I rrigation Water QualityStandards

andSalinity Management

Strategies

B-16674-03

Nearly all waters containdissolved salts and traceelements, many of whichresult from the naturalweathering of the earth’ssurface. In addition,drainage waters from irri-gated lands and effluentfrom city sewage andindustrial waste water canimpact water quality. Inmost irrigation situations,the primary water qualityconcern is salinity levels,since salts can affect boththe soil structure and cropyield. However, a number oftrace elements are found inwater which can limit itsuse for irrigation.

Generally, “salt” is thoughtof as ordinary table salt(sodium chloride). How-

ever, many types of saltsexist and are commonlyfound in Texas waters(Table 1). Most salinityproblems in agricultureresult directly from thesalts carried in the irriga-tion water. The process atwork is illustrated inFigure 1, which shows abeaker of water containinga salt concentration of 1percent. As water evapo-rates, the dissolved saltsremain, resulting in a solu-tion with a higher concen-tration of salt. The sameprocess occurs in soils.Salts as well as other dis-solved substances begin toaccumulate as water evapo-rates from the surface andas crops withdraw water.

Water Analysis:Units, Terms and

Sampling

Numerous parameters areused to define irrigationwater quality, to assesssalinity hazards, and todetermine appropriate man-agement strategies. A com-plete water quality analysiswill include the determina-tion of:

1) the total concentration ofsoluble salts,

2) the relative proportion ofsodium to the othercations,

3) the bicarbonate concen-tration as related to theconcentration of calciumand magnesium, and

4) the concentrations ofspecific elements andcompounds.

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I rrigation Water Quality Standardsand Salinity Management

Guy Fipps*

Table 1. Kinds of salts normally found in irrigation waters, with chemical symbols and approxi -mate proportions of each salt.1 (Longenecker and Lyerly, 1994)

Chemical name Chemical symbol Approximate proportionof total salt content

Sodium chloride NaCl Moderate to large

Sodium sulfate Na2SO4 Moderate to large

Calcium chloride CaCl2 Moderate

Calcium sulfate (gypsum) CaSO4 2H2O Moderate to small

Magnesium chloride MgCl2 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 none1Waters vary greatly in amounts and kinds of dissolved salts. This water typifies many used for irrigation in Texas.

*Associate Professor and ExtensionAgricultural Engineer, Department ofAgricultural Engineering, The TexasA&M University System, CollegeStation, Texas 77843-2117.

The amounts and combina-tions of these substancesdefine the suitability ofwater for irrigation and thepotential for plant toxicity.Table 2 defines commonparameters for analyzingthe suitability of water forirrigation and providessome useful conversions.

When taking water samplesfor laboratory analysis,keep in mind that waterfrom the same source canvary in quality with time.Therefore, samples shouldbe tested at intervalsthroughout the year, partic-ularly during the potentialirrigation period. The Soiland Water Testing Lab atTexas A&M University cando a complete salinityanalysis of irrigation waterand soil samples, and willprovide a detailed computerprintout on the interpreta-tion of the results. Contactyour county Extensionagent for forms and infor-mation or contact the Labat (979) 845-4816.

T wo Types of SaltProblems

Two types of salt problemsexist which are very differ-ent: those associated withthe total salinity and thoseassociated with sodium.Soils may be affected onlyby salinity or by a combi-nation of both salinity andsodium.

Salinity HazardWater with high salinity istoxic to plants and poses asalinity hazard. Soils withhigh levels of total salinityare call saline soils. Highconcentrations of salt inthe soil can result in a“physiological” droughtcondition. That is, eventhough the field appears tohave plenty of moisture,

the plants wilt because theroots are unable to absorbthe water. Water salinity isusually measured by theTDS (total dissolved solids)or the EC (electric conduc-tivity). TDS is sometimesreferred to as the totalsalinity and is measured orexpressed in parts per mil-lion (ppm) or in the equiva-lent units of milligrams perliter (mg/L).

