Irrigation Water Quality

Standards

and

Salinity Management

Strategies

B-1667

4-03
Nearly all waters contain dissolved salts and trace elements,

many of which result from the

natural weathering of the earth’s

surface. In addition, drainage

waters from irrigated lands and

effluent from city sewage and

industrial waste water can impact

water quality. In most irrigation

situations, the primary water quality concern is salinity levels, since

salts can affect both the soil structure and crop yield. However, a

number of trace elements are

found in water which can limit its

use for irrigation.

Generally, “salt” is thought of as

ordinary table salt (sodium chloride). How-ever, many types of

salts exist and are commonly

found in Texas waters (Table 1).

Most salinity problems in agriculture result directly from the salts

carried in the irrigation water.

The process at work is illustrated

in Figure 1, which shows a

beaker of water containing a salt

concentration of 1 percent. As

water evaporates, the dissolved

salts remain, resulting in a solution with a higher concentration

of salt. The same process occurs

in soils. Salts as well as other dissolved substances begin to accumulate as water evaporates from

the surface and as crops withdraw

water.

Water Analysis:

Units, Terms and

Sampling

Numerous parameters are used to

define irrigation water quality, to

assess salinity hazards, and to

determine appropriate management

strategies. A complete water quality analysis will include the determination of:

1) the total concentration of soluble salts,

2) the relative proportion of sodium to the other cations,

3) the bicarbonate concentration as

related to the concentration of

calcium and magnesium, and

3

Irrigation Water Quality Standards

and Salinity Management

Guy Fipps*

Table 1. Kinds of salts normally found in irrigation waters, with chemical symbols and approximate proportions of each salt.

1 (Longenecker and Lyerly, 1994)

Chemical name Chemical symbol Approximate proportion

of total salt content

Sodium chloride NaCl Moderate to large

Sodium sulfate Na2SO4 Moderate to large

Calcium chloride CaCl

2 Moderate

Calcium sulfate (gypsum) CaSO4 2H2O Moderate to small

Magnesium chloride MgCl

2 Moderate

Magnesium sulfate MgS04 Moderate to small

Potassium chloride KCl Small

Potassium sulfate K2SO4 Small

Sodium bicarbonate NaHCO3 Small

Calcium carbonate CaCO3 Very Small

Sodium carbonate Na2CO3 Trace to none

Borates BO-3 Trace to none

Nitrates NO-3 Small to none

1

Waters vary greatly in amounts and kinds of dissolved salts. This water typifies many used for irrigation in Texas.

*Associate Professor and Extension

Agricultural Engineer, Department of

Agricultural Engineering, The Texas A&M

System, College Station, Texas 77843-2117.
4) the concentrations of specific

elements and compounds.

The amounts and combinations of

these substances define the suitability of water for irrigation and

the potential for plant toxicity.

Table 2 defines common parameters for analyzing the suitability of

water for irrigation and provides

some useful conversions.

When taking water samples for

laboratory analysis, keep in mind

that water from the same source

can vary in quality with time.

Therefore, samples should be tested at intervals throughout the

year, particularly during the

potential irrigation period. The

Soil and Water Testing Lab at

Texas A&M University can do a

complete salinity analysis of irrigation water and soil samples, and

will provide a detailed computer

printout on the interpretation of

the results. Contact your county

Extension agent for forms and

information or contact the Lab at

(979) 845-4816.

Two Types of Salt

Problems

Two types of salt problems exist

which are very different: those

associated with the total salinity

and those associated with sodium.

Soils may be affected only by

salinity or by a combination of

both salinity and sodium.

Salinity Hazard

Water with high salinity is toxic

to plants and poses a salinity hazard. Soils with high levels of

total salinity are call saline soils.

High concentrations of salt in the

soil can result in a “physiological” drought condition. That is,

even though the field appears to

have plenty of moisture, the

plants wilt because the roots are

unable to absorb the water. Water

salinity is usually measured by

the TDS (total dissolved solids) or

the EC (electric conductivity).

TDS is sometimes referred to as

the total salinity and is measured

or expressed in parts per million

(ppm) or in the equivalent units of

milligrams per liter (mg/L).

EC is actually a measurement of

electric current and is reported in

one of three possible units as

given in Table 2. Subscripts are

used with the symbol EC to identify the source of the sample.

ECiw is the electric conductivity

of the irrigation water. ECe is the

electric conductivity of the soil as

measured in a soil sample (saturated extract) taken from the root

zone. ECd is the soil salinity of

the saturated extract taken from

below the root zone. ECd is used

to determine the salinity of the

drainage water which leaches

below the root zone.

4

Figure 1. Effect of water evaporation on the concentration of salts in solution. A liter is 1.057 quarts. Ten grams is

.035 ounces or about 1 teaspoonful.

Types of Salinity Problems

affects can lead to

salinity plants saline soil

hazard condition

affects can lead to

sodium soils sodic soil

condition
ingly impervious to water penetration. Fine textured soils, especially those high in clay, are most

subject to this action. Certain

amendments may be required to

maintain soils under high SARs.

Calcium and magnesium, if present in the soil in large enough

quantities, will counter the effects

of the sodium and help maintain

good soil properties.

