zone of a mature citrus tree.

The effect of irrigation is easily observed as sharp increases 

in soil moisture at the 4, 8, and 12-inch depths. However, 

note that the first irrigation increased soil moisture at the 

20-inch depth as well (black arrow), which is below the 

zone of highest root density. Since the goal here was to 

keep the irrigation water in the top 18 inches of soil, the 

grower reduced the duration of subsequent irrigations. 

The steadily decreasing water content at the 20-inch depth 

during the following 2-week period showed that the grower had attained optimum irrigation water management.

9.5.2. Water budgeting

An alternative method to schedule irrigation uses a computer program that estimates grove water consumption 

(evapotranspiration, or ET) from weather data. Reference 

ET and convenient irrigation scheduling management 

tools for all Florida citrus production regions can be found 

on the Florida Automated Weather Network (FAWN) 

website at http://fawn.ifas.ufl.edu. The computer program 

uses the soil water-holding capacity of specific soil series to 

determine field capacity. Irrigation schedules are determined using the strategies and equations described in the 

next section.

9.6. Irrigation strategies to improve nutrient 

uptake and reduce leaching

Developing an irrigation strategy to reduce nutrient leaching has the objective of not applying more irrigation water 

than the root zone can hold. Considering the low waterholding capacity of citrus grove soils, this objective is very 

difficult to accomplish even for the most experienced and 

diligent irrigation managers. The major questions to be 

answered in this procedure are:

•	 How much water can the root zone hold?

•	 What is the maximum irrigation system run time before 

leaching occurs?

Example

A central ridge citrus grove has the following characteristics:

•	 Tree spacing – 121⁄2 ft within the row and 25 ft between 

rows.

•	 Tree canopy diameter – 171⁄2 ft

•	 Root zone depth – 3 ft.

•	 One 16 gal/hr micro-sprinkler per tree with a 16-ft 

diameter wetted pattern.

•	 The citrus root zone is continuous from tree to tree, 

existing both inside and outside of the wetted pattern.

•	 The irrigated system wets approximately 60% of the 

total root zone (Fig. 9.2).

Nutrient leaching risk in this grove is higher within the 

wetted pattern due to potential over-irrigation plus the 

fact that most fertilizers are applied to that zone (Fig. 9.3). 

A good irrigation manager will control this risk with careful water management.
62 

Note: The following depictions of water content changes in the citrus tree root zone (Figs. 9.4 to 9.7) do not 

represent the actual water extraction pattern. The blue 

shading shows 1) approximately where water extraction occurs beneath the canopy, and 2) the relative soil water content with respect to available soil water-holding capacity.

This example starts with the entire grove at field capacity 

moisture content following a heavy rain (Fig. 9.4). The citrus trees begin to remove water from the soil in response to 

the atmospheric ET demand. After several days have passed 

(depending on time of year) the water content in the root 

zone decreases to 50% of available water capacity (Fig. 9.5).

At this point, the grove manager turns on the irrigation 

system and operates it long enough to return the soil in the 

wetted pattern back to field capacity (Fig. 9.6). From this 

point until the next significant rainfall, only the soil water 

content in the irrigated zone can be influenced by the irrigation manager. The water content in the non-irrigated 

zone rapidly decreases to the point where little to no soil 

water can be extracted by the tree.

Fig. 9.2. Scaled diagram of example citrus grove described 

above. 

Fig. 9.3. Irrigated and non-irrigated zones in a citrus grove 

have different leaching potentials that depend on irrigation 

scheduling and fertilizer placement.

Fig. 9.4. The citrus grove at field capacity soil water content 

(time = 0).

Fig. 9.5. The citrus grove several days later, after half of the 

available water has been removed from the root zone. Note that 

water extraction has occurred from both the irrigated and nonirrigated zones.

Fig. 9.6. The citrus grove after irrigation returns the wetted 

zone to field capacity. Note that the non-irrigated zone contains very little available water.
63

If the grove manager operates the irrigation system too 

long and applies more water than the soil can hold, water 

will move beneath the citrus tree roots. If water soluble nutrients like nitrate or potassium are present in the irrigated 

zone during the irrigation period, a portion will leach (Fig. 

9.7).

Fig. 9.7. Excessive irrigation leaches mobile nutrients like 

nitrate and potassium.

How much water can the root zone hold?

From Appendix A, Table A.1:

•	 Central ridge soils – 0.3 to 0.7 inches/ft

•	 Flatwoods soils – 0.3 to 1.2 inches/ft

What is the maximum system run time before leaching 

occurs?

Information needed: In this example:

Soil water-holding capacity 0.6 inches/ft

Maximum allowable depletion 50%

Root zone depth 3 ft

Surface area wetted by microsprinklers 60%

Micro-sprinkler flow rate 16 gal/hr

Tree spacing 121⁄2 ft x 25 ft

Calculations

1. Volume of water the root zone can hold:

0.6 inches/ft × 3 ft deep root zone = 1.8 inches

2. Volume of water to refill at maximum depletion:

1.8 inches × 50% = 0.9 inches

3. Volume of water this represents per tree space:

0.9 inches/tree × 1 ft/12 in × (25 ft × 121⁄2 ft)

× 7.5 gal/cu ft × 60% coverage = 105 gal/tree

4. Maximum system run time:

105 gal ÷ 16 gal/hr emitter flow rate = 6.6 hr

5. Adjust for system delivery efficiency of 90%:

6.6 hr ÷ 0.9 = 7.3 hr

Therefore, the irrigation system should never be run 

longer than about 7 hours for any single cycle, provided 

that the available soil water is at least 50% depleted when 

the irrigation begins. If the soil is less than 50% depleted 

of available water, then the maximum run time decreases 

accordingly.

Considerations

Ideal maximum system run time vs. practical field 

management. There may be management limitations that 

prevent stopping irrigation at or before the ideal maximum run time, such as limitations of the irrigation system 

design or lack of sufficient personnel. Growers should 

evaluate their overall irrigation management and take corrective action if possible.

Theory vs. reality. Calculating maximum run time from 

grove and irrigation system characteristics provides a 

starting point, but the irrigation system or soil may behave 

differently than the model situation. Thus, growers should 

fine-tune the maximum run time in the field.
64 

10.1. Environmental nutrient issues related 

to Florida citrus production

Although N fertilizer is required for tree growth and 

fruit production, there is little net increase in tree size in a 

mature grove. Nitrogen used for leaf growth or taken up by 

a cover crop is largely recycled as leaves drop or the cover 

crop dies, the vegetative material decomposes, and mineralization releases the N for reuse by the tree. This recycled 

N supplies most of the continuing need for new leaves, and 

relatively little fertilizer N is needed for growth. Replacement of the N removed by fruit harvest becomes the main 

N requirement in a mature grove. Figure 10.1 illustrates the 

citrus tree as a component in the environmental N cycle.

Fig. 10.1. The citrus tree as a component of the environmental 

N cycle.

A 600 box/acre crop of oranges removes about 72 lbs of N/

acre from the grove. If this mature grove receives 200 lbs N/

acre annually, approximately 128 lbs of N/acre remains to 

be accounted for after crop removal. The fate of this N is 

not completely understood. Some goes into new roots and 

shoots, and some is taken up by weeds. A portion of the rest 

may be lost by volatilization or denitrification, although 

denitrification in vulnerable soils is minimal. In controlled 

leaching studies, about 40% of the N applied to the soil 

was not recovered even when water was supplied soon after 

fertilizer application. Although unknown mechanisms 

may partially reduce the soil N concentration, a substantial 

portion of the N applied in fertilizer is subject to leaching 

as indicated by elevated N in groundwater beneath some 

groves. Clearly, excess N application should be avoided on 

vulnerable soils where the potential for leaching exists.

In the flatwoods, most soils are slowly-permeable due to the 

presence of spodic and/or argillic horizons (Chapter 2) so 

nitrate leaching to groundwater is less important than on 

the ridge. In addition, nitrate that passes through the root 

zone to the shallow water table can be reduced (denitrified) 

to gaseous N that disperses in the atmosphere.

Surface water contamination by nitrate is more likely to be 

a factor in the flatwoods. Soluble nutrients that move more 

than about 30 inches below ground level become unavailable to flatwoods citrus trees because of their shallow 

rooting depth. Dissolved nutrients (including P) may move 

laterally above the hardpan rather than vertically through 

the restrictive layers. As a result, leached nutrients can move 

into water furrows if rainfall or irrigation is excessive. Nutrients removed as a grove drains are readily used by algae, 

bacteria, and aquatic plants that often clog irrigation/drainage canals and ditches. Excessive plant growth in water 

furrows may be an indication of lateral nitrate movement.

10.2. The BMP era

In the late 1980s, FDEP surveyed the water quality from 

3949 shallow (<100 ft) drinking water wells across the 

state. Nitrate-N was detected in 2483 (63%) of these wells 

and 584 contained a nitrate-N concentration greater than 

the national maximum contamination limit (MCL) of 10 

ppm. Nearly 90% of the high-nitrate wells were located in 

Lake, Polk, and Highlands counties, the heart of Florida’s 

central ridge citrus production area (Fig. 2.7).

In response to water quality concerns, a Nitrogen BMP 

bill was passed by the Florida Legislature in 1994 that 

authorized FDACS “to develop fertilizer BMPs designed 

to meet ground water standards.... These BMPs are not 

mandatory, but if the grower implements the BMPs...., the 

landowner or lessee will not be subject to administrative 

penalties if nitrate ground water standards are violated. 

The Department of Environmental Protection is authorized to conduct field monitoring....”

The definition of a BMP is a practice or combination of 

practices determined by the coordinating agencies, based on 

research, field-testing, and expert review, to be the most effective and practicable on-location means, including economic 

and technological considerations, for improving water qual10. Environmental Issues and Best Management Practices (BMPs)

Brian J. Boman, Thomas A. Obreza, and Kelly T. Morgan

(Includes contributions by Larry K. Jackson, Ashok K. Alva, David P. H. Tucker, and D. V. Calvert)
65

ity in agricultural and urban discharges.

The first BMP that FDACS adopted by rule in response 

to the 1994 law was called “Nitrogen Best Management 

Practices for Florida Ridge Citrus.” It specified N fertilizer 

sources, annual N rates, maximum N rate per application, 

fertilizer application timing, irrigation management, and 

record-keeping for citrus grown on permeable betterdrained sandy soils typical of Florida’s central ridge. The 

purpose was to minimize the risk of nitrate leaching from 

fertilizers to groundwater.

Citrus production BMP development followed in the 

Indian River growing region in response to surface water quality concerns in the Indian River Lagoon and St. 

Lucie Estuary. These BMPs were developed for citrus on 

poorly-drained flatwoods soils. They expanded beyond 

nutrient management to include water volume, sediment transport, pesticides, and aquatic plants. Since then, 

similar BMP manuals have been developed for the Peace 

River and Gulf citrus-growing areas, so essentially all commercial Florida citrus groves can potentially come under 

the auspices of a BMP program if the grower so desires. 

Citrus growers are referred to the FDACS Office of 

Water Policy for more detailed information about BMPs 

(http://www.floridaagwaterpolicy.com/).

The following are the steps growers should take to get 

involved in their regional BMP program:

•	 Assess the grove operation and list BMPs that are already present or will be enacted.