EC is actually a measure-ment of electric current andis reported in one of threepossible units as given inTable 2. Subscripts are usedwith the symbol EC to iden-tify the source of the sam-ple. ECiw is the electric con-ductivity of the irrigationwater. ECe is the electricconductivity of the soil asmeasured in a soil sample(saturated extract) taken

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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 Problemsaffects can lead to

salinity plants saline soilhazard condition

affects can lead to

sodium soils sodic soilcondition

calculated from the ratio ofsodium to calcium andmagnesium. The latter twoions are important sincethey tend to counter theeffects of sodium. Forwaters containing signifi-cant amounts of bicarbon-ate, the adjusted sodiumadsorption ratio (SARadj) issometimes used.

Continued use of water hav-ing a high SAR leads to abreakdown in the physicalstructure of the soil.Sodium is adsorbed andbecomes attached to soilparticles. The soil thenbecomes hard and compactwhen dry and increasinglyimpervious to water pene-tration. Fine textured soils,especially those high inclay, are most subject tothis action. Certain amend-ments may be required tomaintain soils under highSARs. Calcium and magne-sium, if present in the soilin large enough quantities,will counter the effects ofthe sodium and help main-tain good soil properties.

Soluble sodium per cent(SSP) is also used to evalu-ate sodium hazard. SSP isdefined as the ration ofsodium in epm (equivalentsper million) to the totalcation epm multiplied by100. A water with a SSPgreater than 60 per centmay result in sodium accu-mulations that will cause abreakdown in the soil’sphysical properties.

Ions, Trace Elements andOther ProblemsA number of other sub-stances may be found inirrigation water and cancause toxic reactions inplants (Table 3). After sodi-um, chloride and boron are

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Table 2. Terms, units, and useful conversions for understanding water quality analysis reports.

Symbol Meaning Units

Total Salinity

a. EC electric conductivity mmhos/cmµmhos/cmdS/m

b. TDS total dissolved solids mg/Lppm

Sodium Hazard

a. SAR sodium adsorption ratio —

b. ESP exchangeable sodium percentage —

Determination Symbol Unit of measure Atomic weight

Constituents

(1) cationscalcium Ca mol/m3 40.1magnesium Mg mol/m3 24.3sodium Na mol/m3 23.0potassium K mol/m3 39.1

(2) anionsbicarbonate HCO3 mol/m3 61.0sulphate SO4 mol/m3 96.1chloride Cl mol/m3 35.5carbonate CO3 mol/m3 60.0nitrate 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

from the root zone. ECd isthe soil salinity of the satu-rated extract taken frombelow the root zone. ECd isused to determine the salin-ity of the drainage waterwhich leaches below theroot zone.

Sodium HazardIrrigation water containinglarge amounts of sodium isof special concern due tosodium’s effects on the soiland poses a sodiumhazard. Sodium hazard isusually expressed in termsof SAR or the sodiumadsorption ratio. SAR is

of most concern. In certainareas of Texas, boron con-centrations are excessivelyhigh and render waterunsuitable for irrigations.Boron can also accumulatein the soil.

Crops grown on soils hav-ing an imbalance of calci-um and magnesium mayalso exhibit toxic symp-

toms. Sulfate salts affectsensitive crops by limitingthe uptake of calcium andincreasing the adsorptionof sodium and potassium,resulting in a disturbancein the cationic balancewithin the plant. The bicar-bonate ion in soil solutionharms the mineral nutri-tion of the plant through

its effects on the uptakeand metabolism of nutri-ents. High concentrationsof potassium may introducea magnesium deficiencyand iron chlorosis. Animbalance of magnesiumand potassium may betoxic, but the effects of bothcan be reduced by high cal-cium levels.

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Table 3. Recommended limits for constituents in reclaimed water for irrigation. (Adapted from Rowe and Abdel-Magid, 1995)

Constituent Long-term Short-term Remarksuse (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 acidifi-cation 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 ofI rrigation Water

Several different measure-ments are used to classifythe suitability of water forirrigation, including ECiw,the total dissolved solids,and SAR. Some permissiblelimits for classes of irriga-tion water are given inTable 4. In Table 5, the sodi-um hazard of water isranked from low to veryhigh based on SAR values.

