Agricultural Phosphorus

and Eutrophication United States

Department of

Agriculture

Agricultural

Research

Service

ARS–149

July 1999
A.N. Sharpley, T. Daniel, T. Sims, J. Lemunyon, R. Stevens, and R. Parry

United States

Department of

Agriculture

Agricultural

Research

Service

ARS–149

July 1999

Agricultural Phosphorus

and Eutrophication

Sharpley is a soil scientist with the USDA–ARS, Pasture Systems and Watershed

Management Research Laboratory, University Park, PA; Daniel is a professor with the

Department of Agronomy, University of Arkansas, Fayetteville, AR; Sims is a professor with the Department of Plant Science, University of Delaware, Newark, DE;

Lemunyon is an agronomist with the USDA–NRCS, Grazing Lands Technology

Institute, South National Technical Center, Fort Worth, TX; Stevens is an extension

soil scientist with Research and Extension, Washington State University, Prosser,WA;

and Parry is a national program manager with the U.S. Environmental Protection

Agency, Washington, DC.
Abstract

Sharpley, A.N., T. Daniel, T. Sims,

J. Lemunyon, R. Stevens, and R.

Parry. 1999. Agricultural Phosphorus and Eutrophication. U.S. Department of Agriculture, Agricultural

Research Service, ARS–149, 42 pp.

Inputs of phosphorus (P) are essential for profitable crop and animal

agriculture. However, P export in

watershed runoff can accelerate the

eutrophication of receiving fresh

waters. The rapid growth and

intensification of crop and animal

farming in many areas has created

regional imbalances in P inputs in

feed and fertilizer and P output in

farm produce. In many of these

areas, soil P has built up to levels in

excess of crop needs and now has

the potential to enrich surface runoff

with P.

The overall goal of efforts to reduce

P losses from agriculture to water

should be to increase P use efficiency, balance P inputs in feed and

fertilizer into a watershed with P

output in crop and animal produce,

and manage the level of P in the

soil. Reducing P loss in agricultural

runoff may be brought about by

source and transport control

strategies. This includes refining

feed rations, using feed additives to

increase P absorption by animals,

moving manure from surplus to

deficit areas, finding alternative uses

for manure, and targeting conservation practices, such as reduced

tillage, buffer strips, and cover

crops, to critical areas of P export

from a watershed. In these critical

areas high P soils coincide with

parts of the landscape where surface

runoff and erosion potential are

high.

ii
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Keywords: eutrophication, fertilizer,

phosphorus, P input, P output,

runoff

While supplies last, copies of this

publication may be obtained at no

cost from USDA–ARS, Pasture

Systems & Watershed Management

Research Laboratory, Curtin Road,

University Park, PA 16802–3702.

Copies of this publication may be

purchased from the National Technical Information Service, 5285 Port

Royal Road, Springfield, VA 22161;

telephone (703) 605–6000.

iii

Issued July 1999
Introduction ......................................................................................... 1

 Eutrophication .................................................................................. 1

 Agricultural Production .................................................................... 2

Soil Phosphorus ................................................................................... 5

The Loss of Phosphorus in Agricultural Runoff ............................... 10

 Forms and Processes....................................................................... 10

 The Dependence of Agricultural Runoff P on Soil P ..................... 12

Remediation ....................................................................................... 14

 Source Management ....................................................................... 15

 Transport Management ................................................................... 22

 Targeting Remediation ................................................................... 23

Summary............................................................................................ 29

References ......................................................................................... 34

v

Contents
1

Agricultural

Phosphorus and

Eutrophication

Introduction

Eutrophication

Phosphorus (P) is an essential

element for plant and animal growth

and its input has long been recognized as necessary to maintain

profitable crop and animal

production. Phosphorus inputs can

also increase the biological

productivity of surface waters by

accelerating eutrophication.

Eutrophication is the natural aging

of lakes or streams brought on by

nutrient enrichment. This process

can be greatly accelerated by human

activities that increase nutrient

loading rates to water.

Eutrophication has been identified

as the main cause of impaired

surface water quality (U.S. Environmental Protection Agency 1996).

Eutrophication restricts water use

for fisheries, recreation, industry,

and drinking due to the increased

growth of undesirable algae and

aquatic weeds and to oxygen

shortages caused by their death and

decomposition. Associated periodic

surface blooms of cyanobacteria

(blue-green algae) occur in drinking

water supplies and may pose a

serious health hazard to animals and

humans. Recent outbreaks of the

dinoflagellate Pfiesteria piscicida in

the eastern United States, and

Chesapeake Bay tributaries in

particular, have been linked to

excess nutrients in affected waters.

Neurological damage in people

exposed to the highly toxic, volatile

chemical produced by these algae

has dramatically increased public

awareness of eutrophication and the

need for solutions (Burkholder et al.

1992).
2

ances in P inputs and outputs. On

average, only 30 percent of the

fertilizer and feed P input to farming

systems is output in crop and animal

produce. Therefore, when averaged

over the total utilizable agricultural

land area in the United States, an

annual P surplus of 30 lb/acre exists

(National Research Council 1993).

Before World War II, farming

communities tended to be selfsufficient in that enough feed was

produced locally and recycled to

meet animal requirements. After

World War II, increased fertilizer

use in crop production fragmented

farming systems, creating specialized crop and animal operations that

efficiently coexist in different

regions within and among countries.

Since farmers did not need to rely

on manures as nutrient sources (the

primary source until fertilizer

production and distribution became

Eutrophication of most fresh water

around the world is accelerated by P

inputs (Schindler 1977, Sharpley et

al. 1994). Although nitrogen (N)

and carbon (C) are also essential to

the growth of aquatic biota, most

attention has focused on P inputs

because of the difficulty in controlling the exchange of N and C

between the atmosphere and water

and the fixation of atmospheric N

by some blue-green algae. Therefore, P is often the limiting element,

and its control is of prime importance in reducing the accelerated

eutrophication of fresh waters.

When salinity increases, as in

estuaries, N generally becomes the

element controlling aquatic

productivity. However, in

Delaware’s inland bays (coastal

estuaries), nitrate-N leaching has

elevated N concentrations to the

point where P is now the limiting

factor in eutrophication.