Soluble sodium per cent (SSP) is

also used to evaluate sodium hazard. SSP is defined as the ration

of sodium in epm (equivalents per

million) to the total cation epm

multiplied by 100. A water with a

SSP greater than 60 per cent may

result in sodium accumulations

that will cause a breakdown in the

soil’s physical properties.

Ions, Trace Elements

and Other Problems

A number of other substances

may be found in irrigation water

and can cause toxic reactions in

plants (Table 3). After sodium,

chloride and boron are of most

concern. In certain areas of Texas,

boron concentrations are excessively high and render water

unsuitable for irrigations. Boron

can also accumulate in the soil.

Crops grown on soils having an

imbalance of calcium and magnesium may also exhibit toxic

symptoms. Sulfate salts affect

sensitive crops by limiting the

uptake of calcium and increasing

the adsorption of sodium and

potassium, resulting in a disturbance in the cationic balance

within the plant. The bicarbonate

ion in soil solution harms the

mineral nutrition of the plant

through its effects on the uptake

and metabolism of nutrients. High

concentrations of potassium may

introduce a magnesium deficiency

and iron chlorosis. An imbalance

of magnesium and potassium may

be toxic, but the effects of both

can be reduced by high calcium

levels.

5

Table 2. Terms, units, and useful conversions for understanding

water quality analysis reports.

Symbol Meaning Units

Total Salinity

a. EC electric conductivity mmhos/cm

μmhos/cm

dS/m

b. TDS total dissolved solids mg/L

ppm

Sodium Hazard

a. SAR sodium adsorption ratio —

b. ESP exchangeable sodium percentage —

Determination Symbol Unit of measure Atomic weight

Constituents

(1) cations

calcium Ca mol/m3 40.1

magnesium Mg mol/m3 24.3

sodium Na mol/m3 23.0

potassium K mol/m3 39.1

(2) anions

bicarbonate HCO3 mol/m3 61.0

sulphate SO4 mol/m3 96.1

chloride Cl mol/m3 35.5

carbonate CO3 mol/m3 60.0

nitrate NO3 mg/L 62.0

Trace Elements

boron B mg/L 10.8

Conversions

1 dS/m = 1 mmhos/cm = 1000 μmhos/cm

1 mg/L = 1 ppm

TDS (mg/L) ≈ EC (dS/m) x 640 for EC < 5 dS/m

TDS (mg/L ≈ EC (dS/m) x 800 for EC > 5 dS/m

TDS (lbs/ac-ft) ≈ TDS (mg/L) x 2.72

Concentration (ppm) = Concentration (mol/m3) times the atomic weight

Sum of cations/anions

(meq/L) ≈ EC (dS/m) x 10

Key

mg/L = milligrams per liter

ppm = parts per million

dS/m = deci Siemens per meter at 25° C

Sodium Hazard

Irrigation water containing large

amounts of sodium is of special

concern due to sodium’s effects

on the soil and poses a sodium

hazard. Sodium hazard is usually

expressed in terms of SAR or the

sodium adsorption ratio. SAR is

calculated from the ratio of sodium to calcium and magnesium.

The latter two ions are important

since they tend to counter the

effects of sodium. For waters containing significant amounts of

bicarbonate, the adjusted sodium

adsorption ratio (SARadj

) is sometimes used.

Continued use of water having a

high SAR leads to a breakdown

in the physical structure of the

soil. Sodium is adsorbed and

becomes attached to soil particles.

The soil then becomes hard and

compact when dry and increas-
6

Table 3. Recommended limits for constituents in reclaimed water for irrigation. (Adapted from

Rowe and Abdel-Magid, 1995)

Constituent Long-term Short-term Remarks

use (mg/L) use (mg/L)

Aluminum (Al) 5.0 20 Can cause nonproductivity in acid soils, but soils at pH 5.5 to 8.0

will precipitate the ion and eliminate toxicity.

Arsenic (As) 0.10 2.0 Toxicity to plants varies widely, ranging from 12 mg/L for Sudan

grass to less than 0.05 mg/L for rice.

Beryllium (Be) 0.10 0.5 Toxicity to plants varies widely, ranging from 5 mg/L for kale to

0.5 mg/L for bush beans.

Boron (B) 0.75 2.0 Essential to plant growth, with optimum yields for many obtained

at a few-tenths mg/L in nutrient solutions. Toxic to many sensitive

plants (e.g., citrus) at 1 mg/L. Most grasses relatively tolerant at

2.0 to 10 mg/L.

Cadmium (Cd) 0.01 0.05 Toxic to beans, beets, and turnips at concentrations as low as 0.1

mg/L in nutrient solution. Conservative limits recommended.

Chromium (Cr) 0.1 1.0 Not generally recognized as essential growth element. Conservative

limits recommended due to lack of knowledge on toxicity to plants.

Cobalt (Co) 0.05 5.0 Toxic to tomato plants at 0.1 mg/L in nutrient solution. Tends to be

inactivated by neutral and alkaline soils.

Copper (Cu) 0.2 5.0 Toxic to a number of plants at 0.1 to 1.0 mg/L in nutrient solution.

Fluoride (F

–

) 1.0 15.0 Inactivated by neutral and alkaline soils.

Iron (Fe) 5.0 20.0 Not toxic to plants in aerated soils, but can contribute to soil acidification and loss of essential phosphorus and molybdenum.