•	 Submit the summary of practices to FDACS in a 

“Notice of Intent to Implement” BMPs.

•	 Once enrolled, maintain records and provide documentation regarding BMP implementation.

•	 Receive a “presumption of compliance” with water 

quality standards from FDACS.

After enrollment, growers become eligible for cost share 

funding or drainage permit exemptions, depending on 

grove location.

Characteristics of a successful BMP program:

•	 A “cradle to grave” approach.

•	 A stakeholder-driven process from manual development 

through BMP implementation.

•	 Distribution of printed manuals to growers.

•	 Adoption of BMP manual by rule, followed by availability of cost-share for implementation.

•	 Growers keep good production records and use selfassessment tools.

•	 Third-party implementation teams help growers enroll 

and take part in BMPs.

•	 Field studies determine the effectiveness of BMPs.

•	 BMP education is a continuous process.

10.3. General nutrient BMPs for 

citrus production

Nutrient BMPs do not represent exotic or unfamiliar 

fertilizer management practices to modern Florida citrus 

producers. In fact, most BMPs are simple, common-sense, 

“good housekeeping” practices that many grove managers 

already use in their normal caretaking. The following list 

summarizes typical nutrient BMPs found in Florida’s various citrus BMP manuals:

•	 Educate and train field operators who handle, load, or 

apply fertilizers about fertilizer placement, avoiding 

waste, and preventing contamination of open water.

•	 Develop a nutrient management plan based on crop 

nutrient requirements.

•	 Use tissue and soil analysis to make fertilization decisions.

•	 Use appropriate application equipment.

•	 Properly calibrate and maintain application equipment.

•	 Apply fertilizers to target sites.

•	 Avoid high risk applications such as before forecasted 

rainfall, on bare soils with extreme erosion potential, or 

when the water table is near the surface.

•	 Store fertilizer to prevent contamination of nearby 

ground and surface water. Always store fertilizer in areas 

protected from rainfall.

•	 If fertilizer is spilled on the ground, collect it and apply 

as normal. Use a tarp on ground surfaces where fertilizer 

is transferred.

•	 Use caution when loading near ditches, canals, and wells. 

Locate loading activities away from these sites if possible.

•	 Use multiple fertilizer loading and transfer sites to prevent concentration of nutrients in a single area.

•	 Use backflow prevention devices on irrigation and spray 

tank filling systems to prevent entry of nutrients into 

surface or ground water.

•	 Split fertilizer applications throughout the growing 

season.

•	 Use erosion-control practices to minimize soil loss and 

runoff.

•	 When irrigating, try to wet only the root zone. Do not 

over-irrigate.

•	 Add organic matter to the soil whenever possible.

•	 Prevent groundwater contamination by plugging wells 

that are not in use.

•	 Use appropriate fertilizer sources and formulations 

based on nutritional needs, season of year, and anticipated weather conditions to achieve greatest efficiency 

and reduce potential for off-site transport.
66 
67

11.1. Scions

Orlando tangelo

While the nutritional requirements of oranges and grapefruit are well known, the requirements of many commercially grown hybrids are obscure. Orlando tangelos 

may require the higher N and K rates recommended in 

Chapter 8, with emphasis on fall application to prevent 

yellowing and defoliation. Transient yellowing of foliage in 

the winter should not be the sole reason for increasing the 

fertilizer rate.

Honey tangerine

Mature honey tangerines (Murcotts) may require up to 

300 lbs/acre of N and K2

O each year to reduce the incidence of tree collapse during heavy crop years. At least 

one-third of the annual fertilizer rate should be applied in 

the fall to coincide with greater tree stress from the fruit 

load. Some studies have indicated that heavy fruit set leads 

to root starvation and death resulting in reduced mineral 

uptake. Therefore, the fruit crop should be mechanically 

or chemically thinned in heavy crop years with reduced 

rates of fertilization in light crop years.

Navel orange

There is no evidence that Florida navel oranges require 

different fertilizer formulations or additional nutritional 

or foliar N applications compared with other sweet orange 

cultivars. It is prudent to maintain leaf nutrient concentrations within guidelines established for sweet oranges 

without excessive fertilization. Optimum but not excessive 

nutrition will ensure sufficient tree vigor without compromising fruit yield and quality.

11.2. Rootstock/nutrition interaction

Citrus rootstocks influence nutrient uptake and can make 

the difference between a productive and non-productive 

grove. Best known is the poor performance that can occur when trees budded on trifoliate and trifoliate hybrid 

rootstocks like Carrizo citrange and Swingle citrumelo are 

planted on soils with pH > 7. These trees typically exhibit 

Zn, Mn, Fe, and Mg deficiency symptoms. Soils with high 

concentrations of shell, limestone rock, and/or subsoil clay 

are particularly unsuitable for Swingle and Carrizo rootstocks.

The widespread planting of trees on Swingle citrumelo 

rootstock has brought the important influence of rootstock on citrus tree nutrition to the forefront. Trees on 

Swingle rootstock often grow relatively poorly with chlorotic foliage when planted on alkaline soils. Iron deficiency 

has become more prominent with the increased use of 

Swingle rootstock. Trees on Swingle also sometimes show 

general starvation symptoms (of N and other elements) 

during the winter, even though soil temperatures may not 

attain the critical 55° F threshold associated with reduced 

root function. This “winter chlorosis” occurs in spite of 

sufficient soil nutrient supply and supplemental winter 

fertilizer applications.

While rootstock-related nutrient deficiencies can be overcome by appropriate fertilizer applications (although at 

high cost), the only way to alleviate toxicities is by reducing input of phytotoxic elements or the use of less sensitive 

rootstocks. For example, C. macrophylla is highly B tolerant, and trees on Rangpur lime are Cl tolerant.

Cleopatra mandarin is the most Cl tolerant rootstock. 

Sun Chu Sha is efficient in taking up Mg when soil Mg 

availability is low. Trifoliate orange, and to a lesser extent 

citranges and citrumelos, are susceptible to Fe chlorosis on 

high pH (>7) soils. The above rootstocks also accumulate 

B and Cl, and as a result are not tolerant to saline soils.

11.3. Soils high in copper

Repeated use of Cu both as a soil application and as foliar 

fungicide sprays has caused Cu to accumulate to toxic concentrations in some older citrus grove soils with acidic soil 

pH. Trees experiencing Cu toxicity may exhibit Fe chlorosis on the foliage. For most Florida soils in citrus production, the Mehlich 1 soil test Cu interpretations in Table 4.4 

can be used as a rough estimate of potential Cu toxicity.

Although useful as a guide, soil test Cu may not predict 

Cu toxicity well in many cases. Cu is more toxic when 

soil pH is less than 6.0. However, if soil pH is maintained 

above 6.0 by liming, Cu bioavailability is negligible. When 

the soil is found to contain high Cu, take the following 

steps:

•	 Discontinue using Cu except where it is required to 

control fruit and foliage fungal diseases where no alternative control exists.

11. Special Situations

Thomas A. Obreza, Brian J. Boman, Mongi Zekri, Stephen H. Futch, Lawrence R. Parsons, and James J. Ferguson

(Includes contributions by David P. H. Tucker, Heinz Wutscher, Ashok K. Alva, and David V. Calvert)
68 

•	 Lime the soil to pH 6.5 and maintain it there.

•	 Avoid using Swingle citrumelo rootstock, which is Cusensitive.

As Cu is present in the soil solution at low concentration 

and accumulates in feeder roots with time, its extraction 

from roots may be used to diagnose Cu toxicity. Field and 

greenhouse studies have indicated that both total feeder 

root Cu and Cu extracted with 1M hydrochloric acid are 

correlated with extractable Cu in sandy soils. In high Cu 

soils, root Cu concentration is a better indicator of Cu toxicity than leaf Cu. While a critical root Cu concentration is 

not well defined, toxicity in mature groves has been associated with feeder root Cu in the 350 to 800 mg/kg range.

11.4. Calcareous soils

Calcareous soils are alkaline (pH > 7) due to the presence 

of excess calcium carbonate (CaCO3). These soils can contain from 1% to more than 25% CaCO3

 by weight, with 

pH in the range of 7.6 to 8.4. In a Florida calcareous soil, 

the pH is not usually higher than 8.4 regardless of CaCO3

concentration.

Many Florida flatwoods soils contain one or more horizons (layers) that are calcareous. A typical characteristic 

is an alkaline, loamy horizon less than 40 inches deep that 

can be brought to the surface during land preparation for 

citrus planting. Increased nutritional management intensity is often required to successfully grow citrus on calcareous soils. Some sites (e.g. ditch banks) are comprised 

of soils containing considerable amounts of lime rock or 

shell. It may not be economically justifiable to plant these 

sites with certain rootstocks considering the management 

problems and costs involved.

Citrus fertilizer management on calcareous soils differs 

from that on non-calcareous soils because of the effect of 

soil pH on soil nutrient availability and chemical reactions 

that affect the loss or fixation of some nutrients. The presence of CaCO3

 directly or indirectly affects plant availability of N, Mg, K, Mn, Zn, Fe and Cu. The behaviors of 

Fe, Zn, and Mn in high pH soil and recommendations for 

alleviating their deficiency have already been discussed in 

Chapter 8. The remaining discussion deals with N, Mg, 

and K. The availability of soil Cu is also affected but is not 

discussed here because the citrus Cu requirement is normally satisfied through Cu fungicide foliar sprays.

Management of fertilizer nutrients

Soil pH affects biological and chemical reactions involving 

N and can influence plant N use-efficiency. Nitrification 

(the conversion of NH4

+ to NO3-

 by soil bacteria) is more 

rapid in soils with pH between 7 and 8. Volatilization of 

ammoniacal-N fertilizer can be significant if the pH of the 

soil surface is greater than 7 where the fertilizer is applied. 

This condition occurs in calcareous soils or where the 

breakdown of the N fertilizer material produces an alkaline condition (e.g. urea decomposition).

Nitrogen fertilizer should be managed to minimize ammonia volatilization. If rainfall is not imminent following 

application of ammoniacal-N to the surface of a calcareous 

soil, the fertilizer should be immediately moved into the 

soil with irrigation water. Urea applied to the surface of 

any soil should also be irrigated in. Fertigation using either 

of these sources is a suitable application method because 

the N immediately enters the soil.

Low leaf K concentrations are common in groves planted 

on calcareous soils. If low yield, small fruit, fruit splitting, 

and/or creasing are observed, application of additional 

K fertilizer is justified. One approach is to increase the 

N:K2

O fertilizer rate ratio to 1:11⁄4, i.e. apply 25% more 

K2O than normal.

If trees do not respond to soil application, an alternative 

approach to increasing leaf K is foliar sprays of KNO3

 or 

KH2PO4 (Chapter 8). Precautions should be taken to 

avoid foliar burn from high spray concentrations. The N 

applied in the spray is as equally available as soil-applied 

N, so the rate of N applied as KNO3 should be taken into 

account when determining the annual N fertilizer rate.

Low leaf Mg concentration in groves on calcareous soils 

can be addressed by applying foliar sprays of Mg(NO3

)2.