Lake water concentrations of P

above 0.02 ppm generally accelerate

eutrophication. These values are an

order of magnitude lower than P

concentrations in soil solution

critical for plant growth (0.2 to 0.3

ppm), emphasizing the disparity

between critical lake and soil P

concentrations and the importance

of controlling P losses to limit

eutrophication.

Agricultural Production

Confined animal operations are now

a major source of agricultural

income in several states. Animal

manure can be a valuable resource

for improving soil structure and

increasing vegetative cover, thereby

reducing surface runoff and erosion

potential. However, the rapid growth

and intensification of crop and

animal farming in many areas has

created regional and local imbal-
3

less expensive), they could spatially

separate grain and animal production. By 1995, the major animalproducing states imported over 80

percent of their grain for feed

(Lanyon and Thompson 1996). In

fact, less than a third of the grain

produced on farms today is fed on

the farm where it is grown (U.S.

Department of Agriculture 1989).

The potential for P surplus at the

farm scale can increase when

farming systems change from

cropping to intensive animal

production, since P inputs become

dominated by feed rather than

fertilizer. With a greater reliance on

imported feeds, only 27 percent of

the P in purchased feed for a

74,000-layer operation on a 30-acre

farm in Pennsylvania could be

accounted for in farm outputs (table

1). This nutrient budget clearly

shows that the largest input of

nutrients to a poultry farm and,

therefore the primary source of any

on-farm nutrient excess, is in animal

feed. Annual P surpluses of 80 to

110 lb/acre/yr were estimated by

Sims (1997) for a typical poultry

Table 1. Farming system and P balance

Farming system

 P Crop* Dairy † Poultry ‡ Hogs§

 Input - - - - - - - - - - - - - - - lb P/acre/yr - - - - - - - - - - - - - -

Fertilizer 20 10 — —

Feed — 20 1,375 95

 Output –18 –13 – 365 -60

 Balance +2 +17 +1,010 +35

 SOURCE: Lanyon and Thompson (1996) and Bacon et al. (1990).

* 75-acre cash crop farm growing corn and alfalfa.

† 100-acre dairy farm with 65 dairy holsteins averaging 14,500 lb

milk/cow/yr, 5 dry cows, and 35 heifers. Crops were corn for silage

and grain, alfalfa, and rye for forage.

‡ 30-acre poultry farm with 74,000 layers; output includes 335 lb

P/acre/yr in eggs, 20 lb P/acre/yr sold in crops (corn and alfalfa),

and 10 lb P/acre/yr manure exported from the farm.

§ 75-acre farm with 1,280 hogs, output includes 40 lb P/acre/yr

manure exported from the farm.
4

impacts, which in some cases occur

many miles from P sources. By the

time these water-quality impacts are

manifest, remedial strategies are

difficult and expensive to

implement; they cross political and

regional boundaries; and because of

P loading, improvement in water

quality will take a long time.

Nitrogen-based management has

been practiced and advocated by

farm advisers for many years.

Farmers are only now becoming

aware of P issues. Many are confused and feel that science has

misled them or let them down by

not emphasizing the P management

issues. A better understanding of

where P is coming from is needed,

how much P in soil and water is too

much, and how and where these

inputs and losses can be reduced in

order to develop agricultural resource management systems that

sustain production, environmental

quality, and farming communities.

In this publication, P is in its

elemental form, rather than as P2

O5,

which is commonly used in fertilizer

analysis. The conversion factor from

P to P2

O5 is 2.29. When discussing

plant available forms of soil P, as

determined by soil testing laboratories, we will refer to them as “soil

test P” (ppm or mg/kg) and identify

in each case the specific method of

analysis used. Based on a 6-inch soil

depth containing 2 million pounds

of soil, the conversion factor for

ppm to lbs P/acre is 2.0. For more

detailed information on the methods

used for soil P testing, how they

were developed, and why they vary

among regions, see Fixen and Grove

(1990), Sharpley et al. (1994, 1996),

and Sims (1998).

grain farm in Delaware. This

scenario is consistent with other

concentrated animal production

industries, including dairy and

hogs.

Phosphorus accumulation on farms

has built up soil P to levels that

often exceed crop needs. Today,

there are serious concerns that

agricultural runoff (surface and

subsurface) and erosion from high

P soils may be major contributing

factors to surface water eutrophication. Agricultural runoff is all water

draining from an area (field or

watershed) including surface

runoff, subsurface flow, leaching,

and tile drainage processes. Phosphorus loss in agricultural runoff is

not of economic importance to

farmers because it amounts to only

1 or 2 percent of the P applied.

However, P loss can lead to

significant off-site economic
5

Soil Phosphorus

Soil P exists in organic and inorganic forms, but these are not

discrete entities with integrades

among forms occurring (fig. 1).

Organic P consists of undecomposed residues, microbes, and

organic matter in the soil. Inorganic

P is usually associated with Al, Fe,

and Ca (aluminum, iron, and

calcium, respectively) compounds

of varying solubility and availability

to plants. Phosphorus has to be

added to most soils so that there are

adequate levels for optimum crop

growth and yield. However, P can

be rapidly fixed in forms and

therefore unavailable to plants,

depending on soil pH and type (Al,

Fe, and Ca content). Converting

stable forms of soil P to labile or

available forms usually occurs too

slowly to meet crop P requirements

(fig. 1). As a result, soil P tests were

Crop harvest

Manure P Fertilizer P

Labile Labile and fixed

Solution P

Stable

Soil test P

Stable

Organic P Inorganic P

Figure 1.The phosphorus cycle in soil
6

• No manure

• 40 lb P/acre/yr

• 90 lb P/acre/yr

• 110 lb P/acre/yr

Soil test P (ppm)

0

0

50

Soil depth (inches)

100 150 200

10

20

30

40

developed to determine the amount

of plant available P in soil and from

this how much P as fertilizer or

manure should be added to meet

desired crop yield goals.

In most soils, the P content of

surface horizons is greater than that

of the subsoil due to sorption of

added P, greater biological activity,

cycling of P from roots to

aboveground plant biomass, and

more organic material in surface

layers (fig. 2). In reduced tillage

systems, fertilizers and manures are

not incorporated or they are incorporated only to shallow depths,

thereby exacerbating P buildup in

the top 2 to 5 inches of soil. In some

situations, P can easily move

through the soil, as we will discuss

later.