Classification of Salt-Affected Soils

Both ECe and SAR are com-monly used to classify salt-affected soils (Table 6).Saline soils (resulting fromsalinity hazard) normallyhave a pH value below 8.5,are relatively low in sodiumand contain principallysodium, calcium and mag-nesium chlorides and sul-

fates. These compoundscause the white crustwhich forms on the surfaceand the salt streaks alongthe furrows. The com-pounds which cause salinesoils are very soluble inwater; therefore, leachingis usually quite effective inreclaiming these soils.

Sodic soils (resulting fromsodium hazard) generallyhave a pH value between8.5 and 10. These soils arecalled “black alkali soils”due to their darkenedappearance and smooth,slick looking areas causedby the dispersed condition.In sodic soils, sodium hasdestroyed the permanentstructure which tends tomake the soil impervious to

water. Thus, leachingalone will not be effectiveunless the high salt dilu-tion method or amend-ments are used.

Water QualityEffects on Plantsand Crop Yield

Table 7 gives the expectedyield reduction of somecrops for various levels ofsoil salinity as measuredby EC under normal grow-ing conditions, and Table 8gives potential yield reduc-tion due to water salinitylevels. Generally foragecrops are the most resistantto salinity, followed by fieldcrops, vegetable crops, andfruit crops which are gen-erally the most sensitive.

Table 9 lists the chloridetolerance of a number ofagricultural crops. Boronis a major concern in someareas. While a necessarynutrient, high boron levelscause plant toxicity, andconcentrations should notexceed those given in Table10. Some information isavailable on the susceptibil-ity of crops to foliar injuryfrom spray irrigation withwater containing sodium

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Table 4. Permissible limits for classes of irrigation water.

Concentration, total dissolved solids

Classes of water Electrical Gravimetric ppmconductivity µmhos*

Class 1, Excellent 250 175

Class 2, Good 250-750 175-525

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

Class 4, Doubtful2 2,000-3,000 1,400-2,100

Class 5, Unsuitable2 3,000 2,100

*Micromhos/cm at 25 degrees C.1Leaching needed if used2Good drainage needed and sensitive plants will have difficulty obtainingstands

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

and chloride (Table 11). Thetolerance of crops to sodi-um as measured by theexchangeable sodium per-centage (ESP) is given inTable 12.

Salinity and Growth StageMany crops have little toler-ance for salinity duringseed germination, but sig-nificant tolerance duringlater growth stages. Somecrops such as barley, wheatand corn are known to bemore sensitive to salinityduring the early growthperiod than during germi-nation and later growthperiods. Sugar beet and saf-flower are relatively moresensitive during germina-tion, while the tolerance ofsoybeans may increase ordecrease during differentgrowth periods dependingon the variety.

Leaching for SalinityManagement

Soluble salts that accumu-late in soils must be leachedbelow the crop root zone tomaintain productivity.Leaching is the basic man-agement tool for control-ling salinity. Water isapplied in excess of thetotal amount used by thecrop and lost to evapora-tion. The strategy is to keepthe salts in solution andflush them below the rootzone. The amount of waterneeded is referred to as theleaching requirement or theleaching fraction.

Excess water may beapplied with every irriga-tion to provide the waterneeded for leaching. How-ever, the time intervalbetween leachings does notappear to be critical provid-ed that crop tolerances are

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Table 7. Soil salinity tolerance levels1 for dif ferent crops. (Adapted from Ayers and Westcot, 1976)