Acidification to reduce soil pH

Soil acidification can improve nutrient availability in 

calcareous soils by decreasing soil pH. The rate of a soil 

acidifier required to cause a plant response depends on 

the amount of CaCO3

 in the soil. The chance of a positive 

plant response to broadcast applications of an acidifier is 

near zero if lime rock or shell is visible in the root zone. In 

contrast, it is feasible to acidify soils with lower CaCO3

content (e.g. from over-liming) or those that have become 

alkaline from repeated application of high-bicarbonate 

irrigation water.

Soil acidifiers include elemental S and ammonium or 

potassium thiosulfate [(NH4)2S2O3, K2S2O3]. The sulfur in 

these compounds converts to sulfuric acid in the soil that 

neutralizes CaCO3 (Table 11.1) and decreases soil pH. 

Ammonium sulfate [(NH4

)2SO4] acidifies the soil through 

nitrification that releases H+ as NH4

+ converts to NO3-

.
69

Elemental S is the most effective soil acidifier. The powder form can be difficult to handle due to dustiness and 

fire hazard, and can cause severe root burn if not applied 

properly. To overcome these problems, some S products 

have been formulated into porous pellet-like particles that 

are much easier to handle and apply.

Ammonium thiosulfate and potassium thiosulfate are 

clear liquid fertilizers containing S2

O3

2-. They can be 

blended with N, P, and K solutions to form a wide variety 

of N-P-K-S formulations. Thiosulfates are non-corrosive 

and non-hazardous to handle, and are well-adapted to the 

methods used to apply fertilizer solutions. Application of 

ammonium thiosulfate to calcareous soils has been shown 

to increase the amount of extractable Fe in the soil.

The soil within the wetted pattern of a micro-irrigation 

emitter often becomes alkaline when the water source 

contains bicarbonate while the surrounding soil may be 

neutral or acidic. Lowering the soil pH in this situation 

requires an application of acid or acidifying fertilizer to 

the wetted pattern only. Application of acid or thiosulfate 

fertilizer through the irrigation system can be effective in 

treating this problem.

Table 11.1. Materials that can be used to lower soil pH, and 

their acidifying power relative to CaCO3.

Acidifier Amount needed to neutralize 

1000 lbs of pure CaCO3

Elemental sulfur 320 lbs

Concentrated (98%) 

sulfuric acid 68 gal

Ammonium thiosulfate

12-0-0-26S 1600 lbs

Potassium thiosulfate

0-0-25-17S 3800 lbs

Ammonium sulfate

21-0-0 900 lbs

11.5. Saline soils and water

All natural waters and soil solutions contain soluble salts. 

Salt concentration is reported several ways:

•	 Milligrams per liter (mg/L), or parts per million (ppm) 

of total dissolved solids (TDS). The units mg/L and 

ppm are interchangeable.

•	 Electrical conductivity (EC), expressed as deci-Siemens 

per meter (dS/m), millimhos per centimeter (mmhos/

cm), or micromhos per centimeter (μmhos/cm). The 

units dS/m and mmhos/cm are interchangeable, and 

μmhos/cm = mmhos/cm x 1000.

Salts in solution exist as ions that can conduct an electric 

current, so EC increases as dissolved salt concentration 

increases. The EC of Florida waters usually ranges between 

0 and 5 dS/m.

The conversion from EC to TDS depends on the kind of 

salts present in the solution. TDS (in ppm) can be estimated by multiplying EC (in dS/m or mmhos/cm) by 700. 

This conversion factor is an average value appropriate for 

converting the EC of Florida soil extracts and irrigation 

waters to TDS. Many conductivity meters that provide a 

direct salinity reading in ppm have a built-in conversion 

factor in the range of 630 to 640. Care must be taken to 

ensure that measurements made by different conductivity 

meters are comparable.

Vast amounts of salts can be deposited on the soil by longterm irrigation with high-salinity water. For example, 100 

gallons of water at 3000 ppm TDS contains about 2.5 lbs 

of salt. Since the weekly irrigation requirement of a bearing citrus tree can exceed three times this amount, soil salts 

can quickly accumulate. Even 1000 ppm TDS water (containing 0.8 lbs salt in 100 gallons) can create salt stress.

Since soil salt concentration depends on soil water content, soil salinity is often related to a standard saturation 

extract (ECe). The ECe standardizes soil salt concentration to the saturation soil water content. Thus, salinity 

around tree roots may be several times greater than ECe 

when soil moisture is at field capacity or less. In sandy soils, 

where salts are easily leached, management decisions based 

solely on ECe measurement are not advised. ECe of these 

soils only indicates soil salinity at the time of measurement 

and can change rapidly following irrigation or rainfall.

The main citrus tree response to excess salts in soil and irrigation water is growth reduction. Injury symptoms caused 

by saline irrigation water are not usually permanent, but 

affected trees may remain stunted compared with trees not 

receiving salty water especially if they are young.

Salts in solution exert an osmotic effect that reduces water 

availability through both chemical and physical processes. 

Roots are not able to extract as much water from a solution high in salts compared with one low in salts. In effect, 

the trees must expend more energy to move water through 

them, which reduces root growth followed by reductions 

in shoot growth and yield.

The critical salinity concentration will vary with the 

buffering capacity of the soil (that increases with clay and 

organic matter content), the climate, the rootstock used, 

and the soil moisture status. Salinity-induced symptoms 
70 

like reduced root growth, decreased flowering, smaller leaf 

size, and impaired shoot growth are often difficult to assess 

but occur prior to ion toxicity symptoms in the leaves. 

Chloride toxicity, which appears as burned necrotic or 

dry-appearing edges on leaves, is one of the most common 

visible salt injury symptoms. Sodium toxicity symptoms 

are seldom seen in Florida, but sometimes high Na may 

cause an overall leaf “bronzing” accompanied by reductions in growth. As with Cl, high leaf Na can cause nutrient imbalances at much lower concentrations than those 

required to produce visible symptoms. Since Na and Cl 

are highly soluble in soil water, evaluating salinity stress by 

measuring their concentration in the soil has little diagnostic value.

Common citrus rootstocks tolerate soil salinity differently. 

The ranking of rootstocks from most tolerant to least tolerant of salinity is: 1. Cleopatra mandarin; 2. sour orange; 

3. sweet orange; 4. Swingle citrumelo; 5. Carrizo citrange; 

6. rough lemon. Grapefruit trees tend to be less salt tolerant than orange trees.

Fertilizer application frequency directly affects soil solution TDS concentration. A fertilization program that 

frequently applies low rates of soluble salts will normally 

result in less salinity stress than programs applying only 

two or three high-rate doses per year. Use of controlledrelease fertilizers or fertigation can minimize salt stress if 

high-salinity irrigation water must be used. Growers using 

salty irrigation water usually observe a marked improvement in water quality when summer rains begin.

Selecting nutrient sources that have a relatively small osmotic effect on the soil solution can help reduce salt stress. 

The osmotic effect of a fertilizer is defined as its salt index 

relative to sodium nitrate, which arbitrarily has a salt index 

of 100. Phosphorus sources have a low salt index and present little problem. Conversely, N and K sources can have a 

high salt effect (Appendix C). The salt index of inorganic 

and natural organic fertilizers are low compared with 

commonly-used soluble fertilizers. High analysis fertilizers 

may have a lower salt index per unit of plant nutrient than 

lower-analysis fertilizers because they may be made with 

lower salt index materials. Thus, at a given fertilization 

rate, a high-analysis formulation will likely produce less 

salt injury.

Selecting nutrient sources that do not add a potentially 

harmful ion to already high concentrations in irrigation 

water can reduce the likelihood of a salinity problem. The 

Cl-

 in KCl or Na+ in NaNO3 add potentially harmful salts 

to the soil solution. High application rates of fertilizer salts 

can raise soil pH and decrease soil nutrient availability. 

Specific ions can also aggravate nutrient imbalances in soil 

and trees. For example, Na+ displaces K+ and to a lesser extent Ca2+ in soil solutions. Displacement of K+ by Na+ can 

lead to K deficiency and in some cases even Ca2+ deficiency 

in leaves when repeatedly irrigating with water high in 

Na+. Nutrient deficiencies compound the effects of salinity 

stress. Problems can be minimized if sufficient nutrition is 

maintained through either soil or foliar fertilizer application.

Nutrient management with saline irrigation water

•	 Routinely evaluate irrigation water salinity with an EC 

meter. TDS below 1000 ppm is excellent. A salt problem may become evident as TDS increases from 1000 

to 2000 ppm, and is highly likely if TDS exceeds 2000 

ppm.

•	 If excess salts accumulate in the soil, keep the soil moist 

so they are less concentrated.

•	 Fine-textured soils and areas of compacted soils or poor 

drainage may need special management to flush excess 

salts from the root zone.

•	 Do not allow salty water to contact leaves, especially 

when evaporation demand is high.

•	 Use nighttime irrigation whenever possible to minimize 

evaporation and salt deposition.

•	 Choose fertilizer formulations with the lowest salt 

index per unit of plant nutrients.

•	 Increase fertilization frequency, which will help reduce 

the salt content of each application and will aid in preventing excess salt accumulation in the root zone.

•	 Maintain optimum but not excessive nutrient concentrations in the leaves.

•	 Base fertilization rates on the long-term production 

of the grove. Decrease fertilizer rates applied to trees 

irrigated with salty water compared with trees irrigated 

with good quality water, since production is probably 

lower.

•	 Use leaf tissue analysis to detect excessive leaf Na or Cl 

or deficiencies of other elements caused by salt-induced 

nutrient imbalance.

11.6. Using reclaimed water for irrigation

Reclaimed municipal effluent is an excellent citrus irrigation water source as long as it is produced under strict 

quality control. As Florida’s population continues to grow, 

treated wastewater will become increasingly important for 

irrigation.
71

Long term use of large quantities of reclaimed water can 

increase soil pH and soil test P and Ca. Leaf analysis may 

sometimes show increased Na, Cl, and B concentrations 

with no observed tree injury. Differences in uptake of various elements in the water (Table 11.2) can be expected to 

occur among varieties and rootstocks.

Table 11.2. Chemical composition of reclaimed municipal 

effluent from the city of Orlando and Orange county’s Water 

Conserv II project compared with typical well water and drinking water standards (Parsons et al., 2001).

Element or 

parameter

Drinking 

water MCL1

(ppm)

Typical 

well water 

(ppm)

Typical Conserv II 

water (ppm)

EC (μmhos) 781 360 720

Arsenic 0.05 --- <0.005

Boron --- 0.02 <0.25

Cadmium 0.005 --- <0.002

Calcium --- 39 42

Chloride 250 15 75 – 81

Chromium 0.1 --- <0.005

Copper 1.0 0.03 0.002 – 0.05

Lead 0.015 --- <0.003

Magnesium --- 16 9

Manganese 0.05 0.01 0.006 – 0.042

Nickel 0.1 --- 0.01

Nitrate-N 10 3 6 – 7

Phosphorus --- 0.01 1.1

Potassium --- 6 12

Sodium 160 18 50 – 70

Sulfate 250 23 29 – 55

1

Maximum contamination limit.