Figure 2. Soil test P (as Mehlich–3 P) accumulates

at the surface with repeated application of P for 10

years. Note that typical fertilizer P applications for a

corn crop in Oklahoma with a medium soil test P

(20 to 40 ppm Mehlich–3 P) is about 20 lb P/acre.

(Adapted from Sharpley et al. 1984.)
7

Continual long-term application of

fertilizer or manure at levels exceeding crop needs, will increase

soil P levels (fig. 3). In many areas

of intensive, confined animal

production, manures are normally

applied at rates designed to meet

Figure 3. Increase in soil test P from applying more P

than a crop needs each year (as Bray–I P). (Adapted

from a 25-year study by Barber 1979.)

Figure 4. Applying manure to meet crop N

needs (about 200 lb available N/acre) adds

much more P than corn needs.

crop N requirements but to avoid

ground water quality problems

created by leaching of excess N.

This often results in a buildup of

soil test P above amounts sufficient

for optimal crop yields. As illustrated in figure 4, the amount of P

added in average applications of

dairy manure (8 to 10 tons/acre and

0.5 percent P) and poultry litter (4

tons/acre and 1.5 percent P) are

considerably greater than what is

removed in harvested crops; the

result is an accumulation of soil P.

Soil test P (ppm)

Annual P surplus (lb/acre/yr)

Initial soil

test P is 18 ppm

80

60

40

20

0

-20 0 20 40

P removed (lb/acre)

P added

(lb/acre) Dairy manure

Poultry litter

P added in manure or removed by crop

0 50 100 150

Corn
8

In 1997, several state soil test

laboratories reported that the

majority of agricultural soils analyzed had soil test P levels in the

high or above categories, which

require little or no P fertilization. It

is clear from figure 5 that high soil

P levels are a regional problem,

because the majority of agricultural

soils in several states still test

medium or low. For example, most

Great Plains soils still require P for

optimum crop yields. Unfortunately,

problems associated with high soil P

are aggravated by the fact that many

of these agricultural soils are located

in states with sensitive water bodies,

such as the Great Lakes, Lake

Champlain, the Chesapeake and

Delaware Bays, Lake Okeechobee,

the Everglades, and other fresh

water bodies (fig. 5).

Figure 5. A survey of agricultural soils analyzed by state soil

test laboratories in 1997, shows a regional buildup of soil

test P near P-sensitive waters (Fixen 1998).

Percent of samples 

testing high or above

<25%

25 - 50%

>50%

79

70

70

56

65

66

52

77

64

57

65

74

63 51

58

57

58

74

56

53

53

53 70

71

67

74

23

42

43

41

27

29

39

44

50

43

49

48 40 25 40

46
9

Percent in each soil test P category

New Castle Co. DE

Low livestock density

Sussex Co. DE

High livestock

density

Soil test P (ppm)

Low and

 medium: <25

Delaware

75

50

25

0

75

50

25

0

Optimum: 

 25-50

High: >50

Distinct areas of general P deficit

and surplus exist within states and

regions. For example, soil test

summaries for Delaware reveal the

magnitude and localization of high

soil test P levels that can occur in

areas dominated by intensive animal

production (fig. 6). From 1992 to

1996 in Sussex County, Delaware,

with its high concentration of

poultry operations, 87 percent of

fields tested had optimum (25 to 50

ppm) or excessive soil test P (>50

ppm), as determined by Mehlich–1;

whereas, in New Castle County,

with only limited animal production,

72 percent of fields tested were

rated as low (<13 ppm) or medium

(13 to 25 ppm).

Though rapidly built up by applications of P, available soil P decreases

slowly once further applications are

stopped. Therefore, the determination of how long soil test P will

Figure 6. Elevated soil test P levels (as Mehlich–1 P) are

usually localized in areas of confined animal operations.
10

P includes P associated with soil

particles and organic material

eroded during flow events and

constitutes 60 to 90 percent of P

transported in surface runoff from

most cultivated land (Sharpley et al.

1992). Surface runoff from grass,

forest, or noncultivated soils carries

little sediment and is, therefore,

generally dominated by dissolved P.

This dissolved form comes from

the release of P from soil and plant

material (fig. 7). This release occurs

when rainfall or irrigation water

interacts with a thin layer of surface

soil (1 to 2 inches) and plant material before leaving the field as

surface runoff (Sharpley 1985).

Most dissolved P is immediately

available for biological uptake.

Sediment P is not readily available,

but it can be a long-term source of P

for aquatic biota (Sharpley 1993,

Ekholm 1994).

remain above crop sufficiency levels

is of economic and environmental

importance to farmers who must

integrate manure P into sustainable

nutrient management systems. For

example, if a field has a high

potential to enrich agricultural

runoff with P due to excessive soil

P, how long will it be before crop

uptake will lower soil P levels so

that manure can be applied again

without increasing the potential for

P loss? McCollum (1991) estimated

that without further P additions, 16

to 18 years of corn (Zea mays L.)

and soybean [Glycine max (L.)

Merr.] production would be needed

to deplete soil test P (Mehlich–3 P)

(Mehlich 1984) in a Portsmouth fine

sandy loam from 100 ppm to the

agronomic threshold level of 20

ppm.

The Loss of Phosphorus in

Agricultural Runoff

The term “agricultural runoff”

encompasses two processes that

occur in the field—surface runoff

and subsurface flow. In reality these

can be vague terms for describing

very dynamic processes. For

example, surface or overland flow

can infiltrate into a soil during

movement down a slope, move

laterally as interflow, and reappear

as surface flow. In this publication,

agricultural runoff refers to the total

loss of water from a watershed by

all surface and subsurface pathways.

Forms and Processes

The loss of P in agricultural runoff

occurs in sediment-bound and

dissolved forms (fig. 7). Sediment
11

In most watersheds, P export occurs

mainly in surface runoff, rather than

subsurface flow. However, in some

regions, notably the Coastal Plains

and Florida, as well as fields with

subsurface drains, P can be transported in drainage waters. Generally, the concentration of P in water

percolating through the soil profile

is low due to P fixation by Pdeficient subsoils. Exceptions occur

in sandy, acid organic, or peaty soils

with low P fixation or holding

capacities and in soils where the

preferential flow of water can occur

rapidly through macropores and

earthworm holes (Sharpley and

Syers 1979, Bengston et al. 1992).