Lead (Pb) 5.0 10.0 Can inhibit plant cell growth at very high concentrations.

Lithium (Li) 2.5 2.5 Tolerated by most crops at up to 5 mg/L; mobile in soil. Toxic to

citrus at low doses recommended limit is 0.075 mg/L.

Manganese (Mg) 0.2 10.0 Toxic to a number of crops at a few-tenths to a few mg/L in acid

soils.

Molybdenum (Mo) 0.01 0.05 Nontoxic to plants at normal concentrations in soil and water. Can

be toxic to livestock if forage is grown in soils with high levels of

available molybdenum.

Nickel (Ni) 0.2 2.0 Toxic to a number of plants at 0.5 to 1.0 mg/L; reduced toxicity at

neutral or alkaline pH.

Selenium (Se) 0.02 0.02 Toxic to plants at low concentrations and to livestock if forage is

grown in soils with low levels of added selenium.

Vanadium (V) 0.1 1.0 Toxic to many plants at relatively low concentrations.

Zinc (Zn) 2.0 10.0 Toxic to many plants at widely varying concentrations; reduced

toxicity at increased pH (6 or above) and in fine-textured or organic

soils.
Classification of

Irrigation Water

Several different measurements are

used to classify the suitability of

water for irrigation, including

ECiw, the total dissolved solids,

and SAR. Some permissible limits

for classes of irrigation water are

given in Table 4. In Table 5, the

sodium hazard of water is ranked

from low to very high based on

SAR values.

Classification of

Salt-Affected Soils

Both ECe and SAR are commonly

used to classify salt-affected soils

(Table 6). Saline soils (resulting

from salinity hazard) normally

have a pH value below 8.5, are relatively low in sodium and contain

principally sodium, calcium and

magnesium chlorides and sulfates.

These compounds cause the white

crust which forms on the surface

and the salt streaks along the furrows. The compounds which

cause saline soils are very soluble

in water; therefore, leaching is

usually quite effective in reclaiming these soils.

Sodic soils (resulting from sodium hazard) generally have a pH

value between 8.5 and 10. These

soils are called “black alkali

soils” due to their darkened

appearance and smooth, slick

looking areas caused by the dispersed condition. In sodic soils,

sodium has destroyed the permanent structure which tends to

make the soil impervious to

water. Thus, leaching alone will

not be effective unless the high

salt dilution method or amendments are used.

Water Quality

Effects on Plants

and Crop Yield

Table 7 gives the expected yield

reduction of some crops for various levels of soil salinity as measured by EC under normal growing conditions, and Table 8 gives

potential yield reduction due to

water salinity levels. Generally

forage crops are the most resistant

to salinity, followed by field

crops, vegetable crops, and fruit

crops which are generally the

most sensitive.

Table 9 lists the chloride tolerance of a number of agricultural

crops. Boron

is a major concern in some areas.

While a necessary nutrient, high

boron levels cause plant toxicity,

and concentrations should not

exceed those given in Table 10.

Some information is available on

the susceptibility of crops to

foliar injury from spray irrigation

with water containing sodium and

chloride (Table 11). The tolerance

of crops to sodium as measured

by the exchangeable sodium percentage (ESP) is given in Table

12.

7

Table 4. Permissible limits for classes of irrigation water.

Concentration, total dissolved solids

Classes of water Electrical Gravimetric ppm

conductivity μmhos*

Class 1, Excellent 250 175

Class 2, Good 250-750 175-525

Class 3, Permissible1 750-2,000 525-1,400

Class 4, Doubtful

2 2,000-3,000 1,400-2,100

Class 5, Unsuitable2 3,000 2,100

*Micromhos/cm at 25 degrees C. 1

Leaching needed if used

2

Good drainage needed and sensitive plants will have difficulty obtaining

stands

Table 5. The sodium hazard of water based on SAR Values.

SAR values Sodium hazard of water Comments

1-10 Low Use on sodium sensitive crops such as avocados

must be cautioned.

10 - 18 Medium Amendments (such as Gypsum) and leaching needed.

18 - 26 High Generally unsuitable for continuous use.

> 26 Very High Generally unsuitable for use.

Table 6. Classification of salt-affected soils based on analysis of

saturation extracts. (Adapted from James et al., 1982)

Criteria Normal Saline Sodic Saline-Sodic

ECe (mmhos/cm) <4 >4 <4 >4

SAR <13 <13 >13 >13
Salinity and

Growth Stage

Many crops have little tolerance

for salinity during seed germination, but significant tolerance during later growth stages. Some

crops such as barley, wheat and

corn are known to be more sensitive to salinity during the early

growth period than during germination and later growth periods.

Sugar beet and safflower are relatively more sensitive during germination, while the tolerance of

soybeans may increase or

decrease during different growth

periods depending on the variety.

Leaching for Salinity

Management

Soluble salts that accumulate in

soils must be leached below the

crop root zone to maintain productivity. Leaching is the basic

management tool for controlling

salinity. Water is applied in excess

of the total amount used by the

crop and lost to evaporation. The

strategy is to keep the salts in

solution and flush them below the

root zone. The amount of water

needed is referred to as the leaching requirement or the leaching

fraction.