Essential elements in reclaimed water contribute to citrus 

tree nutrition, so it may be possible to reduce fertilizer 

rates if reclaimed water is the sole source of irrigation. For 

example, applying 50 acre-inches/yr of reclaimed water 

containing 10 ppm N would supply 113 lbs N/acre. However, even in low rainfall years, trees in Florida do not need 

50 inches of irrigation, so considerable leaching would 

occur if this volume was applied. Hence, N fertilization is 

still needed when irrigating with reclaimed water.

It has been determined that P and B inputs could be 

reduced or eliminated when using large quantities of 

reclaimed water. Leaf Na, Cl, and B should be routinely 

monitored to avoid their reaching toxic levels under reclaimed water irrigation.

Limited studies have shown that it is feasible to grow 

citrus using citrus processing effluent as an irrigation 

source. However, certain variables must be considered in 

the design and management of irrigation systems for use 

of this water. Daily flow from the processing plant, weekly 

loading depth to the land, and the storage capacity of the 

soil should all be considered when determining the needed 

land area. As with other reclaimed water sources, effluent monitoring procedures at the processing facility are 

needed to ensure that acceptable quality is maintained.

Nutrient concentrations in citrus processing wastewater 

are too low to sustain tree growth (Table 11.3), so supplemental fertilization is necessary. Rootstocks and scion 

varieties should be selected for their tolerance to excess Na 

contained in processing effluent.

Table 11.3. Average chemical composition of citrus processing 

wastewater (Koo, 1973).

Source A Source A Source B Source C

Not 

treated Treated Treated Treated

pH 7.2 7.7 7.8 5.7

ppm

TDS 639 612 412 225

Total N 119 7 8 10

Nitrate-N 2 2 4 3

P 1 2 1 1

K 35 33 22 12

Ca 44 47 32 37

Mg 10 10 7 3

Na 169 137 81 24

Cl 81 48 48 14

Fe 2 1 0.4 16

Mn 0 0 0 0.2

Zn 0.01 0.05 0.03 0.1

B 0 0 0 0.2

11.7. Fertilization strategies for damaged trees

11.7.1. Wind damage

Strong sustained winds from tropical storms or hurricanes 

can damage citrus trees by removing canopy and fruit. 

Root damage may also occur if a storm produces flooding 

rains. The main nutritional factors related to severe canopy 

thinning are loss of leaf N and K reserves and interruption of the natural nutrient recycling that occurs as tree 

residues decompose and mineralize in root zone soil. As a 
72 

result, subsequent vegetative growth may deplete remaining tree and soil reserves of N and K. Additional fertilization beyond the normal program is justified to aid grove 

recovery in this situation, as long as the root system is not 

significantly damaged.

To determine how much additional N and K2

O to apply, start with the assumption that every 10% loss of leaf 

canopy from an average grove represents a loss of about 10 

lbs N/acre, then estimate a target fertilizer rate based on 

the relative amount of leaf canopy that the grove has lost 

and the efficiency of the fertilizer application method to 

be used. Keep the following considerations in mind when 

attempting to replace lost canopy N and K with additional 

fertilizer as the grove recovers:

•	 Reduce the fertilizer rate for smaller trees.

•	 It will take time to re-build a full, healthy canopy.

•	 Distribute additional N and K fertilizer throughout the 

following growing season as normal. Do not front-load 

all of it in the spring.

•	 Consider foliar application in place of soil application.

•	 Remember that bloom and fruit set in the following 

year will depend more on environmental conditions and 

less on fertilizer rates.

When root systems are extensively damaged from prolonged flooding, the tree canopy will recover more slowly. 

In this case, fertilization should be reduced until the root 

system can rejuvenate. Surviving roots are more likely to 

be close to the soil surface, with the lower ones damaged 

or killed. Addition of fertilizer at rates normally applied to 

vigorous trees may further damage roots.

11.7.2. Freeze damage

Trees should not be fertilized or irrigated following a 

severe freeze until the extent of damage is determined and 

re-growth is evident. Stored nutrients in bearing trees on 

a regular fertilizer program can mobilize to new shoots 

and leaves, especially after severe wood damage. No more 

than 50% of the recommended N rate should be applied 

to severely-damaged citrus trees that will not produce a 

crop the following year, provided that optimum tree water 

status is maintained. It may be necessary to apply only N 

and foliar micronutrients. There is strong evidence that 

soil-applied fertilizer may not even be necessary for bearing trees in the first post-freeze season if they have received 

sufficient fertilization in the years prior to the freeze. 

Reduced fertilizer rates may be applied to 2 to 4-year-old 

trees with moderate freeze damage provided no crop is set 

following the freeze.

11.8. Nutritional deficiencies enhanced by 

environmental or pathological factors

Zinc deficiency patterns can be enhanced by citrus blight 

disease. When trees have blight, leaf symptoms will look the 

same as Zn deficiency. Leaves are reduced in size, off-color, 

and small blotches of yellow between green-colored veins in 

the leaf will appear. In many cases, the leaf Zn deficiency pattern may not be evenly distributed within the tree canopy.

Iron deficiency can result from flooding injury to the citrus 

root system. Root damage can occur if the root zone soil is 

flooded for several days in the summer but may take weeks 

to occur in the cooler winter months. The pattern will first 

appear on young, expanding leaves. The leaf turns light 

green, while the veins and midrib remain darker green.

Nitrogen deficiency, or “winter chlorosis,” can occur in late 

winter or early spring when rapid tree growth begins and 

soil temperature is too cool for normal root function and 

nutrient uptake. When this occurs the midrib will begin 

to yellow while the remaining portion of the leaf remains 

darker green.

Phytophthora-induced N deficiency occurs when the 

roots or tree trunk become infected resulting in partial or 

complete girdling that causes the tree to decline. Leaves 

express visual symptoms of vein yellowing typical of N 

deficiency. The deficiency pattern may be associated with 

individual limbs in the case or foot rot or could involve 

the entire tree in the case of root damage. The deficiency 

appears as the ability of the roots or trunk to transport N 

upward into the tree canopy is reduced.

11.9. Organic citrus production

11.9.1. General information

The exclusive use of certified organic nutrient sources in an 

organic production program may not be a viable alternative for large scale Florida commercial citrus production. 

Limitations include an insufficient supply of non-synthetic 

fertilizers (manures, composts, etc.), sometimes slow availability of nutrients with time from these materials, and 

the logistics of their transport, storage, and application. 

For the small producer, organic citrus production may be 

feasible depending on the availability and quality of acceptable sources, customer acceptance of the product, and 

expected returns on investment. Yields, production costs, 

and market returns for organic citrus production have not 

been clearly defined in Florida.

Certified organic groves must have distinct, defined 

boundaries between fields managed organically and those 
73

managed conventionally. Storage facilities and records for 

certified organic fields must be maintained separately from 

non-certified fields. A production unit may be certified 

as organic only if harvest occurs at least 36 months after 

the most recent use of prohibited pesticides or fertilizers. 

Records of all fertilizer and soil amendment use must be 

kept for at least 3 years prior to certification.

Organic fertilization programs for citrus emphasize methods 

to improve soil fertility and health through the use of organic 

fertilizers and soil amendments. Soil management includes 

increasing soil organic matter (humus) content by mowing, 

grazing, growing green manure and N-fixing cover crops in 

row middles, and applying manures, composts, and natural 

fertilizers. Annual nutrient application rates for organicallygrown young trees and bearing trees should be similar to 

those recommended for conventionally-grown trees.

Citrus production operations applying for initial or ongoing organic certification must comply with regulations 

established under the National Organic Program (NOP) 

and applicable organic production regulations. Information about various aspects of the NOP is available on the 

internet at http://www.ams.usda.gov/NOP/NOPhome.

html. This web page provides links to several other web 

pages where additional information can be found.

The U.S. Dept. of Agriculture (USDA) does not provide 

organic certification, but instead accredits state, private, 

and foreign organizations, groups, or persons to become 

certifying agents. FDACS has not assumed this role, so 

growers must choose a private USDA-accredited organic 

certification agency.

11.9.2. Soil fertility and crop nutrient management 

guidelines

(These guidelines are summarized from Section 205.203 of the 

NOP final rule.)

•	 Producers should implement tillage and cultivation practices to maintain or improve the physical, chemical, and 

biological condition of soil and minimize soil erosion.

•	 Producers should manage crop nutrients and soil fertility through rotations, cover crops, and application of 

plant and animal materials.

•	 Producers should manage plants and animal materials 

to maintain or improve soil organic matter in a manner that does not contribute to contamination of crops, 

soil, or water by plant nutrients, pathogenic organisms, 

heavy metals, or residues of prohibited substances. Animal and plant materials include:

o Raw animal manure (feces, urine, other excrement, 

and bedding produced by livestock that has not been 

composted):

	Can be applied to land used for a crop not intended for human consumption.

	Must be incorporated into the soil not less than 

120 days before harvesting a product that comes 

into contact with the soil surface or soil particles.

	Must be incorporated into the soil not less than 90 

days before harvesting a product that does not come 

into contact with the soil surface or soil particles.

o Composted plant and animal materials produced 

through a process that:

	Established an initial carbon-to-nitrogen ratio 

between 25:1 and 40:1; and

	Maintained temperature between 131° F and 170° 

F for 3 days using an in-vessel or static aerated pile 

system, or

	Maintained temperature between 131° F and 170° 

F for 15 days using a windrow composting system, 

turning the materials at least five times.

o Non-composted plant materials.

A producer may apply:

	Crop nutrients or soil amendments allowed for 

use in organic production;

	Mined substances of low solubility;

	Mined substances of high solubility, provided 

that it is used in compliance with the conditions 

established on the National List of non-synthetic 

materials prohibited for crop production;

	Ash obtained from the burning of a plant or 

animal material, provided it has not been treated 

or combined with a prohibited substance or the 

ash is not included on the National List of nonsynthetic substances prohibited for use in organic 

crop production; and

	Plant or animal material that has been chemically 

altered by a manufacturing process, as long as it is 

included on the National List of synthetic substances allowed for use in organic crop production.

A producer may not use:

	Any fertilizer or composted plant and animal 

material that contains a synthetic substance not included on the National List of synthetic substances 

allowed for use in organic crop production.

	Biosolids.

	Burning as a means of disposal for crop residues 

produced on the operation. (Exception: burning 

may be used to suppress the spread of disease.)
74 

12.1. Refereed journal articles

Albrigo, L. Gene. 1999. Effects of foliar applications of 

urea or Nutriphite on flowering and yields of Valencia 

orange trees. Proc. Fla. State Hort. Soc. 112:1-4.

Alva, A. K. 1993. Comparison of Mehlich 3, Mehlich 

1, ammonium bicarbonate-DTPA, 1.0M ammonium 

acetate, and 0.2M ammonium chloride for extraction 

of calcium, magnesium, phosphorus, and potassium for 

a wide range of soils. Commun. Soil Sci. Plant Anal. 

24(7&8):603-612.

Alva, A. K., and S. Paramasivam. 1998. Nitrogen 

management for high yield and quality of citrus in sandy 

soils. Soil Sci. Soc. Am. J. 62:1335-1342.

Alva, A. K., S. Paramasivam, and W. D. Graham. 

1998. Impact of nitrogen management practices on 

nutritional status and yield of Valencia orange trees and 

groundwater nitrate. J. Environ. Qual. 27:904-910.

Alva, A. K., S. Paramavisam, W. D. Graham, and T. 