Irrigation, especially furrow irrigation, can significantly increase the

potential for soil and water contact

and therefore, can increase P loss by

both surface runoff and erosion in

return flows. Furrow irrigation

exposes unprotected surface soil to

the erosive effect of water movement. The process of irrigation also

has the potential to greatly increase

the land area that can serve as a

potential source for P movement, a

fact that is especially important in

the western United States.

Figure 7. Phosphorus can be released from soil and plant material to

surface and subsurface runoff water or lost by erosion.

Subsurface

flow

Tile flow

P leaching

is small

Erosion of

particulate P

Release of soil and plant

P to surface runoff

Total runoff P
12

Surface runoff Subsurface tile drainage

Arkansas

y = 0.32 + 0.0031x

r

2

 = 0.76

New Zealand

y = 0.02 + 0.0033x

r

2

 = 0.85

Oklahoma

y = 0.01 + 0.0031x

r

2

Runoff-dissolved P (mg/L)

 = 0.88

Mehlich-3 soil P, ppm Olsen soil P, ppm

Change point

60 ppm

2

1.5

1

0.5

0

0 100 200 300 400 500

3

2

1

0

0 30 60 90 120

The Dependence of

Agricultural Runoff P

on Soil P

Many studies report that the loss of

dissolved P in surface runoff is

dependent on the P content of

surface soil (fig. 8). In a review of

several studies, Sharpley et al.

(1996) found that the relationship

between surface runoff P and soil P

varies with management. Relationship slopes were flatter for grass

(4.1 to 7.0, mean 6.0) than for

cultivated land (8.3 to 12.5, mean

10.5), but the slopes were too

variable to allow use of a single or

average relationship to recommend

P amendments based on waterquality criteria. Clearly, several soil

and land management factors

influence the relationship between

dissolved P in surface runoff and

soil P.

All in all, the loss of P in subsurface

flow, as well as surface runoff, is

linked to soil P concentration.

Heckrath et al. (1995) found that

soil test P (Olsen P) greater than 60

ppm in the plow layer of a silt loam

caused the dissolved P concentration in tile drainage water to increase dramatically (0.15 to 2.75

mg/L) (fig. 8). They postulated that

this level (60 ppm), which is well

above that needed by major crops for

optimum yield, is a critical point

above which the potential for P

movement in land drains greatly

increases (Ministry of Agriculture,

Food and Fisheries 1994). Similar

studies suggest that this change point

can vary threefold as a function of

site hydrology, relative drainage

volumes, and soil P release characteristics (Sharpley and Syers 1979).

Figure 8. Effect of soil P on the dissolved P concentration of surface runoff from

several pasture watersheds (adapted from Sharpley et al. 1996) and subsurface tile

drainage from Broadbalk fields. (Adapted from Heckrath et al. 1995.)
13

These and similar studies compared

agricultural runoff P to soil P using

traditional soil test methods that

estimate plant availability of soil P.

While these studies show promise in

describing the relationship between

the level of soil P and surface runoff

P, they are limited for several

reasons. First, soil test extraction

methods were developed to estimate

the plant availability of soil P and

may not accurately reflect soil P

release to surface or subsurface

runoff water. Second, although

dissolved P is an important waterquality parameter, it only represents

the dissolved portion of P readily

available for aquatic plant growth. It

does not reflect fixed soil P that can

become available with changing

chemistry in anaerobic conditions.

The final concern is with sampling

depth. It is generally recommended

that soil samples be collected to

plow depth, usually 6 to 8 inches for

routine evaluation of soil fertility. It

is the surface inch or two in direct

contact with runoff that are important when using soil testing to

estimate P loss. Consequently,

different sampling procedures may

be necessary when using a soil test

to estimate the potential for P loss.

To overcome these concerns,

approaches are being developed that

provide a more theoretically sound

estimate, than traditional agronomic

chemical extractants do, of the

amount of P in soil that can be

released to runoff water and the

amount of algal-available P in

runoff (Sharpley 1993).

An approach, developed in the

Netherlands by Breeuwsma and

Silva (1992) to assess P leaching

potential, is to determine soil P

saturation (percent saturation =

available P/P fixation maximum).

This approach is based on the fact

that, as P saturation or the amount

of fixed P increases, more P is

released from soil to surface runoff

or leaching water. This method is

used to limit the loss of P in surface

and ground waters. A critical P

saturation of 25 percent has been

established for Dutch soils as the

threshold value above which the

potential for P movement in surface

and ground waters becomes unacceptable.
14

High P source High transport

Critical source area

Remediation

The overall goal to reduce P losses

from agriculture to water should

increase the efficiency of P use by

balancing P inputs in feed and

fertilizer into a watershed with P

outputs in crop and animal produce

and managing the level of P in the

soil. Reducing P loss in agricultural

runoff may be brought about by

source and transport control

strategies. The transport of P from

agricultural land in surface runoff

and erosion has been reduced;

however, much less attention has

been directed toward source management.

When looking at management to

minimize the environmental impact

of P, there are several important

factors that must be considered. To

cause an environmental problem,

there must be a source of P (that is,

high soil levels, manure or fertilizer

applications, etc.) and it must be

transported to a sensitive location

(that is, for leaching, runoff, erosion, etc.). Problems occur where

these two come together. A high P

source with little opportunity for

transport may not constitute an

Figure 9. Critical source areas for P loss from a watershed occur

where areas of high soil P and transport potential coincide.

environmental threat. Likewise, a

situation where there is a high

potential for transport but no source

of P to move is also of little threat.

Management should focus on the

areas where these two conditions

intersect. These areas are called

“critical source areas” (fig. 9).
15

Source Management

Reducing off-farm inputs of P in

feed

Manipulation of dietary P intake by

animals may help balance farm P

input and output in animal operations because feed inputs are often

the major cause of P surplus (table

1). Morse et al. (1992) recorded a

17-percent reduction in P excretion

when the daily P intake of dairy

cows was reduced from 82 to 60 g/

day. In some areas of the United

States and Europe, reductions in

concentrate P are being implemented to reduce the amounts of P

excreted. Therefore, there is a clear

indication that amounts of excreted

P can be reduced by carefully

matching dietary P inputs to animal

requirements.
16

It is common to supplement poultry

and hog feed with mineral forms of

P because of the low digestibility of

phytin, the major P compound in

grain. This supplementation contributes to P enrichment of animal

manures and litters. Enzyme additives for animal feed are being

tested to increase the efficiency of P

uptake from grain during digestion.