Excess water may be applied with

every irrigation to provide the

water needed for leaching. However, the time interval between

leachings does not appear to be

critical provided that crop tolerances are not exceeded. Hence,

leaching can be accomplished

with each irrigation, every few

irrigations, once yearly, or even

longer depending on the severity

of the salinity problem and salt

tolerance of the crop. An occasional or annual leaching event

where water is ponded on the surface is an easy and effective

method for controlling soil salinity. In some areas, normal rainfall

provides adequate leaching.

8

Table 7. Soil salinity tolerance levels

1 for different crops.

(Adapted from Ayers and Westcot, 1976)

Yield potential, ECe

Crop 100% 90% 75% 50% Maximum ECe

Field crops

Barley

a 8.0 10.0 13.0 18.0 28

Bean (field) 1.0 1.5 2.3 3.6 7

Broad bean 1.6 2.6 4.2 6.8 12

Corn 1.7 2.5 3.8 5.9 10

Cotton 7.7 9.6 13.0 17.0 27

Cowpea 1.3 2.0 3.1 4.9 9

Flax 1.7 2.5 3.8 5.9 10

Groundnut 3.2 3.5 4.1 4.9 7

Rice (paddy) 3.0 3.8 5.1 7.2 12

Safflower 5.3 6.2 7.6 9.9 15

Sesbania 2.3 3.7 5.9 9.4 17

Sorghum 4.0 5.1 7.2 11.0 18

Soybean 5.0 5.5 6.2 7.5 10

Sugar beet 7.0 8.7 11.0 15.0 24

Wheat

a 6.0 7.4 9.5 13.0 20

Vegetable crops

Bean 1.0 1.5 2.3 3.6 7

Beet

b 4.0 5.1 6.8 9.6 15

Broccoli 2.8 3.9 5.5 8.2 14

Cabbage 1.8 2.8 4.4 7.0 12

Cantaloupe 2.2 3.6 5.7 9.1 16

Carrot 1.0 1.7 2.8 4.6 8

Cucumber 2.5 3.3 4.4 6.3 10

Lettuce 1.3 2.1 3.2 5.2 9

Onion 1.2 1.8 2.8 4.3 8

Pepper 1.5 2.2 3.3 5.1 9

Potato 1.7 2.5 3.8 5.9 10

Radish 1.2 2.0 3.1 5.0 9

Spinach 2.0 3.3 5.3 8.6 15

Sweet corn 1.7 2.5 3.8 5.9 10

Sweet potato 1.5 2.4 3.8 6.0 11

Tomato 2.5 3.5 5.0 7.6 13

Forage crops

Alfalfa 2.0 3.4 5.4 8.8 16

Barley hay

a 6.0 7.4 9.5 13.0 20

Bermudagrass 6.9 8.5 10.8 14.7 23

Clover, Berseem 1.5 3.2 5.9 10.3 19

Corn (forage) 1.8 3.2 5.2 8.6 16

Harding grass 4.6 5.9 7.9 11.1 18

Orchard grass 1.5 3.1 5.5 9.6 18

Perennial rye 5.6 6.9 8.9 12.2 19

Sudan grass 2.8 5.1 8.6 14.4 26

Tall fescue 3.9 5.8 8.61 3.3 23

Tall wheat grass 7.5 9.9 13.3 19.4 32

Trefoil, big 2.3 2.8 3.6 4.9 8

Trefoil, small 5.0 6.0 7.5 10.0 15

Wheat grass 7.5 9.0 11.0 15.0 22
Determining Required

Leaching Fraction

The leaching fraction is commonly

calculated using the following

relationship:

ECiw

LF = (1)

ECe

where

LF = leaching fraction

- the fraction of

applied irrigation

water that must

be leached

through the root

zone

ECiw = electric conductivity of the irrigation water

ECe = the electric conductivity of the

soil in the root

zone

Equation (1) can be used to determine the leaching fraction necessary to maintain the root zone at a

targeted salinity level. If the

amount of water available for

leaching is fixed, then the equation

can be used to calculate the salinity level that will be maintained in

the root zone with that amount of

leaching. Please note that equation

(1) simplifies a complicated soil

water process. ECe should be

checked periodically and the

amount of leaching adjusted

accordingly.

Based on this equation, Table 13

lists the amount of leaching needed for different classes of irrigation waters to maintain the soil

salinity in the root zone at a

desired level. However, additional

water must be supplied because of

the inefficiencies of irrigation systems (Table 14), as well as to

remove the existing salts in the

soil.

9

Table 7. Soil salinity tolerance levels

1 for different crops.

(continued)

Yield potential, ECe

Crop 100% 90% 75% 50% Maximum ECe

Fruit crops

Almond 1.5 2.0 2.8 4.1 7

Apple, Pear 1.7 2.3 3.3 4.8 8

Apricot 1.6 2.0 2.6 3.7 6

Avocado 1.3 1.8 2.5 3.7 6

Date palm 4.0 6.8 10.9 17.9 32

Fig, Olive,

Pomegranate 2.7 3.8 5.5 8.4 14

Grape 1.5 2.5 4.1 6.7 12

Grapefruit 1.8 2.4 3.4 4.9 8

Lemon 1.7 2.3 3.3 4.8 8

Orange 1.7 2.3 3.2 4.8 8

Peach 1.7 2.2 2.9 4.1 7

Plum 1.5 2.1 2.9 4.3 7

Strawberry 1.0 1.3 1.8 2.5 4

Walnut 1.7 2.3 3.3 4.8 8

1

Based on the electrical conductivity of the saturated extract taken from a

root zone soil sample (ECe) measured in mmhos/cm. a

During germination and seedling stage ECe should not exceed 4 to 5

mmhos/cm except for certain semi-dwarf varieties. b

During germination ECe should not exceed 3 mmhos/cm.