A. Wheaton. 2003. Best nitrogen and irrigation 

management practices for citrus production in sandy 

soils. Water, Air, and Soil Pollution 143:139-154.

Alva, A. K., S. Paramasivam, T. A. Obreza, and A. W. 

Schumann. 2006. Nitrogen best management practice 

for citrus trees I. Fruit yield, quality, and leaf nutritional 

status. Scientia Horticulturae 107:233-244.

Alva, A. K., S. Paramasivam, A. Fares, T. A. Obreza, and 

A. W. Schumann. 2006. Nitrogen best management 

practice for citrus trees II. Nitrogen fate, transport, 

and components of N budget. Scientia Horticulturae 

109:223-233.

Boman, B. J., M. Zekri, and E. Stover. 2005. Managing 

salinity in citrus. HortTechnology 15(1):108-113.

Hanselman, T. A., D. A. Graetz, and T. A. Obreza. 

2004. A comparison of in situ methods for measuring 

net nitrogen mineralization rates of organic soil 

amendments. J. Env. Qual. 33:1098-1105.

He, Z. L., D. V. Calvert, A. K. Alva, D. J. Banks, and Y. C. 

Li. 2003. Thresholds of leaf nitrogen for optimum fruit 

production and quality in grapefruit. Soil Sci. Soc. Am. 

J. 67:583-588.

Lamb, S. T., W. D. Graham, C. B. Harrison, and A. K. 

Alva. Impact of alternative citrus management practices 

on groundwater nitrate in the central Florida ridge I. 

Field investigation. Trans. ASAE 42:1653-1668.

Lea-Cox, J. D., and J. P. Syvertsen. 1995. Nitrogen uptake 

by Citrus leaves. J. Amer. Soc. Hort. Sci. 120:505-509.

Mattos, Jr., D., D. A. Graetz, and A. K. Alva. 2003. 

Biomass distribution and nitrogen-15 partitioning 

in citrus trees on a sandy Entisol. Soil Sci. Soc. Am. J. 

67:555-563.

Mattos, Jr., D., J. A. Quaggio, H. Cantarella, and A. K. 

Alva. 2003. Nutrient content of biomass components 

of Hamlin sweet orange trees. Scientia Agricola 

60(1):155-160.

Obreza, T. A., R. E. Rouse, and J. B. Sherrod. 1999. 

Economics of controlled-release fertilizer use on young 

citrus trees. J. Prod. Agric. 12:69-73.

Obreza, T. A., and R. E. Rouse. 2006. Long-term response 

of ‘Hamlin’ orange trees to controlled-release fertilizers. 

HortScience 41(2):423-426.

Orbovic, V., D. Achor, P. Petracek, and J. P. Syvertsen. 

2001. Air temperature, humidity, and leaf age affect 

penetration of urea through grapefruit leaf cuticles. J. 

Amer. Soc. Hort. Sci. 126:44-50. 

Ozores-Hampton, M., T. A. Obreza, and G. Hochmuth. 

1998. Using composted wastes on Florida vegetable 

crops. HortTechnology 8:130-137.

Paramasivam, S., and A. K. Alva. 1997. Leaching of 

nitrogen forms from controlled-release nitrogen 

fertilizers. Commun. Soil Sci. Plant Anal. 28:1663-1674.

Paramasivam, S., A. K. Alva, K. H. Hostler, G. W. 

Easterwood, and J. S. Southwell. 2000. Fruit nutrient 

accumulation of four orange varieties during fruit 

development. J. Plant Nutrition 23(3):313-327.

Paramasivam, S., A. K. Alva, A. Fares, and K. S. Sajwan. 

2001. Estimation of nitrate leaching in an Entisol 

under optimum citrus production. Soil Sci. Soc. Am. J. 

65:914-921.

12. References and Further Reading

(Note: References in Sections 12.1 through 12.3 are the information sources from which 

new material in this second edition of “Nutrition of Florida Citrus Trees” was obtained.)
75

Parsons, L. R., T. A. Wheaton, and W. S. Castle. 2001. 

High application rates of reclaimed water benefit 

citrus tree growth and fruit production. HortScience 

36:1273-1277.

Parsons, L. R., and B. J. Boman. 2006. Best management 

practices for Florida citrus. HortTechnology 

16(3):389-393.

Schumann, A. W., W. M. Miller, Q. U. Zaman, K. H. 

Hostler, S. Buchanon, and S. Cugati. 2006. Variable 

rate granular fertilization of citrus groves: spreader 

performance with single-tree prescription zones. 

Applied Engineering in Agriculture 22(1):19-24.

Wang, F. L., and A. K. Alva. 1996. Leaching of nitrogen 

from slow-release urea sources in sandy soils. Soil Sci. 

Soc. Am. J. 60:1454-1458.

Zekri, M., L. R. Parsons, and R. C. J. Koo. 2000. Irrigation 

of Florida citrus orchards with reclaimed municipal 

wastewater. Proc. International Soc. Citriculture. IX 

Congr. 1:554-558.

12.2. Non-refereed journal articles

Boman, B. J. 1997. Effectiveness of fall potassium sprays 

on enhancing grapefruit size. Proc. Fla. State Hort. Soc. 

110:1-7.

Boman, B. J., and J. W. Hebb. 1998. Post bloom and 

summer foliar K effects on grapefruit size. Proc. Fla. 

State Hort. Soc. 111:128-135.

Boman, B. J. 2001. Foliar nutrient sprays influence yield 

and size of ‘Valencia’ orange. Proc. Fla. State Hort. Soc. 

114:83-88.

Futch, S. H. (ed.). 2003. Nutrient management for 

optimum citrus tree growth and yield – short course 

proceedings. Fla. Coop. Extension Serv., Gainesville, FL. 

Koo, 1973. Irrigation of citrus with citrus processing waste 

water. Proc. Fla. State Hort. Soc. 86:233-237.

Koo, R. C. J. 1988. Fertilization and irrigation effects 

on fruit quality. p. 35-42. In J. J. Ferguson and W. F. 

Wardowski (eds.). Factors affecting fruit quality – citrus 

short course proceedings. Fla. Coop. Extension Serv., 

Gainesville, FL.

Morgan, K. T., T. A. Obreza, and T. A. Wheaton. 

2006. The basis for mature citrus nitrogen fertilizer 

recommendations. Proc. Fla. State Hort. Soc. 

119:168-171.

Sartain, J. B. 1978. Adaptability of the double-acid 

extractant to Florida soils. Soil Crop Sci. Soc. Proc. 

37:204-208.

Schumann, A. W., A. Fares, A. K. Alva, and S. 

Paramasivam. 2003. Response of ‘Hamlin’ orange to 

fertilizer source, annual rate, and irrigated area. Proc. Fla. 

State Hort. Soc. 116:256-260.

Schumann, A. W., K. H. Hostler, S. M. Buchanon, and 

Q. Zaman. 2006. Relating citrus canopy size and yield 

to precision fertilization. Proc. Fla. State Hort. Soc. 

119:148-154.

12.3. Extension publications, articles, and books

Boman, B. J. 2002. Water and Florida Citrus: Use, 

Regulation, Irrigation, Systems, and Management. Univ. 

of Florida-IFAS SP 281.

Lovatt, C. J., and R. L. Mikkelsen. 2006. Phosphite 

fertilizers: What are they? Can you use them? What can 

they do? Better Crops Plant Food 90(4):11-13.

Obreza, T. A. 2004. Maintenance guide for Florida 

microirrigation systems. Univ. of Florida-IFAS Circular 

1449.

12.4. Historical documents

Koo, R. C. J., C. A. Anderson, I. Stewart, D. P. H. 

Tucker, D. V. Calvert, and H. K. Wutscher. 1984. 

Recommended fertilizers and nutritional sprays for 

citrus. Fla. Coop. Extension Serv. Bulletin 536D.

Tucker, D. P. H., A. K. Alva, L. K. Jackson, and T. A. 

Wheaton. 1995. Nutrition of Florida citrus trees. Univ. 

of Florida-IFAS Publication SP 169.
76 

Acid-forming fertilizer – A fertilizer capable of 

increasing soil acidity, derived principally from the 

nitrification of ammonium salts by soil bacteria.

Adsorption - the adhesion in an extremely thin layer of 

molecules (as of gases, solutes, or liquids) to the surfaces of 

solid bodies or liquids with which they are in contact.

Association of American Plant Food Control Officials

– An organization of officers and their deputies charged by 

law with regulating the sale of fertilizers, and of research 

workers employed by state or federal agencies engaged 

in the investigation of fertilizers. Its object is to promote 

uniform and effective legislation, definitions, and rulings, 

and to enforce the laws relating to the control of sale and 

distribution of fertilizers and fertilizer materials.

Basic fertilizer – A fertilizer capable of decreasing soil 

acidity.

Biuret – A phytotoxic impurity formed when urea 

molecules condense (combine) during fertilizer 

manufacture.

Brand – Term, design, or trademark used in connection 

with one or several grades of commercial fertilizer.

Bulk fertilizer – A fertilizer distributed in a non-packaged 

form, usually in semi-trailers.

Clear liquid fertilizer – A fertilizer in which the N-P-K 

and other materials are completely dissolved.

Commercial fertilizer – Any substance containing one or 

more recognized plant nutrients that is designed for use or 

claimed to have value in promoting plant growth or that 

is designed for use or claimed to have value in controlling 

soil acidity or alkalinity (except non-manipulated animal 

and vegetable manures).

Complete fertilizer – A mixed fertilizer that contains 

the three major plant nutrient elements: nitrogen, 

phosphorus, and potassium.

Coning – The formation of a pyramidal pile or cone of dry 

bulk mixed fertilizer such as may occur while being loaded 

into a holding hopper or transport vehicle and cause 

separation and segregation of the fertilizer components.

Dealer – Any person, other than the manufacturer, 

who offers for sale, sells, barters, or otherwise supplies 

commercial fertilizer.

Deconing – Any accepted process employed by a 

manufacturer that will prevent or minimize coning.

Deficiency – The amount of nutrient found by analysis 

less than that guaranteed, which may result from lack of 

nutrient ingredients or from lack of uniformity.

Dry bulk blending – The process of mechanically mixing 

solid fertilizer materials.

Excess – The amount found by analysis over and above 

that guaranteed on the label.

Fertilizer – Any substance containing one or more 

recognized plant nutrients that is used for its plant 

nutrient content. Unprocessed animal and vegetable 

manures, marl, lime, limestone, wood ashes, and other 

products are exempt from this definition.

Fertilizer formula – An expression of the quantity and 

analysis of the materials in a mixed fertilizer.

Fertilizer grade – The percentages in mixed fertilizer of 

total nitrogen (N), available phosphoric acid (P2

O5), and 

the soluble potash (K2O), stated in whole numbers in the 

same terms, order, and percentages as in the “guaranteed 

analysis” form (15-5-15, for example). Mixed fertilizer 

containing a total of 5% or less of total N, P2O5, and 

K2O may be guaranteed in other than whole percentages; 

however, a minimum guarantee shall be established by 

rule.