Development of such enzymes that

would be cost-effective in terms of

animal weight gain may reduce the

P content of manure. One method is

to use phytase, an enzyme that

enhances the efficiency of P

recovery from phytin in grains fed

to poultry and hogs. Another

promising method is to develop

grain varieties that are lower in

phytin.

A third method is to increase the

quantity of P in corn that is available to animals by reducing the

amount of phytate produced by

corn. This would decrease phytateP, which contributes as much as 85

percent of P in corn grain, and

increase inorganic P concentrations

in grain. Ertl et al. (1998) manipulated the genes controlling phytate

formation in corn and showed that

phytate-P concentrations in lowphytic-acid corn grain were as much

as 51 percent less than in wild-type

grain (table 2). Subsequently, P

concentrations in the litter of poultry

fed the low-phytic-acid grain were

23 percent less than those in the

litter of poultry fed the wild-type

corn grain. Therefore, the use of

low-phytate corn in poultry feed

can increase the availability of P

and other phytate-bound minerals

and proteins.

Table 2. Total P and phytate-P concentrations in wildtype and low-phytic-acid corn and subsequent P concentrations in the litter of poultry fed both types of grain

Corn type Grain Poultry litter

Total P Phytate-P Total P

- - - - - - - - - - - - - - -lb P/ton- - - - - - - - - - - - - - -

Wild type 7.6 6.4 5.6

Low phytic acid 7.8 2.6 4.4

SOURCE: Adapted from Ertl et al. (1998).
17

Soil P management and estimating threshold levels for environmental risk assessment

The long-term use of commercial

fertilizers has increased the P status

of many agricultural soils to optimum or excessive levels. The goal

of P fertilization was to remove soil

P supply as a limitation to agricultural productivity; however, for

many years actions taken to achieve

this goal did not consider the

environmental consequences of P

loss from soil to water. The constraint on P buildup in soils from

commercial fertilizer use was

usually economic, with most

farmers recognizing that soil tests

for P accurately indicated when to

stop applying fertilizer P. Some

“insurance” fertilization has always

occurred, particularly in high-value

crops, such as vegetables, tobacco,

and sugarcane. However, the use of

commercial fertilizers alone would

not be expected to grossly overfertilize soils because farmers would

cease applying fertilizer P when it

became unprofitable. Today’s

dilemma is caused by the realization

that soils considered optimum in

soil test P (or perhaps only slightly

overfertilized) from a crop production perspective may still provide

environmentally significant quantities of soluble and sediment P in

surface runoff and erosion.

Environmental concern has forced

many states to consider developing

recommendations for P applications

and watershed management based

on the potential for P loss in agricultural runoff. A major difficulty is

the identification of a threshold soil

test P level to estimate when soil P

becomes high enough to result in

unacceptable P enrichment of

agricultural runoff. Table 3 gives

examples from several states, along

with agronomic threshold concentrations for comparison.

Environmental threshold levels

range from 2 times (Michigan) to

4 times (Texas) the agronomic

thresholds.

Soil test results for environmental

purposes must be interpreted

carefully. The comments given on

soil test reports—low, medium,

optimum, high, and so forth—were

established based on the expected

response of a crop to P. However,

one cannot assume a direct relationship between the soil test calibration

for crop response to P and runoff

enrichment potential. In other

words, one cannot accurately project

that a soil test level above an

expected crop response level

exceeds crop needs and therefore is

potentially polluting. What will be
18

Table 3. Threshold soil test P values and P management recommendations

Threshold values, ppm

Soil test P Management recommendations

State Agronomic* Environmental method to protect water quality

Arkansas 50 150 Mehlich-3 At or above 150 ppm soil P:

Apply no more P, provide buffers next to streams,

overseed pastures with legumes to aid P removal, and

provide constant soil cover to minimize erosion.

Delaware 25 50 Mehlich-3 Above 50 ppm soil P:

Apply no more P until soil P is significantly reduced.

Idaho 12 50 Olsen Sandy soils, above 50 ppm soil P:

100 Silt loam soils, above 100 ppm soil P:

Apply no more P until soil P is significantly reduced.

Ohio 40 150 Bray-1 Above 150 ppm soil P:

Reduce erosion and reduce or eliminate P additions.

Oklahoma 30 130 Mehlich-3 30 to 130 ppm soil P:

Halve P rate on >8% slopes.

130 to 200 lb/acre soil P:

Halve P rate and reduce surface runoff and erosion.

Above 200 lb/acre soil P:

Amount of P applied should not exceed that taken up

by the crop and removed as harvested produce.
19

Table 3. Threshold soil test P values and P management recommendations (continued)

Threshold values, ppm

Soil test P Management recommendations

State Agronomic* Environmental method to protect water quality

Michigan 40 75 Bray-1 Below 75 ppm soil P:

P application not to exceed crop removal.

Above 75 ppm soil P:

Apply no P from any source.

Texas 44 200 Texas Above 200 ppm soil P:

A&M Amount of P applied should not exceed that taken up by

the crop and removed as harvested produce.

Wisconsin 20 75 Bray-1 Below 75 ppm soil P:

Rotate to P-demanding crops and reduce P additions.

Above 75 ppm soil P:

Discontinue P applications.

SOURCE: Adapted from Sharpley et al. (1996).

* Agronomic threshold concentrations are average values for nonvegetable crops; actual values vary

with soil and crop type. Vegetables have higher agronomic P requirements.
20

Soil test P

Crop yield

P loss in surface runoff

Critical

value

for yield

Critical

value

for P loss

Low Optimum High

Low Medium High

crucial in terms of managing P

based in part on soil test levels will

be the interval between the critical

or threshold soil P value for crop

yield and runoff P (fig. 10).