Table 8. Irrigation water salinity tolerances

1 for different crops.

(Adapted from Ayers and Westcot, 1976)

Yield potential, ECiw

Crop 100% 90% 75% 50%

Field crops

Barley 5.0 6.7 8.7 12.0

Bean (field) 0.7 1.0 1.5 2.4

Broad bean 1.1 1.8 2.0 4.5

Corn 1.1 1.7 2.5 3.9

Cotton 5.1 6.4 8.4 12.0

Cowpea 0.9 1.3 2.1 3.2

Flax 1.1 1.7 2.5 3.9

Groundnut 2.1 2.4 2.7 3.3

Rice (paddy) 2.0 2.6 3.4 4.8

Safflower 3.5 4.1 5.0 6.6

Sesbania 1.5 2.5 3.9 6.3

Sorghum 2.7 3.4 4.8 7.2

Soybean 3.3 3.7 4.2 5.0

Sugar beet 4.7 5.8 7.5 10.0

Wheat 4.0 4.9 6.4 8.7

Vegetable crops

Bean 0.7 1.0 1.5 2.4

Beet 2.7 3.4 4.5 6.4

Broccoli 1.9 2.6 3.7 5.5
10

Table 8. Irrigation water salinity tolerances

1 for different crops.

(continued)

Yield potential, ECiw

Crop 100% 90% 75% 50%

Cabbage 1.2 1.9 2.9 4.6

Cantaloupe 1.5 2.4 3.8 6.1

Carrot 0.7 1.1 1.9 3.1

Cucumber 1.7 2.2 2.9 4.2

Lettuce 0.9 1.4 2.1 3.4

Onion 0.8 1.2 1.8 2.9

Pepper 1.0 1.5 2.2 3.4

Potato 1.1 1.7 2.5 3.9

Radish 0.8 1.3 2.1 3.4

Spinach 1.3 2.2 3.5 5.7

Sweet corn 1.1 1.7 2.5 3.9

Sweet potato 1.0 1.6 2.5 4.0

Tomato 1.7 2.3 3.4 5.0

Forage crops

Alfalfa 1.3 2.2 3.6 5.9

Barley hay 4.0 4.9 6.3 8.7

Bermudagrass 4.6 5.7 7.2 9.8

Clover, Berseem 1.0 2.1 3.9 6.8

Corn (forage) 1.2 2.1 3.5 5.7

Harding grass 3.1 3.9 5.3 7.4

Orchard grass 1.0 2.1 3.7 6.4

Perennial rye 3.7 4.6 5.9 8.1

Sudan grass 1.9 3.4 5.7 9.6

Tall fescue 2.6 3.9 5.7 8.9

Tall wheat grass 5.0 6.6 9.0 13.0

Trefoil, big 1.5 1.9 2.4 3.3

Trefoil, small 3.3 4.0 5.0 6.7

Wheat grass 5.0 6.0 7.4 9.8

Fruit crops

Almond 1.0 1.4 1.9 2.7

Apple, Pear 1.0 1.6 2.2 3.2

Apricot 1.1 1.3 1.8 2.5

Avocado 0.9 1.2 1.7 2.4

Date palm 2.7 4.5 7.3 12.0

Fig, Olive,

Pomegranate 1.8 2.6 3.7 5.6

Grape 1.0 1.7 2.7 4.5

Grapefruit 1.2 1.6 2.2 3.3

Lemon 1.1 1.6 2.2 3.2

Orange 1.1 1.6 2.2 3.2

Peach 1.1 1.4 1.9 2.7

Plum 1.0 1.4 1.9 2.8

Strawberry 0.7 0.9 1.2 1.7

Walnut 1.1 1.6 2.2 3.2

1

Based on the electrical conductivity of the irrigation water (ECiw) measured

in mmhos/cm.

Subsurface Drainage

Very saline, shallow water tables

occur in many areas of Texas.

Shallow water tables complicate

salinity management since water

may actually move upward into

the root zone, carrying with it dissolved salts. Water is then extracted by crops and evaporation, leaving behind the salts.

Shallow water tables also contribute to the salinity problem by

restricting the downward leaching

of salts through the soil profile.

Installation of a subsurface

drainage system is about the only

solution available for this situation. The original clay tiles have

been replaced by plastic tubing.

Modern drainage tubes are covered by a “sock” made of fabric

to prevent clogging of the small

openings in the plastic tubing.

A schematic of a subsurface

drainage system is shown in

Figure 2. The design parameters

are the distance between drains

(L) and the elevation of the drains

(d) above the underlying impervious or restricting layer. Proper

spacing and depth maintain the

water level at an optimum level,

shown here as the distance m

above the drain tubes. The USDA

Natural Resources Conservation

Service (NRCS) has developed

drainage design guidelines that

are used throughout the United

States. A drainage computer

model developed by Wayne

Skaggs at North Carolina State

University, DRAINMOD, is also

widely used throughout the world

for subsurface drainage design.