Fertilizer material – A fertilizer that either:

•	 contains important quantities of no more than

one of the primary plant nutrients nitrogen (N), 

phosphoric acid (P2O5), and potash (K2O); or

•	 has 85% or more of its plant nutrient content present

in the form of a single chemical compound; or

•	 is derived from a plant or animal residue or byproduct or natural material deposit that has been 

processed in such a way that its content of plant 

nutrients has not been materially changed except by 

purification and concentration.

Fertilizer ratio – Refers to the relative percentages of N, 

P2

O5, and K2O (a 15-5-15 has a 3-1-3 ratio).

Filler – A “make-weight” material added to a mixed 

fertilizer or fertilizer material to make up the difference 

between the weight of the added ingredients required to 

supply the plant nutrients in a ton of a given analysis and 

2000 lbs.

13. Glossary
77

Fluid fertilizer – Clear or suspension liquid fertilizers.

Granulation – The process of manufacturing fertilizer 

particles of reasonably uniform size and stability.

Label – A display of written, printed, or graphic matter 

upon the immediate container of any commercial fertilizer 

or accompanying same when moved in bulk.

Manufacturer – A person engaged in the business of 

importing, preparing, mixing, blending, or manufacturing 

commercial fertilizer for sale, either to direct consumers or 

through other media of distribution.

Marl – A friable earthy deposit consisting of clay and 

calcium carbonate.

Mixed fertilizer – A fertilizer containing any combination 

or mixtures of commercial fertilizers designed for use or 

claimed to have value in promoting plant growth.

Non-acid-forming, or “neutral” fertilizer – A fertilizer 

that is guaranteed to leave neither an acidic nor a basic 

residue in the soil.

Official sample – Any sample of commercial fertilizer 

taken by FDACS or its representative, in accordance with 

the provisions of the fertilizer law.

Organic – A material containing carbon and one or more 

elements, other than hydrogen and oxygen, essential for 

plant growth. When the term “organic” is used on the 

label, it shall be qualified as either “synthetic organic” or 

“natural organic,” with the percentage of each specified. 

When the term “organic” is used, it must be clearly 

indicated that it refers only to the nitrogen or other 

applicable portion of the fertilizer.

• “Natural organic” is a byproduct from processing of 

animal or vegetable substances that contain sufficient 

plant nutrients to be of value as fertilizers.

•	 “Synthetic organic” is a material that is manufactured

chemically (by synthesis) from its elements or other 

chemicals, as contrasted to those found ready-made 

in nature.

Percent – Indicates percentage by weight.

Primary plant nutrient – Any form of nitrogen, 

phosphoric acid, or potash, or any combination of these 

substances.

Registrant – The person who registers commercial 

fertilizer under the provisions of the fertilizer law.

Secondary plant nutrient – Any element or substance 

useful as plant nutrient other than the primary plant 

nutrients.

Slow or controlled-release fertilizer – A fertilizer 

containing a plant nutrient in a form that delays its 

availability for plant uptake and use after application, or 

which extends its availability to the plant significantly 

longer than a reference “rapidly available nutrient 

fertilizer,” like ammonium nitrate or urea, ammonium 

phosphate, or potassium chloride. When slow or 

controlled-release nutrient is claimed or advertised, the 

guarantee for such a nutrient shall be shown as a footnote 

and shall be expressed as percent of actual nutrient. When 

a slowly released nutrient is less than 15% of the guarantee 

for either total nitrogen, available phosphoric acid, or 

soluble potash, as appropriate, the label shall bear no 

reference to such designations.

Soil amendment – A material applied to improve or 

enhance soil characteristics for plant growth.

Sorb – To take up and hold by either adsorption or 

absorption.

Specialty fertilizer – Commercial fertilizer in packages 

sold or offered for sale for home use.

Suspension fertilizer – A fertilizer in which some of the 

fertilizer materials are suspended as fine particles.

Tolerance – The variation authorized by law or regulation 

from the guaranteed analysis.

Unit of plant nutrient – 1% of a ton, or 20 lbs.

Water insoluble nitrogen – Nitrogen not soluble in 

water. All organic nitrogen soluble in water shall be 

classified as “water-soluble organic nitrogen.” However, 

soluble organic nitrogen derived from urea may be 

classified either as “urea nitrogen” or “water-soluble 

organic nitrogen,” at the option of the registrant. Nitrogen 

in the nitrate or ammoniacal forms shall be so classified.
78 
79

14. Appendices
80 
81

Appendix A. Physical and chemical properties of typical Florida citrus soils.

Table A.1. Typical root zone1 soil physical and chemical properties for common soil series found in Florida citrus groves.

Physical properties Chemical properties

Soil texture Organic 

matter

Water-holding capacity

pH

Cation 

exchange 

capacity Soil series Sand Silt Clay inches per 

ft

inches in 

the root 

% zone meq/100 g

ENTISOLS

Astatula 98.5 0.75 0.75 0.5 – 1.0 0.3 – 0.6 0.9 – 1.8 4.5 – 6.5 2 – 4

Basinger 98.5 0.5 1.0 0.5 – 1.0 0.4 – 0.8 0.6 – 1.2 3.6 – 7.3 2 – 4

Candler 97.5 1.25 1.25 0.5 – 1.0 0.3 – 0.7 0.9 – 2.1 4.5 – 6.0 2 – 4

Tavares 97.0 1.5 1.5 0.5 – 1.0 0.3 – 0.6 0.9 – 1.8 3.6 – 6.0 2 – 4

ALFISOLS

Boca 94.0 3.0 3.0 1.0 – 2.5 0.4 – 0.9 0.6 – 1.4 5.1 – 8.4 6 – 10

Holopaw 94.0 3.5 2.5 1.0 – 2.5 0.7 – 1.2 1.1 – 1.8 5.1 – 7.3 3 – 7

Pineda 96.0 2.5 1.5 0.5 – 2.0 0.3 – 0.6 0.5 – 0.9 5.6 – 7.3 2 – 6

Riviera 96.5 2.0 1.5 0.5 – 2.0 0.6 – 1.0 0.9 – 1.5 4.5 – 6.5 2 – 6

Winder 85.0 6.0 9.0 1.0 – 3.0 0.7 – 1.2 1.1 – 1.8 5.6 – 7.8 14 – 18

SPODOSOLS

Immokalee 98.5 1.0 0.5 1.0 – 2.0 0.4 – 0.8 0.6 – 1.2 3.6 – 6.0 2 – 6

Myakka 98.5 1.0 0.5 1.0 – 2.0 0.4 – 0.8 0.6 – 1.2 3.6 – 6.5 2 – 6

Oldsmar 98.0 1.5 0.5 1.0 – 2.0 0.3 – 0.6 0.5 – 0.9 3.6 – 7.3 2 – 6

Pomona 96.0 3.5 0.5 1.0 – 2.0 0.4 – 1.0 0.6 – 1.5 3.6 – 5.5 2 – 6

Smyrna 97.0 2.5 0.5 1.0 – 3.0 0.4 – 0.8 0.6 – 1.2 3.6 – 7.3 2 – 6

Wabasso 97.5 1.5 1.0 1.0 – 2.0 0.3 – 0.6 0.5 – 0.9 4.5 – 7.0 2 – 6

1

Top 36 inches of soil for central ridge Entisols and top 18 inches of soil for flatwoods Alfisols, Spodosols, and Entisols.

Table A.2. Physical and chemical properties of the subsurface diagnostic layers of typical Alfisols and Spodosols found in Florida 

flatwoods citrus groves. These layers may reside in an undisturbed state beneath the root zone, or they may be partially excavated 

and mixed into the root zone soil during the bedding process.

Physical properties Chemical properties

Soil texture Organic 

matter

Water-holding 

capacity pH

Cation 

exchange 

capacity Soil series Layer

Sand Silt Clay

% inches per ft meq/100 g

ALFISOLS

Boca Loamy 81.0 4.0 15.0 0.3 – 1.2 1.2 – 1.8 5.1 – 8.4 16 – 24

Holopaw Loamy 80.0 7.0 13.0 0.2 – 0.4 1.8 – 2.4 5.1 – 8.4 11 – 22

Pineda Loamy 77.0 3.5 19.5 0.1 – 0.3 1.2 – 1.8 5.1 – 8.4 4 – 18

Riviera Loamy 77.0 4.5 18.5 0.2 – 0.3 1.4 – 1.8 6.1 – 8.4 9 – 24

Winder Loamy 80.0 4.0 16.0 0.1 – 0.3 1.2 – 1.8 6.6 – 8.4 12 – 26

SPODOSOLS

Immokalee Organic 95.0 2.5 2.5 2.5 – 3.8 1.2 – 3.0 3.3 – 4.4 14 – 25

Myakka Organic 90.5 5.0 4.5 2.8 – 4.5 1.2 – 2.4 4.0 – 4.7 13 – 18

Oldsmar Organic 92.0 3.5 4.5 1.8 – 3.0 1.2 – 1.8 4.7 – 5.3 7 – 15

Pomona Organic 93.0 5.5 1.5 1.0 – 1.5 1.2 – 1.8 4.0 – 4.7 5 – 15

Smyrna Organic 90.5 5.0 4.5 3.3 – 3.9 1.2 – 1.8 4.3 – 4.7 19 – 21

Wabasso Organic 93.0 2.0 5.0 1.8 – 2.1 1.2 – 1.8 4.7 – 5.2 5 – 12
82 

Appendix B. Nutrient concentrations of fertilizer materials.

Table B.1. Conventional and slow-release sources.

Material Percentage composition Availability N P2

O5 K2O Ca Mg S Other

Conventional sources

Ammonium molybdate 54 Mo Rapid

Ammonium nitrate 34 Rapid

Ammonium polyphosphate 10 34 Rapid

Ammonium sulfate 21 23 Rapid

Ammonium thiosulfate 12 26 Rapid

Borax 10 – 15 B Rapid

Calcitic limestone 32 Slow

Calcium ammonium nitrate 27 6 Rapid

Calcium nitrate 15.5 20 Rapid

Calcium sulfate (gypsum) 23 18 Moderate

Copper sulfate 12 25 – 35 Cu Rapid

Diammonium phosphate 18 46 0 – 2 Rapid

Dolomitic limestone 8 - 20 22 Ca Slow

Iron (ferrous) sulfate 20 Fe Rapid

Iron oxy-sulfate 45 – 50 Fe Slow

Iron DTPA 10 Fe Rapid

Iron EDTA 9 – 12 Fe Rapid

Iron EDDHA 6 Fe Rapid

Iron HEDTA 5 – 9 Fe Rapid

Iron humate 25 – 28 Fe Moderate

Iron sucrate 50 Fe Moderate

Magnesium oxide 56 Moderate

Magnesium sulfate 10 Rapid

Manganese oxide 41 – 68 Mn Moderate

Manganese sulfate 13 24 Mn Moderate

Monoammonium phosphate 11 48 0 – 2 Rapid

Phosphoric acid 54 Rapid

Phosphorous acid 40 - 60 Moderate

Potassium chloride 60 44 Cl Rapid

Potassium-magnesium sulfate 22 11 22 Moderate

Potassium nitrate 13 48 Rapid

Potassium phosphite 28 26 Moderate

Potassium sulfate 46 18 Rapid

Potassium thiosulfate 25 17 Rapid

Sodium molybdate 39 Mo Rapid

Sulfur, elemental 30 - 99 Moderate

Superphosphate, ordinary 20 20 10 - 14 Rapid

Superphosphate, concentrated 46 16 1 - 2 Rapid

(continued on p. 83)
83

Material Percentage composition Availability N P2

O5 K2O Ca Mg S Other

Urea 46 Rapid

Urea-ammonium nitrate 28 - 32 Rapid

Zinc oxide 50 – 78 Zn Moderate

Zinc sulfate 12 21 – 36 Zn Moderate

Zinc EDTA 9 – 14 Zn Rapid

Zinc HEDTA 9 Zn Rapid

Slow-release sources

Sulfur-coated urea 32 - 38 3 - 6 mo.

