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Irrigation Water Quality Standards and Salinity Management Strategies B-1667 4-03

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Irrigation Water QualityStandards

andSalinity Management

Strategies

B-16674-03

Nearly all waters contain dis-solved salts and trace elements,many of which result from thenatural weathering of the earth’ssurface. In addition, drainagewaters from irrigated lands andeffluent from city sewage andindustrial waste water can impactwater quality. In most irrigationsituations, the primary water qual-ity concern is salinity levels, sincesalts can affect both the soil struc-ture and crop yield. However, anumber of trace elements arefound in water which can limit itsuse for irrigation.

Generally, “salt” is thought of asordinary table salt (sodium chlo-ride). How-ever, many types ofsalts exist and are commonlyfound in Texas waters (Table 1).Most salinity problems in agricul-ture result directly from the saltscarried in the irrigation water.The process at work is illustratedin Figure 1, which shows abeaker of water containing a saltconcentration of 1 percent. Aswater evaporates, the dissolvedsalts remain, resulting in a solu-tion with a higher concentrationof salt. The same process occursin soils. Salts as well as other dis-solved substances begin to accu-mulate as water evaporates fromthe surface and as crops withdrawwater.

Water Analysis:Units, Terms and

Sampling

Numerous parameters are used todefine irrigation water quality, toassess salinity hazards, and todetermine appropriate managementstrategies. A complete water quali-ty analysis will include the deter-mination of:

1) the total concentration of solu-ble salts,

2) the relative proportion of sodi-um to the other cations,

3) the bicarbonate concentration asrelated to the concentration ofcalcium and magnesium, and

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Irrigation 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 Texas A&MSystem, College Station, Texas 77843-2117.

4) the concentrations of specificelements and compounds.

The amounts and combinations ofthese substances define the suit-ability of water for irrigation andthe potential for plant toxicity.Table 2 defines common parame-ters for analyzing the suitability ofwater for irrigation and providessome useful conversions.

When taking water samples forlaboratory analysis, keep in mindthat water from the same sourcecan vary in quality with time.Therefore, samples should be test-ed at intervals throughout theyear, particularly during thepotential irrigation period. TheSoil and Water Testing Lab atTexas A&M University can do acomplete salinity analysis of irri-gation water and soil samples, andwill provide a detailed computerprintout on the interpretation ofthe results. Contact your countyExtension agent for forms andinformation or contact the Lab at(979) 845-4816.

Two Types of SaltProblems

Two types of salt problems existwhich are very different: thoseassociated with the total salinityand those associated with sodium.Soils may be affected only bysalinity or by a combination ofboth salinity and sodium.

Salinity HazardWater with high salinity is toxicto plants and poses a salinity haz-ard. Soils with high levels oftotal salinity are call saline soils.High concentrations of salt in thesoil can result in a “physiologi-cal” drought condition. That is,even though the field appears tohave plenty of moisture, theplants wilt because the roots areunable to absorb the water. Watersalinity is usually measured by

the TDS (total dissolved solids) orthe EC (electric conductivity).TDS is sometimes referred to asthe total salinity and is measuredor expressed in parts per million(ppm) or in the equivalent units ofmilligrams per liter (mg/L).

EC is actually a measurement ofelectric current and is reported inone of three possible units asgiven in Table 2. Subscripts areused with the symbol EC to iden-tify the source of the sample.ECiw is the electric conductivityof the irrigation water. ECe is theelectric conductivity of the soil asmeasured in a soil sample (satu-rated extract) taken from the rootzone. ECd is the soil salinity ofthe saturated extract taken frombelow the root zone. ECd is usedto determine the salinity of thedrainage water which leachesbelow the root zone.

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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

ingly impervious to water pene-tration. Fine textured soils, espe-cially those high in clay, are mostsubject to this action. Certainamendments may be required tomaintain soils under high SARs.Calcium and magnesium, if pre-sent in the soil in large enoughquantities, will counter the effectsof the sodium and help maintaingood soil properties.