Seed Placement

Obtaining a satisfactory stand is

often a problem when furrow irrigating with saline water. Growers

sometimes compensate for poor

germination by planting two or

three times as much seed as normally would be required.
11

However, planting procedures can

be adjusted to lower the salinity in

the soil around the germinating

seeds. Good salinity control is

often achieved with a combination

of suitable practices, bed shapes

and irrigation water management.

In furrow-irrigated soils, planting

seeds in the center of a single-row,

raised bed places the seeds exactly

where salts are expected to concentrate (Figure 3). This situation

can be avoided using “salt ridges.”

With a double-row raised planting

bed, the seeds are placed near the

shoulders and away from the area

of greatest salt accumulation.

Alternate-furrow irrigation may

help in some cases. If alternate

furrows are irrigated, salts often

can be moved beyond the single

seed row to the non-irrigated side

of the planting bed. Salts will still

accumulate, but accumulation at

the center of the bed will be

reduced.

With either single- or double-row

plantings, increasing the depth of

the water in the furrow can

improve germination in saline

soils. Another practice is to use

sloping beds, with the seeds planted on the sloping side just above

the water line (Fig. 3b). Seed and

plant placement is also important

with the use of drip irrigation.

Typical wetting patterns of drip

emitters and micro-sprinklers are

shown in Figure 4. Salts tend to

move out and upward, and will

accumulate in the areas shown.

Other Salinity

Management

Techniques

Techniques for controlling salinity

that require relatively minor

changes are more frequent irrigations, selection of more salt-tolerant crops, additional leaching, preplant irrigation, bed forming and

seed placement. Alternatives that

require significant changes in

management are changing the irrigation method, altering the water

supply, land-leveling, modifying

the soil profile, and installing subsurface drainage.

Residue Management

The common saying “salt loves

bare soils” refers to the fact that

exposed soils have higher evaporation rates than those covered by

residues. Residues left on the soil

surface reduce evaporation. Thus,

less salts will accumulate and rainfall will be more effective in providing for leaching.

More Frequent

Irrigations

Salt concentrations increase in the

soil as water is extracted by the

crop. Typically, salt concentrations

are lowest following an irrigation

and higher just before the next irrigation. Increasing irrigation frequency maintains a more constant

moisture content in the soil. Thus,

more of the salts are then kept in

solution which aids the leaching

process. Surge flow irrigation is

often effective at reducing the

minimum depth of irrigation that

can be applied with furrow irrigation systems. Thus, a larger number of irrigations are possible

using the same amount of water.

Figure 2. A subsurface drainage system. Plastic draintubes are located a distance (L) apart.
12

Figure 3a. Single-row versus double-row beds showing areas of salt accumulation following a heavy irrigation with

salty water. Best planting position is on the shoulders of the double-row bed.

Figure 3b. Pattern of salt build-up as a function of seed placement, bed shape and irrigation water quality.
13

With proper placement, drip irrigation is very effective at flushing

salts, and water can be applied

almost continuously. Center pivots

equipped with LEPA water applicators offer similar efficiencies and

control as drip irrigation at less

than half the cost. Both sprinkler

and drip provide more control and

flexibility in scheduling irrigation

than furrow systems.

Preplant Irrigation

Salts often accumulate near the

soil surface during fallow periods,

particularly when water tables are

high or when off-season rainfall is

below normal. Under these conditions, seed germination and

seedling growth can be seriously

reduced unless the soil is leached

before planting.

Changing Surface

Irrigation Method

Surface irrigation methods, such as

flood, basin, furrow and border are

usually not sufficiently flexible to

permit changes in frequency of

irrigation or depth of water applied

per irrigation. For example, with

furrow irrigation it may not be

possible to reduce the depth of

water applied below 3-4 inches.

As a result, irrigating more frequently might improve water availability to the crop but might also

waste water. Converting to surge

flow irrigation may be the solution

for many furrow systems.

Otherwise a sprinkler or drip irrigation system may be required.

Chemical Amendments

In sodic soils (or sodium affected

soils), sodium ions have become

attached to and adsorbed onto the

soil particles. This causes a breakdown in soil structure and results

in soil sealing or “cementing,”

making it difficult for water to

infiltrate. Chemical amendments

are used in order to help facilitate

the displacement of these sodium

ions. Amendments are composed

Table 9. Chloride tolerance of agricultural crops. Listed in order

of tolerancea. (Adapted from Tanji. 1990)

Maximum Cl

-

concentration

b

without loss in yield

Crop mol/m

3

ppm

Strawberry 10 350

Bean 10 350

Onion 10 350

Carrot 10 350

Radish 10 350

Lettuce 10 350

Turnip 10 350

Rice, paddy

c 30d 1,050

Pepper 15 525

Clover, strawberry 15 525

Clover, red 15 525

Clover, alsike 15 525

Clover, ladino 15 525

Corn 15 525

Flax 15 525

Potato 15 525

Sweet potato 15 525

Broad bean 15 525

Cabbage 15 525

Foxtail, meadow 15 525

Celery 15 525

Clover, Berseem 15 525

Orchardgrass 15 525

Sugarcane 15 525

Trefoil, big 20 700

Lovegras 20 700

Spinach 20 700

Alfalfa 20 700

Sesbania

c 20 700

Cucumber 25 875

Tomato 25 875

Broccoli 25 875

Squash, scallop 30 1,050

Vetch, common 30 1,050

Wild rye, beardless 30 1,050

Sudan grass 30 1,050

Wheat grass, standard crested 35 1,225

Beet, red

c 40 1,400

Fescue, tall 40 1,400

Squash, zucchini 45 1,575

Harding grass 45 1,575

Cowpea 50 1,750

Trefoil, narrow-leaf bird’s foot 50 1,750
References

Ayres, R.S. and D.W. Westcot.