Urea formaldehyde 38 - 40 6 - 12 mo.

Isobutylidene diurea (IBDU) 31 3 - 6 mo.

Table B.2. Organic sources.

Material Percentage composition Availability N P2

O5 K2O Ca Mg S Other

Organic sources

Organiform 30 Slow

Municipal biosolids 3 - 7 1 - 7 0.5 - 1 Moderate

Activated biosolids 6 2 0.5 Moderate

Blood meal 8 - 13 2 1 Rapid

Bone meal 1 - 4 18 - 34 Slow

Cottonseed meal 6 3 1 Medium

Fish meal 9 4 - 6 Rapid

Alfalfa meal 2.5 0.3 2 Medium

Soybean meal 7 1.5 2 Medium

Poultry manure 3 3 Rapid (depends on bedding)

Poultry litter pellets 4 2 2 Rapid

Poultry manure compost 5 3 2 Rapid

Cow manure 1 1 Rapid (if fresh)

Table B.1. (continued)
84 

Appendix C. Salt index of fertilizer materials.

Table C.1. Salt index of water-soluble fertilizers.

Salt index

Material and analysis Per equal weights of materials

(Basis: sodium nitrate = 100)

Per unit (20 lbs) of 

plant nutrients

Nitrogen

Ammonium nitrate, 34% N 104.0 3.059

Ammonium sulfate, 21% N, 24% S 68.3 3.252

Calcium nitrate,15.5% N 65.0 4.194

Sodium nitrate, 16% N 100.0 6.060

Urea 74.4 1.618

Urea-ammonium nitrate solution, 28% N 63.0 2.250

Urea-ammonium nitrate solution, 32% N 71.1 2.221

Phosphate

Diammonium phosphate, 18% N, 46% P2

O5 29.2 0.456

Superphosphate, ordinary, 20% P2

O5 7.8 0.390

Superphosphate, concentrated, 45% P2

O5 10.1 0.224

Ammonium polyphosphate, 10% N, 34% P2

O5 20.0 0.455

Potassium

Potassium chloride, 60% K2

O 116.2 1.936

Potassium hydroxide, 83% K2

O --- 1.015

Potassium nitrate, 13% N, 44% K2

O 69.5 1.219

Potassium sulfate, 50% K2

O, 18% S 42.6 0.852

Monopotassium phosphate, 52% P2

O5, 34% K2O 8.4 0.097

Potassium-magnesium sulfate, 22% K2

O, 11% Mg, 22% S 43.4 1.971

Table C.2. Example salt index calculation for a dry fertilizer.

15-4-15 dry granular Nutrient units Salt index

Material Nutrient 

conc.

lbs/ton N P2

O5 K2O per unit (20 lbs) 

(from table C.1) in the formula

Ammonium nitrate 34% N 882 15 3.059 45.9

Conc. superphosphate 45% P2

O5 178 4 0.224 0.9

Potassium chloride 60% K2

O 500 15 1.936 29.0

Filler 440

Total 2000 15 4 15 75.8

Table C.3. Example salt index calculation for a solution fertilizer.

8-0-8 solution Nutrient units Salt index

Material % Nutrient lbs/ton N P2

O5 K2O per unit (20 lbs) 

(from table C.1) in the formula

Ammonium nitrate 34% N 329 5.6 3.059 17.1

Potassium nitrate 13% N

44% K2

O 364 2.4 8 1.219 9.8

Water 1307

Total 2000 8 0 8 26.9
85

Appendix D. Solubility of fertilizer sources and common fertilizer solutions.

Table D.1. Water solubility of fertilizer sources.

Material Amount (lbs) that will dissolve in 1 gallon of water

Cold water (32° F)1 Hot water (212° F)1

Ammonium nitrate, NH4NO3 9.8 72.7

Ammonium sulfate, (NH4

)

2

SO4 5.9 8.7

Borax, Na2

B4O7•10H2O 0.17 14.2

Calcium carbonate (limestone), CaCO3 0.00013 (77° F) 0.00016 (167° F)

Calcium nitrate, Ca(NO3

)

2

•4H2O 10.1 (64° F) 31.4

Calcium sulfate (gypsum), CaSO4

•2H2O 0.020 0.019

Copper sulfate, CuSO4

•5H2O 2.6 17.0

Diammonium phosphate, (NH4

)

2

HPO4 4.8 (50° F) 8.8 (158° F)

Ferric sulfate, Fe2

(SO4)

3

•9H2O 36.7 Decomposes

Ferrous sulfate, FeSO4

•7H2O 1.3 4.1 (122° F)

Magnesium sulfate, MgSO4

•7H2O 5.9 (68° F) 7.6 (104° F)

Manganese sulfate, MnSO4

•4H2O 8.8 9.3 (129° F)

Monocalcium phosphate, Ca(H2

PO4)

2

•2H2O 0.15 (86° F) Decomposes

Potassium chloride, KCl 2.9 (68° F) 4.7

Potassium nitrate, KNO3 1.1 20.6

Potassium sulfate, K2

SO4 1.0 (77° F) 2.0

Sodium molybdate, Na2

MoO4 3.7 7.0

Sodium nitrate, NaNO3 7.7 (77° F) 15.0

Urea, CO(NH2

)

2 6.5 (41° F) 62.8

Zinc sulfate, ZnSO4

•7H2O 8.1 (68° F) 55.4

1

Temperatures of cold and hot water are 32° F and 212° F, respectively, unless otherwise noted.
86 

Table D.2. Common fertilizer solutions.

Analysis Density Materials and formulation Additional 

nutrients N P K

% lbs/gallon

Component solutions

21 0 0 10.8 Ammonium nitrate solution

9 0 0 10.4 Ammonium sulfate solution 10 S

10 34 0 11.8 Ammonium polyphosphate

12 0 0 11.0 Ammonium thiosulfate 26 S

17 0 0 12.6 Calcium ammonium nitrate solution 9 Ca

9 0 0 11.3 Calcium nitrate solution 11 Ca

32 0 0 11.1 Urea ammonium nitrate solution

0 54 0 14.5 Phosphoric acid, merchant grade

3 0 11 9.7 Potassium nitrate solution

0 0 62 16.5 Potassium chloride solution

7 0 0 11.3 Magnesium nitrate 6 Mg

7 0 0 13.4 Manganese nitrate 15 Mn

7 0 0 13.3 Zinc nitrate 17 Zn

Fertilizer solutions

5 0 10 10.0 Ammonium nitrate, potassium chloride

5 0 10 10.5 Ammonium nitrate, potassium chloride, magnesium nitrate Micronutrients

8 0 8 9.8 Ammonium nitrate, potassium nitrate

8 0 8 9.7 Ammonium nitrate, potassium chloride

8 0 8 11.6 Calcium nitrate, potassium nitrate

8 0 8 12.0 Calcium nitrate, potassium chloride

8 0 8 10.2 Ammonium nitrate, potassium chloride, magnesium nitrate 1 Mg

8 2 8 10.3 Ammonium nitrate, potassium nitrate, phosphoric acid

8 2 8 10.0 Ammonium nitrate, potassium chloride, phosphoric acid

8 4 8 10.3 Ammonium nitrate, potassium nitrate, phosphoric acid

8 4 8 10.0 Ammonium nitrate, potassium chloride, phosphoric acid

9 0 9 10.2 Ammonium nitrate, potassium nitrate

9 0 9 10.2 Ammonium nitrate, potassium chloride

9 2 9 10.7 Ammonium nitrate, potassium nitrate, phosphoric acid

9 2 9 10.6 Ammonium nitrate, potassium chloride, phosphoric acid

9 4 9 10.7 Ammonium nitrate, potassium nitrate, phosphoric acid

9 4 9 10.6 Ammonium nitrate, potassium chloride, phosphoric acid

10 0 10 10.4 Ammonium nitrate, potassium nitrate

10 0 10 10.3 Ammonium nitrate, potassium chloride
87

The following table summarizes the above calculations.

Table E.1. Components of a fertilizer blend with 16-5-16-3.6 Mg nutrient ratio.

Material

Total material 

weight N P2

O5 K2O Mg

lbs per ton

Diammonium phosphate (18-46-0) 174 31 80

Potassium-magnesium sulfate (0-0-22-11Mg) 655 144 72

Ammonium nitrate (34-0-0) 850 289

Potassium chloride (0-0-60) 293 176

Filler 28

Totals 2000 320 80 320 72

1. Assume that the annual fertilizer recommendation for 

a citrus grove is 160 lbs of N, 40 lbs of P2O5, 160 lbs 

of K2O, and 36 lbs of Mg per acre, and that the yearly 

rate will be split into four applications.

2. The nutrients required per application are 40 lbs of N, 

10 lbs of P2O5, 40 lbs of K2O, and 9 lbs of Mg per acre. 

The fertilizer blend to be used contains 16% N, 4% 

P2

O5, 16% K2O, and 3.6% Mg.

3. The amount of this fertilizer needed is: (40 lbs N per 

acre) ÷ (0.16) = 250 lbs per acre. If the grove is 80 

acres, then the amount of fertilizer to order is: (250 lbs 

per acre) × (80 acres) = 20,000 lbs = 10 tons.

4. One ton of this fertilizer contains 320 lbs of N, 80 

lbs of P2

O5, 320 lbs of K2O, and 72 lbs of Mg. In this 

example, 1 ton will be blended using the following 

materials: Ammonium nitrate (34% N), diammonium 

phosphate, or DAP (18% N, 46% P2O5), potassium 

chloride (60% K2O); and potassium-magnesium sulfate, or SPM (22% K2

O, 11% Mg).

5. The P fertilizer is supplied by only one source, DAP. 

The amount needed is: (80 lbs P2

O5) ÷ (0.46) = 174 

lbs DAP.

6. The Mg fertilizer is also supplied by only one source, 

SPM. The amount needed is: (72 lbs Mg) ÷ (0.11) = 

655 lbs SPM.

7. In addition to supplying P, DAP also supplies some N: 

(174 lbs DAP) × (0.18) = 31 lbs N.

8. The balance of the N, to be obtained from ammonium 

nitrate, is: (320 lbs N) – (31 lbs N) = 289 lbs N. Thus, 

the amount needed is: (289 lbs N) ÷ (0.34) = 850 lbs 

ammonium nitrate.

9. In addition to supplying Mg, SPM also supplies some 

K2

O: (655 lbs SPM) × (0.22) = 144 lbs K2O.

10. The balance of the K2

O, to be obtained from potassium chloride, is: (320 lbs K2

O) – (144 lbs K2O) = 176 

lbs K2O. Thus, the amount needed is: (176 lbs K2O) ÷ 

(0.60) = 293 lbs potassium chloride.