There is reluctance on the part of

most soil testing programs to

establish upper threshold limits for

soil test P. Reasons range from the

fact that soil tests were not originally designed or calibrated for

environmental purposes to an

unjustified reliance upon soil test P

alone by environmental regulatory

agencies. Refusing to participate in

the debate on the appropriateness of

critical limits for soil test P is

extremely shortsighted and may

force others with less expertise to

set the limits that are so important to

the soil testing and agricultural

community. A foresighted stance

acknowledges that agronomically

based soil tests can play a role in

Figure 10. As soil P increases, so does crop yield and the potential for P

loss in surface runoff. The interval between the critical soil P value for

yield and runoff P will be important for P management.
21

environmental management of soil

P but are only a first step in a more

comprehensive approach. This

awareness will enhance the credibility of soil testing programs and

improve the contribution they make

to the agricultural community.

Manure management

Farm advisers and resource planners

are recommending that P content of

manure and soil be determined by

soil test laboratories before land

application of manure. This is

important because without such

determinations, farmers and their

advisers tend to underestimate the

nutritive value of manure. Soil test

results can also demonstrate the

positive and negative long-term

effects of manure use and the time

required to build up or deplete soil

nutrients. For instance, soil tests can

help a farmer identify the soils in

need of P fertilization, those where

moderate manure applications may

be made, and fields where no

manure applications need to be

made for crop yield response.

Commercially available manure

amendments, such as slaked lime or

alum, can reduce ammonia (NH3

)

volatilization, leading to improved

animal health and weight gains;

reduce the solubility of P in poultry

litter by several orders of magnitude; and decrease dissolved P,

metal, and hormone concentrations

in surface runoff (Moore and Miller

1994, Moore et al. 1995, Nichols et

al. 1997). Also, the dissolved P

concentration (11 mg/L) of surface

runoff from fescue treated with

alum-amended litter was much

lower than that from fescue treated

with unamended litter (83 mg/L)

(Shreve et al. 1995). Perhaps the

most important benefit of manure

amendments for air and water

quality would be an increase in the

N:P ratio of manure via reduced N

loss because of NH3

 volatilization.

An increased N:P ratio of manure

would more closely match crop N

and P requirements.

A mechanism should be established

to facilitate movement of manures

from surplus to deficit areas. At the

moment, manures are rarely transported more than 10 miles from

where they are produced. However,

mandatory transport of manure from

farms with surplus nutrients to

neighboring farms where nutrients

are needed faces several significant

obstacles. First, it must be shown

that manure-rich farms are unsuitable for manure application based

on soil properties, crop nutrient

requirements, hydrology, actual P

movement, and sensitive water

bodies. Then, it must be shown that
22

the recipient farms are more suitable

for manure application. The greatest

success with redistribution of

manure nutrients is likely to occur

when the general goals of nutrient

management set by a national (or

state) government are supported by

consumers, local governments, the

farm community, and the animal

industry. This may initially require

incentives to facilitate subsequent

transport of manures from one area

to another. Again, this may be a

short-term alternative if N-based

management is used to apply the

transported manures. If this happens, soil P in areas receiving

manures may become excessive in 3

to 5 years.

Innovative methods are being used

by some farmers to transport

manure. For example, grain or feed

trucks and railcars are transporting

dry manure back to the grain source

area instead of returning empty

(Collins et al. 1988). In Delaware, a

local poultry trade organization has

established a manure bank network

that puts manure-needy farmers in

contact with manure-rich poultry

growers. Even so, large-scale

transportation of manure from

producing to non-manure-producing

areas is not occurring.

Composting, another potential tool,

may also be considered a management tool to improve manure

distribution. Although it tends to

increase the P concentration of

manures, composting reduces the

volume of manures and therefore

transportation costs. Additional

markets are also available for

composted materials. As the value

of clean water and the cost of

sustainable manure management is

realized, it is expected that alternative entrepreneurial uses for manure

will be developed, become more

cost-effective, and create expanding

markets.

Transport Management

Phosphorus loss via surface runoff

and erosion may be reduced by

conservation tillage and crop

residue management, buffer strips,

riparian zones, terracing, contour

tillage, cover crops, and impoundments (settling basins). Basically,

these practices reduce the impact of

rainfall on the soil surface, reduce

runoff volume and velocity, and

increase soil resistance to erosion.

None of these measures, however,

should be relied on as the sole or

primary practice to reduce P losses

in agricultural runoff.

Most of these practices are generally

more efficient at reducing sediment

P than dissolved P. Several re-
23

searchers report little decrease in

lake productivity with reduced P

inputs following implementation of

conservation measures (Young and

DePinto 1982, Gray and Kirkland

1986). Many times the impact of

remedial measures used to help

improve poor water quality will be

slow because lake and stream

sediments can be a long-term source

of P in waters even after inputs

from agriculture are reduced.

Therefore, immediate action may be

needed to reduce future problems.

Targeting Remediation

Threshold soil P levels are being

proposed to guide P management

recommendations. In most cases,

agencies that seek these levels hope

to uniformly apply a threshold value

to areas and states under their

domain. However, it is too

simplistic to use threshold soil P

levels as the sole criterion to guide P

management and P applications. For

example, adjacent fields having

similar soil test P levels but differing susceptibilities to surface runoff

and erosion, due to contrasting

topography and management,

should not have similar P management recommendations. Also, it has

been shown that in some agricultural watersheds, 90 percent of

annual algal-available P export from

watersheds comes from only 10

percent of the land area during a few

relatively large storms (Pionke et al.

1997). For example, more than 75

percent of annual water discharge

from watersheds in Ohio (Edwards

and Owens 1991) and Oklahoma

(Smith et al. 1991) occurred during

one or two severe storms. These

events contributed over 90 percent

of annual total P export (0.2 and 5.6

lb/acre/yr in Ohio and Oklahoma,

respectively). Therefore, threshold

soil P values will have little meaning unless they are used in conjunction with an estimate of a site’s

potential for surface runoff and

erosion.

A sounder approach advocated by

researchers and an increasing

number of advisers is to link areas

of surface runoff and high soil P

content in a watershed (fig. 11).

Preventing P loss is now taking on

the added dimension of defining,

targeting, and remediating source

areas of P where high soil P levels

coincide with high surface runoff

and erosion potentials. This approach addresses P management at

multifield or watershed scales.

Furthermore, a comprehensive P

management strategy must address

down-gradient water-quality impacts, such as the proximity of Psensitive waters. Conventionally

applied remediations may not
24

Soil test P >100 ppm

Area of high surface runoff potential

Figure 11. The principle of source-area

management to more effectively target

measures to reduce P export in surface

runoff from watersheds.