1976. Water Quality for

Agriculture. Irrigation and

Drainage Paper No. 29. Food

and Agriculture Organization

of the United Nations. Rome.

Cuena, R.H. 1989. Irrigation

System Design. Prentice Hall,

Englewood Cliffs, NJ. 552pp.

Hoffman, G.S., R.S. Ayers, E.J.

Doering and B.L. McNeal.

1980. Salinity in Irrigated

Agriculture. In: Design and

Operation of Farm Irrigation

Systems. M.E. Jensen, Editor.

ASAE Monograph No. 3. St.

Joseph, MI. 829pp.

James, D.W., R.J. Hanks and J.H.

Jurinak. 1982. Modern

Irrigated Soils. John Wiley

and Sons, NY.

Jensen, M.E. (Editor). 1980.

Design and Operation of

Farm Irrigation Systems.

American Society of

Agricultural Engineers, St.

Joseph MI. 829pp.

Longenecker, D.E. and P.J.

Lyerly. 1974. B-876 Control

of Soluble Salts in Farming

and Gardening. Texas

Agricultural Experiment

Station, Texas A&M

University System, College

Station. June. 36pp.

Pair, C.H. (editor). 1983.

Irrigation. The Irrigation

Assoc., Arlington, VA. 680pp.

Rowe, D.R. and I.M. AbdelMagid. 1995. Handbook of

Wastewater Reclamation and

Reuse. CRC Press, Inc.

550pp.

Stewart, B.A. and D.R. Nielsen.

1990. Irrigation of

Agricultural Crops. American

Society of Agronomy.

1,218pp.

Tanji, K.K. 1990. Agricultural

Salinity Assessment and

Management. American

Society of Civil Engineers.

14

Table 9. Chloride tolerance of agricultural crops. Listed in order

of tolerancea. (continued)

Maximum Cl

-

concentration

b

without loss in yield

Crop mol/m

3

ppm

Ryegrass, perennial 55 1,925

Wheat, Durum 55 1,925

Barley (forage)

c 60 2,100

Wheat

c 60 2,100

Sorghum 70 2,450

Bermudagrass 70 2,450

Sugar beet

c 70 2,450

Wheat grass, fairway crested 75 2,625

Cotton 75 1,625

Wheat grass, tall 75 2,625

Barley

c 80 2,800 a

These data serve only as a guideline to relative tolerances among crops.

Absolute tolerances vary, depending upon climate, soil conditions and

cultural practices. b

Cl

– concentrations in saturated-soil extracts sampled in the rootzone.

c

Less tolerant during emergence and seedling stage.

d

Values for paddy rice refer to the Cl

– concentration in the soil water during

the flooded growing conditions.

of sulphur in its elemental form or

related compounds such as sulfuric acid and gypsum. Gypsum also

contains calcium which is an

important element in correcting

these conditions. Some chemical

amendments render the natural

calcium in the soil more soluble.

As a result, calcium replaces the

adsorbed sodium which helps

restore the infiltration capacity of

the soil. Polymers are also beginning to be used for treating sodic

soils.

It is important to note that use of

amendments does not eliminate

the need for leaching. Excess

water must still be applied to leach

out the displaced sodium.

Chemical amendments are only

effective on sodium-affected soils.

Amend-ments are ineffective for

saline soil conditions and often

will increase the existing salinity

problem. Table 15 lists the most

common amendments. The irrigation books listed under the

References section present equations that are used to determine

the amount of amendments needed

based on soil analysis results.

Pipe Water Delivery

Systems Stabilize

Salinity

As illustrated in Fig. 1, any open

water is subject to evaporation

which leads to higher salt concentrations in the water. Evaporation

rates from water surfaces often

exceed 0.25 inch a day during

summer in Texas. Thus, the salinity content of irrigation water will

increase during the entire time

water is transported through irrigation canals or stored in reservoirs.

Replacing irrigation ditches with

pipe systems will help stabilize

salinity levels. In addition, pipe

systems, including gated pipe and

lay-flat tubing, reduce water lost

to canal seepage and increase the

amount of water available for

leaching.
15

Manuals and Reports on

Engineering Practice Number

71. 619pp.

van der Leeden, F., F.L. Troise and

D.K. Todd. 1990. The Water

Encyclopedia. Lewis

Publishers. 808pp.

Figure 4. Typical wetting patterns and areas of salt accumulation with drip emitters and micro-sprinklers sprayers.
16

Table 10. Limits of boron in irrigation water. (Adapted from Rowe and Abdel-Magid, 1995)

A. Permissible Limits (Boron in parts per million)

Class of water Crop group

Sensitive Semitolerant Tolerant

Excellent <0.33 <0.67 <1.00

Good 0.33 to 0.67 0.67 to 1.33 1.00 to 2.00

Permissible 0.67 to 1.00 1.33 to 2.00 2.00 to 3.00

Doubtful 1.00 to 1.25 2.00 to 2.50 3.00 to 3.75

Unsuitable >1.25 >2.5 >3.75

B. Crop groups of boron tolerance (in each plant group, the first names are considered as being more

tolerant; the last names, more sensitive).