Yield potential, ECe

Crop 100% 90% 75% 50% Maximum ECe

Field cropsBarleya 8.0 10.0 13.0 18.0 28Bean (field) 1.0 1.5 2.3 3.6 7Broad bean 1.6 2.6 4.2 6.8 12Corn 1.7 2.5 3.8 5.9 10Cotton 7.7 9.6 13.0 17.0 27Cowpea 1.3 2.0 3.1 4.9 9Flax 1.7 2.5 3.8 5.9 10Groundnut 3.2 3.5 4.1 4.9 7Rice (paddy) 3.0 3.8 5.1 7.2 12Safflower 5.3 6.2 7.6 9.9 15Sesbania 2.3 3.7 5.9 9.4 17Sorghum 4.0 5.1 7.2 11.0 18Soybean 5.0 5.5 6.2 7.5 10Sugar beet 7.0 8.7 11.0 15.0 24Wheata 6.0 7.4 9.5 13.0 20Vegetable cropsBean 1.0 1.5 2.3 3.6 7Beetb 4.0 5.1 6.8 9.6 15Broccoli 2.8 3.9 5.5 8.2 14Cabbage 1.8 2.8 4.4 7.0 12Cantaloupe 2.2 3.6 5.7 9.1 16Carrot 1.0 1.7 2.8 4.6 8Cucumber 2.5 3.3 4.4 6.3 10Lettuce 1.3 2.1 3.2 5.2 9Onion 1.2 1.8 2.8 4.3 8Pepper 1.5 2.2 3.3 5.1 9Potato 1.7 2.5 3.8 5.9 10Radish 1.2 2.0 3.1 5.0 9Spinach 2.0 3.3 5.3 8.6 15Sweet corn 1.7 2.5 3.8 5.9 10Sweet potato 1.5 2.4 3.8 6.0 11Tomato 2.5 3.5 5.0 7.6 13Forage cropsAlfalfa 2.0 3.4 5.4 8.8 16Barley haya 6.0 7.4 9.5 13.0 20Bermudagrass 6.9 8.5 10.8 14.7 23Clover, Berseem 1.5 3.2 5.9 10.3 19Corn (forage) 1.8 3.2 5.2 8.6 16Harding grass 4.6 5.9 7.9 11.1 18Orchard grass 1.5 3.1 5.5 9.6 18Perennial rye 5.6 6.9 8.9 12.2 19Sudan grass 2.8 5.1 8.6 14.4 26Tall fescue 3.9 5.8 8.61 3.3 23Tall wheat grass 7.5 9.9 13.3 19.4 32Trefoil, big 2.3 2.8 3.6 4.9 8Trefoil, small 5.0 6.0 7.5 10.0 15Wheat grass 7.5 9.0 11.0 15.0 22

not exceeded. Hence, leach-ing can be accomplishedwith each irrigation, everyfew irrigations, once yearly,or even longer dependingon the severity of the salini-ty problem and salt toler-ance of the crop. An occa-sional or annual leachingevent where water is pondedon the surface is an easyand effective method forcontrolling soil salinity. Insome areas, normal rainfallprovides adequate leaching.

Determining RequiredLeaching FractionThe leaching fraction iscommonly calculated usingthe following relationship:

ECiwLF = (1)

ECe

where

LF = leaching fraction - the fraction of applied irrigation water that must be leached through the root zone

ECiw =electric conductiv-ity of the irriga-tion water

ECe = the electric con-ductivity of the soil in the root zone

Equation (1) can be used todetermine the leaching frac-tion necessary to maintainthe root zone at a targetedsalinity level. If the amountof water available for leach-ing is fixed, then the equa-tion can be used to calculatethe salinity level that will bemaintained in the root zonewith that amount of leach-ing. Please note that equa-tion (1) simplifies a compli-cated soil water process. ECeshould be checked periodi-

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Table 7. Soil salinity tolerance levels1 for dif ferent crops. (continued)

Yield potential, ECe

Crop 100% 90% 75% 50% Maximum ECe

Fruit cropsAlmond 1.5 2.0 2.8 4.1 7Apple, Pear 1.7 2.3 3.3 4.8 8Apricot 1.6 2.0 2.6 3.7 6Avocado 1.3 1.8 2.5 3.7 6Date palm 4.0 6.8 10.9 17.9 32Fig, Olive,

Pomegranate 2.7 3.8 5.5 8.4 14Grape 1.5 2.5 4.1 6.7 12Grapefruit 1.8 2.4 3.4 4.9 8Lemon 1.7 2.3 3.3 4.8 8Orange 1.7 2.3 3.2 4.8 8Peach 1.7 2.2 2.9 4.1 7Plum 1.5 2.1 2.9 4.3 7Strawberry 1.0 1.3 1.8 2.5 4Walnut 1.7 2.3 3.3 4.8 81Based on the electrical conductivity of the saturated extract taken from aroot zone soil sample (ECe) measured in mmhos/cm.

aDuring germination and seedling stage ECe should not exceed 4 to 5mmhos/cm except for certain semi-dwarf varieties.

bDuring germination ECe should not exceed 3 mmhos/cm.