Areas most vulnerable to P loss

Integrated P and N management
25

produce the desired results and may

prove to be an inefficient and poor

cost-effective approach to the

problem if this source-area perspective to target application of P

fertility, surface runoff, and erosion

control technology is not used.

A simple P index has been developed by the U.S. Department of

Agriculture, Agricultural Research

Service, in cooperation with several

research scientists, as a screening

tool for use by field staffs, watershed planners, and farmers to rank

the vulnerability of fields as sources

of P loss in surface runoff

(Lemunyon and Gilbert 1993). The

index accounts for and ranks

transport and source factors controlling P loss in surface runoff and

sites where the risk of P movement

is expected to be higher than that of

others (table 4). Each site characteristic affecting P loss is weighted

according to the assumption that

certain characteristics have a

relatively greater effect on potential

P loss than others. Each user must

establish a range of P loss potential

values for different geographic

areas. An assessment of site vulnerability to P loss in surface runoff is

made by selecting the rating value

for each site characteristic from the

P index. Each rating is multiplied

by the appropriate weighting factor.

The P index value for the site is the

sum of weighted values of all site

characteristics, which can then be

used to categorize the site’s vulnerability to P loss (table 5).

The index is a tool for field personnel to identify agricultural areas or

practices that have the greatest

potential to accelerate eutrophication. It can be used to identify

management options available to

land users and will allow them

flexibility in developing remedial

strategies. The first step is to

determine the P index for soils

adjacent to sensitive waters and

prioritize the efforts needed to

reduce P losses. Then, management

options appropriate for soils with

different P index ratings can be

implemented. General recommendations are given in table 6; however,

P management is very site specific

and requires a well-planned, coordinated effort among farmers, extension agronomists, and soil conservation specialists.

Farm N inputs can usually be more

easily balanced with plant uptake

than P inputs can, particularly where

confined animal operations exist. In

the past, separate strategies for

either N or P were developed and

implemented at farm or watershed

scales. Because N and P have

different chemistry and flow path-
26

Table 4. The P index

Site characteristic Loss rating (value)

(Weighting factor)

None (0) Low (1) Medium (2) High (4) Very high (8)

Transparent Factors

Soil erosion (1.5) N/A < 5 tons/acre 5 to 10 tons/acre 10 to 15 tons/acre > 15 tons/acre

Irrigation erosion (1.5) N/A Infrequent irrigation Moderate irrigation Frequent irrigation Frequent irrigation

on well-drained on soils with on soils with on soils with

soils slopes < 5% slopes of 2 to 5% slopes > 5%

Soil runoff class (0.5) N/A Very low or low Medium Optimum Excessive

Distance from > 1,000 ft 1,000 to 500 ft 500 to 200 ft 200 to 30 ft < 30 ft

watercourse (1.0)
27

Table 4. The P index (continued)

Site characteristic Loss rating (value)

(Weighting factor)

None (0) Low (1) Medium (2) High (4) Very high (8)

Source Factors

Soil test P (1.0) N/A Low Medium Optimum Excessive

P fertilizer application None < 15 16 to 40 41 to 65 > 65

rate, lb P/acre (0.75) applied

P fertilizer application None Placed with Incorporated Incorporated > 3 Surface applied to

method (0.5) applied planter deeper immediately months before crop pasture or applied

than 2 inches before crop or surface applied > 3 months

< 3 months before crop

before crop

Organic P source None < 15 16 to 40 41 to 65 > 65

application rate, lb applied

P/acre (1.0)

Organic P source None Injected deeper Incorporated Incorporated > 3 Surface applied to

application method (0.5) applied than 2 inches immediately months before crop pasture or applied

before planting or surface applied > 3 months

< 3 months before crop

before crop
28

Table 5. Generalized interpretation of the P index

P index General vulnerability to P loss

< 8 Low potential for P loss. If current farming practices are

maintained, there is a low probability of adverse impacts on

surface waters.

8 to 14 Medium potential for P loss. The chance for adverse impacts

on surface waters exists, and some remediation should be taken

to minimize the probability of P loss.

15 to 32 High potential for P loss and adverse impacts on surface waters.

Soil and water conservation measures and P management plans

are needed to minimize the probability of P loss.

> 32 Very high potential for P loss and adverse impacts on surface

waters. All necessary soil and water conservation measures and a

P management plan must be implemented to minimize the P loss.

ways through soils and watersheds,

these narrowly targeted strategies

often conflict and lead to compromised water quality. For example,

manure application based on crop N

requirements to minimize nitrate

leaching to ground water often

results in excess soil P and enhances

potential P losses in surface runoff.

In contrast, reducing surface runoff

losses of P via conservation tillage

can increase water infiltration into

the soil profile and enhance nitrate

leaching.

For P, a primary strategy is to

minimize surface runoff and particulate transport. In most cases,

decreasing P loss by plant cover,

crop residues, tillage and planting

along contours, and buffer zones has

a neutral or beneficial impact on

nitrate loss. Some exceptions are

practices that promote water infiltration, which tend to increase leach-
29

ing, and tillage practices that do not

incorporate P fertilizers and manures into the soil. No-till is commonly recommended as a conservation measure for eroding cropland.

Conversion to no-till is followed by

decreased soil and total N and P loss

in surface runoff and increased

nitrate leaching and algal-available

P transport (Sharpley and Smith

1994).

Nitrogen losses can occur from any

location in a watershed, so remedial

strategies for N can be applied to the

whole watershed. Phosphorus losses

usually occur from areas prone to

surface runoff; therefore, the most

effective P strategy would be to (1)

avoid excessive soil P buildup in the

whole watershed and thereby limit

losses in subsurface flow, and (2)

use more stringent measures for the

most vulnerable sites to minimize

loss of P in surface runoff.

Development of sound remedial

measures should consider these

conflicting impacts of conservation

practices on resultant water quality.

Clearly, a technically sound framework must be developed that includes critical sources of N and P

export from agricultural watersheds

so that optimal strategies at farm

and watershed scales can be implemented to best manage N and P.

Summary

The overall goal to reduce P losses

from agriculture should be to

balance off-farm inputs of P in feed

and fertilizer with outputs in products and to manage soils in ways

that retain crop nutrient resources.

Source and transport control strategies can provide the basis for

increasing P-use efficiency in

agricultural systems.