Appendix E. Fertilizer mixture formulation example.

The following illustrates the procedure a bulk blending plant uses to formulate a fertilizer mixture. 

The formula is only an example and should not be regarded as a recommendation.
88 

Appendix F. Example determination of the fertilizer 

requirement for bearing citrus trees.

We have a 40 acre block of 10-year old Hamlin orange trees on Carrizo citrange rootstock with an average yield of 567 

boxes/acre during the past 3 years. How much fertilizer should the grove receive this year? Assume that we want to apply 

dry fertilizer material in three equal split applications.

1. Determine the annual N fertilizer requirement from 

the recommendations in Chapter 8, which for this 

example is 180 lbs/acre. The rate selection assumes 

healthy trees and optimum leaf N as indicated by leaf 

analysis.

2. Determine the annual K2

O requirement. The rate 

recommendation for K2

O in most cases will be the 

same as for N. Assuming trees in good condition and 

optimum leaf analysis for K, we will use 180 lbs K2O/

acre/year.

3. Determine the need (if any) for P2

O5 from Table 

8.3. We will assume that our grove tests very low in 

Mehlich 1 P and a leaf test shows P in the low range. 

Therefore, we will supply P2O5 at 72 lbs P2O5/acre/

year based on the previous year’s yield.

4. Determine if Mg is needed by inspecting leaves for 

deficiency symptoms, analyzing leaf tissue, and/or 

testing the soil. If tests reveal a deficiency, it could be 

corrected by including Mg in the fertilizer. If soil pH is 

below 5.3, dolomite should be applied to raise the pH 

to 6.0 – 6.5. We will assume that no Mg is needed.

5. Determine other nutritional deficiencies or excesses 

by inspecting leaves and confirming with leaf analysis. 

In this grove, corrections (if any) will be made by applying a foliar nutritional spray that is not part of the 

routine soil-applied fertilizer program.

6. Establish the ratio of the fertilizer mix to be used, 

assigning a value of 1 to the N rate. In this example 

the relative values are N = 1, P2

O5 = 0.4, and K2O = 1, 

so the ratio is 1-0.4-1.

7. Choose a fertilizer analysis that will provide the 

desired ratio. Examples of analyses that will provide a 

1-0.4-1 ratio would be 10-4-10, 15-6-15, or 20-8-20. 

In this example, we will use a 10-4-10 fertilizer.

8. Determine the application frequency and distribution 

of the fertilizer in each application. This publication 

recommends applying at least one-half of the annual fertilizer rate between January and June. In this 

example we will fertilize three times during the year, 

applying one-third in February, one-third in May, and 

one-third in October. This schedule supplies twothirds of the fertilizer during the January-June period.

9. Determine how many lbs/acre of 10-4-10 fertilizer 

are needed for each application to deliver the required 

amounts of N, P2

O5, and K2O. Only the N value needs 

to be determined since P2O5 and K2O will be present 

in the appropriate amounts in the 10-4-10 mixed 

fertilizer.

The annual N fertilizer rate requirement is 180 lbs/

acre, divided as follows:

February application (1/3) = 60 lbs/acre.

May application (1/3) = 60 lbs/acre.

October application (1/3) = 60 lbs/acre.

10. Determine how much 10-4-10 fertilizer to apply to 

each acre to achieve the required N, P2

O5, and K2O 

rates.

Divide 60 lbs by 10%: (60 ÷ 0.10) = 600. Thus, we 

will apply 600 lbs of 10-4-10 to each acre in February, 

May, and October.

11. Multiply the lbs/acre of fertilizer needed by the 

number of acres in the grove to get amount needed per 

application.

600 lbs/acre of 10-4-10 × 40 acres = 24,000 lbs 

 = 12 tons/application

Total amount of fertilizer needed for the year 

 = 12 tons/application × 3 applications = 36 tons.
89

A1 Symptoms originate only on new growth, but often 

persist in mature growth.

B1 Leaves uniform in color; growth reduced; internodes shortened, giving a bushy appearance.

 C1 Leaves usually large and dark green. Shoots 

long and willowy in early stages, may have 

short and bushy secondary growth following dieback of long shoots; gum blisters may 

form along vigorous shoots at base of each 

petiole; multiple buds or sprouts may form 

at the nodes; fruit may show gum in tips of 

locules and brownish eruption on peel surface 

(exanthema)......COPPER

 C2 New leaves pale green, turning yellowgreen as they enlarge; growth is sparse......

NITROGEN

 C3 New growth is drab green, lusterless, sparse, 

with some misshapen leaves; fruit has gum 

deposits in the albedo peel layer......BORON

B2 Leaves with chlorosis patterns

 C1 Leaves reduced in size, pointed, narrow, with 

sharply contrasting bright yellow mottling on 

a green background......ZINC

 C2 Leaves approximately normal in size and 

shape.

 D1 Pale green mottle over entire leaf; or, 

mottle may be a marbled pattern with 

dark green color following a crooked 

network of veins with light green color 

in between......MANGANESE

 D2 Feather-like straight green veins on a 

light green or yellow background; in 

severe cases, leaves may be totally yellow, 

reduced in size, and twigs may die on the 

outer end of branches......IRON

A2 Symptoms originating on mature leaves, with young 

leaves appearing normal or nearly so.

B1 Pattern formed by fading of chlorophyll in localized areas, with gradual enlargement with time.

 C1 Fading of chlorophyll starts in basal part of 

leaf between midrib and lateral leaf margin; 

spread is usually outward, leaving a green 

“wedge” pattern at the base of the leaf; 

however, it may be inward, causing a yellow wedge; entire leaf may fade to a golden 

bronze color......MAGNESIUM

 C2 Fading of chlorophyll starts along lateral leaf 

margins and moves inward about halfway 

to midrib with an irregular front margin......

CALCIUM

 C3 Fading of chlorophyll starts as blotches in distal half of leaf; blotches are pale yellow at first, 

but deepen to bronze as they spread and coalesce; foliage is drab, fruit is greatly reduced 

in size but of good quality......POTASSIUM

 C4 Chlorophyll fading in spots randomly distributed over the leaf blade; spots develop 

brown centers with a yellow or orange halo; 

spots range from one-quarter to one-half inch 

in diameter and appear only in the fall......

MOLYBDENUM

B2 Fading of chlorophyll not localized.

 C1 Fading of leaf to dull green and eventually to 

orange-yellow; in extreme cases, burned tips 

or spots may develop; fruit is coarse, spongy, 

and hollow-centered with thickened peel and 

above-normal acid......PHOSPHORUS

 C2 General pale green to yellow foliage color 

with whitish veins; fruit is sparse and palecolored both externally and internally; 

quality is good, but juice content is low......

NITROGEN

Appendix G. Key to citrus nutrient deficiency symptoms. 

(Excerpted with minor modification from the book Nutrition of Fruit Crops. 1966. Horticultural publications, New Brunswick, 

NJ. Permission granted by the author of the article “Citrus Nutrition,” P. F. Smith, and the editor of the book, N. F. Childers.)

Many citrus nutrient deficiency symptoms are distinctive and can be diagnosed by skillful observation without the benefit 

of leaf analysis. Initial symptoms of nutrient deficiencies can be transient. As they become more severe, symptoms begin to 

intensify and will become permanent until corrected. Severe deficiencies in most cases are not easily corrected on the current 

crop of leaves and fruit, and correction may not be possible until new growth starts. To the untrained eye, injury from residual 

herbicides or other soil-applied chemicals may be confused with nutritional disorders. Multiple deficiencies may be encountered where a distinctive pattern is not readily recognized. In such cases leaf analysis is essential in interpreting the condition.
90 

Mongi Zekri

Mongi Zekri

Fig. H1. Nitrogen deficiency. Top: Moderate leaf N 

deficiency. Center: Severe leaf N-deficiency. Bottom: 

Severely N-deficient leaf at left compared with increasing 

N status to the right.

P deficient

Normal

IPNI

Fig. H2. Phosphorus deficiency. Note the thicker peel 

and hollow core of P-deficient fruit compared with normal 

fruit.

Appendix H. Color plates.
91

K deficient Normal

Mongi Zekri

Thomas Obreza

Thomas Obreza

Fig. H3. Potassium deficiency. Top: K deficiency 

produces smaller fruit with smoother peel and higher 

color compared with normal fruit. Center: Leaf chlorosis 

caused by K deficiency. Bottom: Severe K deficiency can 

cause leaf and twig death.

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Fig. H4. Sulfur deficiency. General leaf chlorosis that 

looks similar to N deficiency.

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Mongi Zekri

Fig. H5. Magnesium deficiency. Chlorosis begins at the 

leaf margins and moves inward as the severity increases, 

producing a “Christmas tree” effect.
92 

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Mongi Zekri

Thomas Obreza

Fig. H6. Iron deficiency. Mild to severe from top to 

bottom. The major symptom is inter-veinal chlorosis. In 

severe cases, leaves are small and almost white, with twig 

die-back.

Mongi Zekri

Mongi Zekri

Fig. H7. Zinc deficiency. The major symptoms are interveinal chlorosis and smaller than normal leaves.
93

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Mongi Zekri

Fig. H8. Manganese deficiency. The main symptom is 

inter-veinal chlorosis of normal-size leaves.

Mongi Zekri

Fig. H9. Copper deficiency. Top: “Ammoniation” of fruit. 

Center: Gum pockets and twig dieback. Bottom: Vigorous, 

drooping branches and unusually dark green leaves with a 

“bowing up” of the midrib characterize Cu-N imbalance.
94 

Fig. H10. Boron deficiency. Top: External symptoms 

on grapefruit. Center: Internal symptoms on grapefruit 

(thick peel, gum pockets in peel, gumming around core). 

Bottom: Close-up of gum pockets in peel.

Mongi Zekri

Fig. H11. Molybdenum deficiency. The main symptom is 

large yellow spots.
95

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Fig. H12. Salt damage. Top: Burn on leaf edges compared 

with normal leaf at right. Center: Salinity-induced 

chlorosis. Bottom: Severe leaf burn in the field.

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Mongi Zekri

Mongi Zekri

Fig. H13. Biuret toxicity (top and center). Urea spray 

burn (bottom).
96

Mongi Zekri

Michael Rogers

J.M. Bove and M. Garnier

Mongi Zekri

Fig. H14. Do not confuse the blotchy mottle (light and 

dark green patches) pattern of citrus greening disease 

with nutrient deficiency symptoms. The pattern is not 

symmetrical on opposite sides of the mid-vein. (Contrast 

with nutritional deficiencies that usually exhibit a 

symmetrical pattern in relation to the midrib.)

COOPERATIVE EXTENSION SERVICE, UNIVERSITY OF FLORIDA, INSTITUTE OF FOOD AND AGRICULTURAL SCIENCES, 

Larry R. Arrington, Director, in cooperation with the United States Department of Agriculture, publishes this information to further the purpose 

of the May 8 and June 30, 1914 Acts of Congress; and is authorized to provide research, educational information, and other services only to 

individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, 

marital status, national origin, political opinions, or affiliations. This information was published January 2008 as SL 253, Florida Cooperative 

Extension Service.

SL 253

Nutrition of Florida Citrus Trees