Table 8. Irrigation water salinity tolerances1 for dif ferent crops. (Adapted from Ayers and Westcot, 1976)

Yield potential, ECi w

Crop 100% 90% 75% 50%

Field cropsBarley 5.0 6.7 8.7 12.0Bean (field) 0.7 1.0 1.5 2.4Broad bean 1.1 1.8 2.0 4.5Corn 1.1 1.7 2.5 3.9Cotton 5.1 6.4 8.4 12.0Cowpea 0.9 1.3 2.1 3.2Flax 1.1 1.7 2.5 3.9Groundnut 2.1 2.4 2.7 3.3Rice (paddy) 2.0 2.6 3.4 4.8Safflower 3.5 4.1 5.0 6.6Sesbania 1.5 2.5 3.9 6.3Sorghum 2.7 3.4 4.8 7.2Soybean 3.3 3.7 4.2 5.0Sugar beet 4.7 5.8 7.5 10.0Wheat 4.0 4.9 6.4 8.7Vegetable cropsBean 0.7 1.0 1.5 2.4Beet 2.7 3.4 4.5 6.4Broccoli 1.9 2.6 3.7 5.5

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Table 8. Irrigation water salinity tolerances1 for dif ferent crops. (continued)

Yield potential, ECi w

Crop 100% 90% 75% 50%

Cabbage 1.2 1.9 2.9 4.6Cantaloupe 1.5 2.4 3.8 6.1Carrot 0.7 1.1 1.9 3.1Cucumber 1.7 2.2 2.9 4.2Lettuce 0.9 1.4 2.1 3.4Onion 0.8 1.2 1.8 2.9Pepper 1.0 1.5 2.2 3.4Potato 1.1 1.7 2.5 3.9Radish 0.8 1.3 2.1 3.4Spinach 1.3 2.2 3.5 5.7Sweet corn 1.1 1.7 2.5 3.9Sweet potato 1.0 1.6 2.5 4.0Tomato 1.7 2.3 3.4 5.0Forage cropsAlfalfa 1.3 2.2 3.6 5.9Barley hay 4.0 4.9 6.3 8.7Bermudagrass 4.6 5.7 7.2 9.8Clover, Berseem 1.0 2.1 3.9 6.8Corn (forage) 1.2 2.1 3.5 5.7Harding grass 3.1 3.9 5.3 7.4Orchard grass 1.0 2.1 3.7 6.4Perennial rye 3.7 4.6 5.9 8.1Sudan grass 1.9 3.4 5.7 9.6Tall fescue 2.6 3.9 5.7 8.9Tall wheat grass 5.0 6.6 9.0 13.0Trefoil, big 1.5 1.9 2.4 3.3Trefoil, small 3.3 4.0 5.0 6.7Wheat grass 5.0 6.0 7.4 9.8Fruit cropsAlmond 1.0 1.4 1.9 2.7Apple, Pear 1.0 1.6 2.2 3.2Apricot 1.1 1.3 1.8 2.5Avocado 0.9 1.2 1.7 2.4Date palm 2.7 4.5 7.3 12.0Fig, Olive,

Pomegranate 1.8 2.6 3.7 5.6Grape 1.0 1.7 2.7 4.5Grapefruit 1.2 1.6 2.2 3.3Lemon 1.1 1.6 2.2 3.2Orange 1.1 1.6 2.2 3.2Peach 1.1 1.4 1.9 2.7Plum 1.0 1.4 1.9 2.8Strawberry 0.7 0.9 1.2 1.7Walnut 1.1 1.6 2.2 3.21Based on the electrical conductivity of the irrigation water (ECiw) measuredin mmhos/cm.

cally and the amount ofleaching adjusted accord-ingly.

Based on this equation,Table 13 lists the amount ofleaching needed for differ-ent classes of irrigationwaters to maintain the soilsalinity in the root zone ata desired level. However,additional water must besupplied because of theinefficiencies of irrigationsystems (Table 14), as wellas to remove the existingsalts in the soil.

Subsurface Drainage

Very saline, shallow watertables occur in many areasof Texas. Shallow watertables complicate salinitymanagement since watermay actually move upwardinto the root zone, carryingwith it dissolved salts.Water is then extracted bycrops and evaporation,leaving behind the salts.