Sensitive Semitolerant Tolerant

(1.0 mg/L of Boron) (2.0 mg/L of Boron) (4.0 mg/L of Boron)

Pecan

Walnut (Black, Persian, or English)

Jerusalem artichoke

Navy bean

American elm

Plum

Pear

Apple

Grape (Sultania and Malaga)

Kadota fig

Persimmon

Cherry

Peach

Apricot

Thornless blackberry

Orange

Avocado

Grapefruit

Lemon

(0.3 mg/L of Boron)

Sunflower (native)

Potato

Cotton (Acala and Pima)

Tomato

Sweetpea

Radish

Field pea

Ragged Robin rose

Olive

Barley

Wheat

Corn

Milo

Oat

Zinnia

Pumpkin

Bell pepper

Sweet potato

Lima bean

(1.0 mg/L of Boron)

Athel (Tamarix aphylla)

Asparagus

Palm (Phoenix canariensis)

Date palm (P. dactylifera)

Sugar beet

Mangel

Garden beet

Alfalfa

Gladiolus

Broad bean

Onion

Turnip

Cabbage

Lettuce

Carrot

(2.0 mg/L of Boron)

Table 11. Relative susceptibility of crops to foliar injury from

saline sprinkling waters. (Tanji, 1990)

Na or Cl concentration (mol/m3) causing foliar injurya

<5 5-10 10-20 >20

Almond Grape Alfalfa Cauliflower

Apricot Pepper Barley Cotton

Citrus Potato Corn Sugar beet

Plum Tomato Cucumber Sunflower

Safflower

Sesame

Sorghum a

Foliar injury is influenced by cultural and environmental conditions. These

data are presented only as general guidelines for daytime sprinkling.
17

Table 12. Tolerance of Various Crops to Exchangeable-Sodium Percentage. (James et al., 1982)

Tolerance to ESP Growth Responsible

(range at which affected) Crop Under Field Conditons

Extremely sensitive Deciduous fruits Sodium toxicity symptoms even at

(ESP = 2-10) Nuts low ESP values

Citrus

Avocado

Sensitive Beans Stunted growth at low ESP values

(ESP = 10-20) even though the physical condition

of the soil may be good

Moderately tolerant Clover Stunted growth due to both

(ESP = 20-40) Oats nutritional factors and adverse soil

Tall fescue conditions

Rice

Dallisgrass

Tolerant Wheat Stunted growth usually due to

(ESP = 40-60) Cotton adverse physical conditions of soil

Alfalfa

Barley

Tomatoes

Beets

Most tolerant Crested and Fairway wheatgrass Stunted growth usually due to

(ESP > 60) Tall wheatgrass adverse physical conditions of soil

Rhodes grass

Table 13. Leaching requirement* as related to the electrical conductivities of the irrigation and

drainage water.

Electrical conductivity of Leaching requirement based on the indicated maximum values for the

irrigation water (mmhos/cm) conductivity of the drainage water at the bottom of the root zone

4 mmhos/cm 8 mmhos/cm 12 mmhos/cm 16 mmhos/cm

Percent Percent Percent Percent

0.75 13.3 9.4 6.3 4.7

1.00 25.0 12.5 8.3 6.3

1.25 31.3 15.6 10.4 7.8

1.50 37.5 18.7 12.5 9.4

2.00 50.0 25.0 16.7 12.5

2.50 62.5 31.3 20.8 15.6

3.00 75.0 37.5 25.0 18.7

5.00 — 62.5 41.7 31.2

*Fraction of the applied irrigation water that must be leached through the root zone expressed as percent.
18

Table 15. Various amendments for reclaiming sodic soil and amount

equivalent to gypsum.

Amendment Physical description Amount equivalent

100% gypsum

Gypsum* White mineral 1.0

Sulfur

† Yellow element 0.2

Sulfuric acid* Corrosive liquid 0.6

Lime sulfur* Yellow-brown solution 0.8

Calcium carbonate† White mineral 0.6

Calcium chloride* White salt 0.9

Ferrous sulfate* Blue-green salt 1.6

Pyrite† Yellow-black mineral 0.5

Ferric sulfate* Yellow-brown salt 0.6

Aluminum sulfate* Corrosive granules 1.3

*Suitable for use as a water or soil amendment. †

Suitable only for soil application.

Table 14. Typical overall on-farm efficiencies for various types of irrigation systems.

System Overall efficiency (%)

Surface 50-80

a. average 50

b. land leveling and delivery pipeline meeting design standards 70

c. tailwater recovery with (b) 80

d. surge 60-90*

Sprinkler (moving and fixed systems) 55-85

LEPA (low pressure precision application) 95-98

Drip 80-90**

*Surge has been found to increase efficiencies 8 to 28% over non-surge furrow systems.

**Drip systems are typically designed at 90% efficiency, short laterals (100 feet) or systems with pressure compensating emitters may have higher efficiencies.

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