Soluble sodium per cent (SSP) isalso used to evaluate sodium haz-ard. SSP is defined as the rationof sodium in epm (equivalents permillion) to the total cation epmmultiplied by 100. A water with aSSP greater than 60 per cent mayresult in sodium accumulationsthat will cause a breakdown in thesoil’s physical properties.

Ions, Trace Elementsand Other ProblemsA number of other substancesmay be found in irrigation waterand can cause toxic reactions inplants (Table 3). After sodium,chloride and boron are of mostconcern. In certain areas of Texas,boron concentrations are exces-sively high and render waterunsuitable for irrigations. Boroncan also accumulate in the soil.

Crops grown on soils having animbalance of calcium and magne-sium may also exhibit toxicsymptoms. Sulfate salts affectsensitive crops by limiting theuptake of calcium and increasingthe adsorption of sodium andpotassium, resulting in a distur-bance in the cationic balancewithin the plant. The bicarbonateion in soil solution harms themineral nutrition of the plantthrough its effects on the uptakeand metabolism of nutrients. Highconcentrations of potassium mayintroduce a magnesium deficiencyand iron chlorosis. An imbalanceof magnesium and potassium maybe toxic, but the effects of bothcan be reduced by high calciumlevels.

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

Symbol Meaning Units

Total Salinitya. EC electric conductivity mmhos/cm

µmhos/cmdS/m

b. TDS total dissolved solids mg/Lppm

Sodium Hazarda. SAR sodium adsorption ratio —

b. ESP exchangeable sodium percentage —

Determination Symbol Unit of measure Atomic weightConstituents(1) cations

calcium 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 Elementsboron B mg/L 10.8

Conversions1 dS/m = 1 mmhos/cm = 1000 µmhos/cm1 mg/L = 1 ppmTDS (mg/L) ≈ EC (dS/m) x 640 for EC < 5 dS/mTDS (mg/L ≈ EC (dS/m) x 800 for EC > 5 dS/mTDS (lbs/ac-ft) ≈ TDS (mg/L) x 2.72Concentration (ppm) = Concentration (mol/m3) times the atomic weight

Sum of cations/anions(meq/L) ≈ EC (dS/m) x 10Keymg/L = milligrams per literppm = parts per milliondS/m = deci Siemens per meter at 25° C

Sodium HazardIrrigation water containing largeamounts of sodium is of specialconcern due to sodium’s effectson the soil and poses a sodiumhazard. Sodium hazard is usuallyexpressed in terms of SAR or thesodium adsorption ratio. SAR iscalculated from the ratio of sodi-um to calcium and magnesium.The latter two ions are importantsince they tend to counter the

effects of sodium. For waters con-taining significant amounts ofbicarbonate, the adjusted sodiumadsorption ratio (SARadj) is some-times used.

Continued use of water having ahigh SAR leads to a breakdownin the physical structure of thesoil. Sodium is adsorbed andbecomes attached to soil particles.The soil then becomes hard andcompact when dry and increas-

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Table 3. Recommended limits for constituents in reclaimed water for irrigation. (Adapted fromRowe 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 8.0will precipitate the ion and eliminate toxicity.

Arsenic (As) 0.10 2.0 Toxicity to plants varies widely, ranging from 12 mg/L for Sudangrass 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 to0.5 mg/L for bush beans.

Boron (B) 0.75 2.0 Essential to plant growth, with optimum yields for many obtainedat a few-tenths mg/L in nutrient solutions. Toxic to many sensitiveplants (e.g., citrus) at 1 mg/L. Most grasses relatively tolerant at2.0 to 10 mg/L.

Cadmium (Cd) 0.01 0.05 Toxic to beans, beets, and turnips at concentrations as low as 0.1mg/L in nutrient solution. Conservative limits recommended.

Chromium (Cr) 0.1 1.0 Not generally recognized as essential growth element. Conservativelimits 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 beinactivated 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 tocitrus 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 acidsoils.