Future advisory programs should

reinforce the fact that all fields do

not contribute equally to P export

from watersheds. Most P export

comes from only a small portion of

the watershed as a result of relatively few storms. Although soil P

content is important in determining

the concentration of P in agricultural

runoff, surface runoff and erosion

potential often override soil levels in

determining P export. If water or

soil do not move from a field or

below the root zone, then P will not

move. Clearly, management systems

will be most effective if targeted to

the hydrologically active source

areas in a watershed that operate

during a few major storms.

Manure management recommendations will have to account for site

vulnerability to surface runoff and

erosion, as well as soil P content,

because not all soils and fields have
30

Table 6. Management options to minimize nonpoint source pollution

of surface waters by soil P

Phosphorus index Management options

< 8 Soil testing: Test soils for P at least every 3 years to monitor buildup or decline in soil P.

(Low)

Soil conservation: Follow good soil conservation practices. Consider effects of changes in tillage practices

or land use on potential for increased transport of P from site.

Nutrient management: Consider effects of any major changes in agricultural practices on P loss before

implementing them on the farm. Examples include increasing the number of animal units on a farm or

changing to crops with a high demand for fertilizer P.

8 to 14 Soil testing: Test soils for P at least every 3 years to monitor buildup or decline in soil P. Conduct a more

(Medium) comprehensive soil testing program in areas identified by the P index as most sensitive to P loss by surface

runoff, subsurface flow, and erosion.

Soil conservation: Implement practices to reduce P loss by surface runoff, subsurface flow, and erosion in

the most sensitive fields (that is, reduced tillage, field borders, grassed waterways, and improved irrigation

and drainage management).

Nutrient management: Any changes in agricultural practices may affect P loss; carefully consider the

sensitivity of fields to P loss before implementing any activity that will increase soil P. Avoid broadcast

applications of P fertilizers and apply manures only to fields with low P index values.
31

Table 6. Management options to minimize nonpoint source pollution

of surface waters by soil P (continued)

Phosphorus index Management options

15 to 32 Soil testing: A comprehensive soil testing program should be conducted on the entire farm to determine fields

(High) that are most suitable for further additions of P. For fields that are excessive in P, estimates of the time required

to deplete soil P to optimum levels should be made for use in long-range planning.

Soil conservation: Implement practices to reduce P loss by surface runoff, subsurface flow, and erosion in the

most sensitive fields (that is, reduced tillage, field borders, grassed waterways, and improved irrigation and

drainage management). Consider using crops with high P removal capacities in fields with high P index values.

Nutrient management: In most situations involving fertilizer P, only a small amount used in starter fertilizers

is needed. Manure may be in excess on the farm and should only be applied to fields with lower P index

values. A long-term P management plan should be considered.

> 32 Soil testing: For fields that are excessive in P, estimate the time required to deplete soil P to optimum levels

(Very high) for use in long-range planning. Consider using new soil testing methods that provide more information on

environmental impact of soil P.

Soil conservation: Implement practices to reduce P loss by surface runoff, subsurface flow, and erosion in the

most sensitive fields (that is, reduced tillage, field borders, grassed waterways, and improved irrigation and

drainage management). Consider using crops with high P removal capacities in fields with high P index values.

Nutrient management: Fertilizer and manure P should not be applied for 3 years or more. A comprehensive,

long-term P management plan must be developed and implemented.
32

the same potential to transfer P to

surface runoff and leaching. As a

result, threshold soil P levels should

be indexed against P transport

potential, with lower values for P

source areas than for areas not

contributing to water export.

Phosphorus applications at recommended rates can reduce P loss in

agricultural runoff via increased

crop uptake and cover. It is of vital

importance that management

practices are implemented to

minimize soil P buildup in excess of

crop requirements, reduce surface

runoff and erosion, and improve

capability to identify fields that are

major sources of P loss to surface

waters.

Overall—

• management systems should

balance P inputs and outputs at

farm and watershed scales;

• source and transport controls

should target and identify critical

source areas of P export from

watersheds; and

• farmers should link threshold soil

P levels that guide manure

applications with site vulnerability

to P loss.

Consideration of all these factors

will be needed to develop extension

and demonstration projects that

educate farmers, the animal

industry, and the general public

about what is actually involved in

ensuring clean water. It is hoped this

will help overcome the common

misconception that diffuse or

nonpoint sources are too difficult,

costly, or variable to control or

target substantial reductions (fig.

12).

Efforts to implement defensible

remedial strategies that minimize P

loss from agricultural land will

require interdisciplinary research

involving soil scientists,

hydrologists, agronomists,

limnologists, and animal scientists.

Development of guidelines to

implement such strategies will also

require consideration of the

socioeconomic and political impacts

of any management changes on

rural and urban communities and of

the mechanisms by which change

can be achieved in a diverse and

dispersed community of land users.
33

Soils are infinite sinks for P. Research

shows that soils cannot indefinitely fix

applied P. Continued applications of P

beyond crop requirements, a common

scenario where organic wastes have been

heavily used in agriculture, are a major

cause of soil P saturation.

Crop N requirements should drive

manure management. Basing manure

management on mature N and crop N needs

can lead to undesirably high P applications

due to the unfavorable N:P ratios of most

manures and crop requirements.

Phosphorus does not move through the

soil. While most P losses occur with

surface runoff, P may move through soils

with combinations of low P-fixing capacities, high percolation potential, or high soil

test P contents.

Erosion control will stop P losses in

runoff. Erosion control is not the sole

answer; reduction of dissolved P loss in

runoff can only be achieved by minimizing

P loss at the source and implementing

practices that reduce total P in runoff.

By controlling point sources we can

solve water quality problems. Although

point source inputs have been reduced in

many areas, nonpoint source inputs now

contribute to a greater share of water quality

problems.

Most management practices are

permanent solutions. In most cases the

only permanent solution to reducing P losses

is balancing farm and watershed P inputs

and outputs.

Figure 12. Several myths about P still exist:

Phosphorus management strategies can

be universally applied. All fields and

water bodies are not created equal; management plans for P and best management

practices must be tailored to site

vulnerability to P loss and proximity of Psensitive waters.

We don't know enough about agricultural P. We know a lot about how P

reacts with soil and is transferred to runoff,

but we have not adequately disseminated

this information to land users and state and

Federal agencies.
34

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