Shallow water tables alsocontribute to the salinityproblem by restricting thedownward leaching of saltsthrough the soil profile.Installation of a subsurfacedrainage system is aboutthe only solution availablefor this situation. Theoriginal clay tiles have beenreplaced by plastic tubing.Modern drainage tubes arecovered by a “sock” made offabric to prevent cloggingof the small openings inthe plastic tubing.

A schematic of a subsurfacedrainage system is shownin Figure 2. The designparameters are the distancebetween drains (L) and theelevation of the drains (d)above the underlyingimpervious or restrictinglayer. Proper spacing and

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depth maintain the waterlevel at an optimum level,shown here as the distancem above the drain tubes.The USDA NaturalResources ConservationService (NRCS) has devel-oped drainage designguidelines that are usedthroughout the UnitedStates. A drainage comput-er model developed byWayne Skaggs at NorthCarolina State University,DRAINMOD, is also widelyused throughout the worldfor subsurface drainagedesign.

Seed Placement

Obtaining a satisfactorystand is often a problemwhen furrow irrigatingwith saline water. Growerssometimes compensate forpoor germination by planti-ng two or three times asmuch seed as normallywould be required.However, planting proce-dures can be adjusted to

lower the salinity in the soilaround the germinatingseeds. Good salinity controlis often achieved with acombination of suitablepractices, bed shapes andirrigation water manage-ment.

In furrow-irrigated soils,planting seeds in the centerof a single-row, raised bedplaces the seeds exactlywhere salts are expected toconcentrate (Figure 3). Thissituation can be avoidedusing “salt ridges.” With adouble-row raised plantingbed, the seeds are placednear the shoulders andaway from the area ofgreatest salt accumulation.Alternate-furrow irrigationmay help in some cases. Ifalternate furrows are irri-gated, salts often can bemoved beyond the singleseed row to the non-irrigat-ed side of the planting bed.Salts will still accumulate,but accumulation at thecenter of the bed will bereduced.

With either single- or dou-ble-row plantings, increas-ing the depth of the waterin the furrow can improvegermination in saline soils.Another practice is to usesloping beds, with the seedsplanted on the sloping sidejust above the water line(Fig. 3b). Seed and plantplacement is also importantwith the use of drip irriga-tion. Typical wetting pat-terns of drip emitters andmicro-sprinklers are shownin Figure 4. Salts tend tomove out and upward, andwill accumulate in the areasshown.

Other SalinityManagementTechniques

Techniques for controllingsalinity that require rela-tively minor changes aremore frequent irrigations,selection of more salt-toler-ant crops, additional leach-

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

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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.

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ing, preplant irrigation, bedforming and seed place-ment. Alternatives thatrequire significant changesin management are chang-ing the irrigation method,altering the water supply,land-leveling, modifying thesoil profile, and installingsubsurface drainage.

Residue ManagementThe common saying “saltloves bare soils” refers tothe fact that exposed soilshave higher evaporationrates than those covered byresidues. Residues left onthe soil surface reduce evap-oration. Thus, less salts willaccumulate and rainfall willbe more effective in provid-ing for leaching.

More Frequent Irrigations Salt concentrations increasein the soil as water isextracted by the crop.Typically, salt concentra-tions are lowest followingan irrigation and higherjust before the next irriga-tion. Increasing irrigationfrequency maintains a moreconstant moisture contentin the soil. Thus, more ofthe salts are then kept insolution which aids theleaching process. Surgeflow irrigation is often effec-tive at reducing the mini-mum depth of irrigationthat can be applied with fur-row irrigation systems.Thus, a larger number ofirrigations are possibleusing the same amount ofwater.

With proper placement, dripirrigation is very effective atflushing salts, and watercan be applied almost con-tinuously. Center pivotsequipped with LEPA waterapplicators offer similar effi-ciencies and control as drip

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/m3

ppm

Strawberry 10 350

Bean 10 350

Onion 10 350

Carrot 10 350

Radish 10 350

Lettuce 10 350

Turnip 10 350

Rice, paddyc

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

Sesbaniac

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, redc

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

trate. Chemical amend-ments are used in order tohelp facilitate the displace-ment of these sodium ions.Amendments are composedof sulphur in its elementalform or related compoundssuch as sulfuric acid andgypsum. Gypsum also con-tains calcium which is animportant element in cor-recting these conditions.Some chemical amendmentsrender the natural calciumin the soil more soluble. Asa result, calcium replacesthe adsorbed sodium whichhelps restore the infiltra-tion capacity of the soil.Polymers are also begin-ning to be used for treatingsodic soils.

It is important to note thatuse of amendments doesnot eliminate the need forleaching. Excess watermust still be applied toleach out the displacedsodium. Chemical amend-ments are only effective onsodium-affected soils.Amend-ments are ineffec-tive for saline soil condi-tions and often willincrease the existing salini-ty problem. Table 15 liststhe most common amend-ments. The irrigation bookslisted under the Referencessection present equationsthat are used to determinethe amount of amendmentsneeded based on soil analy-sis results.

Pipe Water DeliverySystems Stabilize SalinityAs illustrated in Fig. 1, anyopen water is subject toevaporation which leads tohigher salt concentrationsin the water. Evaporationrates from water surfacesoften exceed 0.25 inch aday during summer inTexas. Thus, the salinity

14

Table 9. Chloride tolerance of agricultural crops. Listed in order of tolerancea. (continued)

Maximum Cl-concentration

b

without loss in yield

Crop mol/m3

ppm

Ryegrass, perennial 55 1,925

Wheat, Durum 55 1,925

Barley (forage)c

60 2,100

Wheatc

60 2,100

Sorghum 70 2,450

Bermudagrass 70 2,450

Sugar beetc

70 2,450

Wheat grass, fairway crested 75 2,625

Cotton 75 1,625

Wheat grass, tall 75 2,625

Barleyc

80 2,800aThese data serve only as a guideline to relative tolerances among crops.Absolute tolerances vary, depending upon climate, soil conditions and cultural practices.

bCl–

concentrations in saturated-soil extracts sampled in the rootzone.cLess tolerant during emergence and seedling stage.dValues for paddy rice refer to the Cl

–concentration in the soil water during

the flooded growing conditions.

irrigation at less than halfthe cost. Both sprinkler anddrip provide more controland flexibility in schedulingirrigation than furrow sys-tems.

Preplant IrrigationSalts often accumulate nearthe soil surface during fal-low periods, particularlywhen water tables are highor when off-season rainfallis below normal. Underthese conditions, seed ger-mination and seedlinggrowth can be seriouslyreduced unless the soil isleached before planting.

Changing SurfaceIrrigation MethodSurface irrigation methods,such as flood, basin, furrowand border are usually notsufficiently flexible to per-mit changes in frequency of

irrigation or depth of waterapplied per irrigation. Forexample, with furrow irri-gation it may not be possi-ble to reduce the depth ofwater applied below 3-4inches. As a result, irrigat-ing more frequently mightimprove water availabilityto the crop but might alsowaste water. Converting tosurge flow irrigation maybe the solution for manyfurrow systems. Otherwisea sprinkler or drip irriga-tion system may berequired.

Chemical AmendmentsIn sodic soils (or sodiumaffected soils), sodium ionshave become attached toand adsorbed onto the soilparticles. This causes abreakdown in soil structureand results in soil sealingor “cementing,” making itdifficult for water to infil-

15

content of irrigation waterwill increase during theentire time water is trans-ported through irrigationcanals or stored in reser-

voirs. Replacing irrigationditches with pipe systemswill help stabilize salinitylevels. In addition, pipe sys-tems, including gated pipe

and lay-flat tubing, reducewater lost to canal seepageand increase the amount ofwater available for leaching.

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 TolerantExcellent <0.33 <0.67 <1.00Good 0.33 to 0.67 0.67 to 1.33 1.00 to 2.00Permissible 0.67 to 1.00 1.33 to 2.00 2.00 to 3.00Doubtful 1.00 to 1.25 2.00 to 2.50 3.00 to 3.75Unsuitable >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)

PecanWalnut (Black, Persian, or English)Jerusalem artichokeNavy beanAmerican elmPlumPearAppleGrape (Sultania and Malaga)Kadota figPersimmonCherryPeachApricotThornless blackberryOrangeAvocadoGrapefruitLemon

(0.3 mg/L of Boron)

Sunflower (native)PotatoCotton (Acala and Pima)TomatoSweetpeaRadishField peaRagged Robin roseOliveBarleyWheatCornMiloOatZinniaPumpkinBell pepperSweet potatoLima bean

(1.0 mg/L of Boron)

Athel (Tamarix aphylla)AsparagusPalm (Phoenix canariensis)Date palm (P. dactylifera)Sugar beetMangelGarden beetAlfalfaGladiolusBroad beanOnionTurnipCabbageLettuce 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 CauliflowerApricot Pepper Barley CottonCitrus Potato Corn Sugar beetPlum Tomato Cucumber Sunflower

SafflowerSesameSorghum

aFoliar injury is influenced by cultural and environmental conditions. Thesedata 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

CitrusAvocado

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 conditionsRiceDallisgrass

Tolerant Wheat Stunted growth usually due to(ESP = 40-60) Cotton adverse physical conditions of soil

AlfalfaBarleyTomatoesBeets

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 theirrigation 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 Percent0.75 13.3 9.4 6.3 4.71.00 25.0 12.5 8.3 6.31.25 31.3 15.6 10.4 7.81.50 37.5 18.7 12.5 9.42.00 50.0 25.0 16.7 12.52.50 62.5 31.3 20.8 15.63.00 75.0 37.5 25.0 18.75.00 — 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 1.0Sulfur† Yellow element 0.2Sulfuric acid* Corrosive liquid 0.6Lime sulfur* Yellow-brown solution 0.8Calcium carbonate† White mineral 0.6Calcium chloride* White salt 0.9Ferrous sulfate* Blue-green salt 1.6Pyrite† Yellow-black mineral 0.5Ferric sulfate* Yellow-brown salt 0.6Aluminum 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.S ystem O verall efficiency (%)

Surface 50-80a. average 50b. land leveling and delivery pipeline meeting design standards 70c. tailwater recovery with (b) 80d. surge 60-90*

Sprinkler (moving and fixed systems) 55-85LEPA (low pressure precision application) 95-98Drip 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 compen-

sating emitters may have higher efficiencies.

References

Ayres, R.S. and D.W.Westcot. 1976. WaterQuality for Agriculture.Irrigation and DrainagePaper No. 29. Food andAgricultureOrganization of theUnited Nations. Rome.

Cuena, R.H. 1989.Irrigation SystemDesign. Prentice Hall,Englewood Cliffs, NJ.552pp.

Hoffman, G.S., R.S. Ayers,E.J. Doering and B.L.McNeal. 1980. Salinityin Irrigated Agriculture.In: Design andOperation of FarmIrrigation Systems.M.E. Jensen, Editor.ASAE Monograph No. 3.St. Joseph, MI. 829pp.

James, D.W., R.J. Hanksand J.H. Jurinak. 1982.Modern Irrigated Soils.John Wiley and Sons,NY.

Jensen, M.E. (Editor). 1980.Design and Operation ofFarm IrrigationSystems. AmericanSociety of AgriculturalEngineers, St. JosephMI. 829pp.

Longenecker, D.E. and P.J.Lyerly. 1974. B-876Control of Soluble Saltsin Farming andGardening. TexasAgricultural ExperimentStation, Texas A&MUniversity System,College Station. June.36pp.

Pair, C.H. (editor). 1983.Irrigation. TheIrrigation Assoc.,Arlington, VA. 680pp.

Rowe, D.R. and I.M. Abdel-Magid. 1995. Handbookof WastewaterReclamation and Reuse.CRC Press, Inc. 550pp.

Stewart, B.A. and D.R.Nielsen. 1990. Irrigationof Agricultural Crops.American Society ofAgronomy. 1,218pp.

Tanji, K.K. 1990.Agricultural SalinityAssessment andManagement. AmericanSociety of CivilEngineers. Manuals andReports on EngineeringPractice Number 71.619pp.

van der Leeden, F., F.L.Troise and D.K. Todd.1990. The WaterEncyclopedia. LewisPublishers. 808pp.

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