Molybdenum (Mo) 0.01 0.05 Nontoxic to plants at normal concentrations in soil and water. Canbe toxic to livestock if forage is grown in soils with high levels ofavailable molybdenum.

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

Selenium (Se) 0.02 0.02 Toxic to plants at low concentrations and to livestock if forage isgrown 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; reducedtoxicity at increased pH (6 or above) and in fine-textured or organicsoils.

Classification ofIrrigation Water

Several different measurements areused to classify the suitability ofwater for irrigation, includingECiw, the total dissolved solids,and SAR. Some permissible limitsfor classes of irrigation water aregiven in Table 4. In Table 5, thesodium hazard of water is rankedfrom low to very high based onSAR values.

Classification ofSalt-Affected Soils

Both ECe and SAR are commonlyused to classify salt-affected soils(Table 6). Saline soils (resultingfrom salinity hazard) normallyhave a pH value below 8.5, are rel-atively low in sodium and containprincipally sodium, calcium andmagnesium chlorides and sulfates.These compounds cause the whitecrust which forms on the surface

and the salt streaks along the fur-rows. The compounds whichcause saline soils are very solublein water; therefore, leaching isusually quite effective in reclaim-ing these soils.

Sodic soils (resulting from sodi-um hazard) generally have a pHvalue between 8.5 and 10. Thesesoils are called “black alkalisoils” due to their darkenedappearance and smooth, slicklooking areas caused by the dis-persed condition. In sodic soils,sodium has destroyed the perma-nent structure which tends tomake the soil impervious towater. Thus, leaching alone willnot be effective unless the highsalt dilution method or amend-ments are used.

Water QualityEffects on Plantsand Crop Yield

Table 7 gives the expected yieldreduction of some crops for vari-ous levels of soil salinity as mea-sured by EC under normal grow-ing conditions, and Table 8 givespotential yield reduction due towater salinity levels. Generallyforage crops are the most resistantto salinity, followed by fieldcrops, vegetable crops, and fruitcrops which are generally themost sensitive.

Table 9 lists the chloride toler-ance of a number of agriculturalcrops. Boronis a major concern in some areas.While a necessary nutrient, highboron levels cause plant toxicity,and concentrations should notexceed those given in Table 10.Some information is available onthe susceptibility of crops tofoliar injury from spray irrigationwith water containing sodium andchloride (Table 11). The toleranceof crops to sodium as measuredby the exchangeable sodium per-centage (ESP) is given in Table12.

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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 avocadosmust 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 ofsaturation 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 andGrowth StageMany crops have little tolerancefor salinity during seed germina-tion, but significant tolerance dur-ing later growth stages. Somecrops such as barley, wheat andcorn are known to be more sensi-tive to salinity during the earlygrowth period than during germi-nation and later growth periods.Sugar beet and safflower are rela-tively more sensitive during ger-mination, while the tolerance ofsoybeans may increase ordecrease during different growthperiods depending on the variety.

Leaching for SalinityManagement

Soluble salts that accumulate insoils must be leached below thecrop root zone to maintain pro-ductivity. Leaching is the basicmanagement tool for controllingsalinity. Water is applied in excessof the total amount used by thecrop and lost to evaporation. Thestrategy is to keep the salts insolution and flush them below theroot zone. The amount of waterneeded is referred to as the leach-ing requirement or the leachingfraction.

Excess water may be applied withevery irrigation to provide thewater needed for leaching. How-ever, the time interval betweenleachings does not appear to becritical provided that crop toler-ances are not exceeded. Hence,leaching can be accomplishedwith each irrigation, every fewirrigations, once yearly, or evenlonger depending on the severityof the salinity problem and salttolerance of the crop. An occa-sional or annual leaching eventwhere water is ponded on the sur-face is an easy and effectivemethod for controlling soil salini-ty. In some areas, normal rainfallprovides adequate leaching.

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Table 7. Soil salinity tolerance levels1 for different 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

Determining RequiredLeaching FractionThe leaching fraction is commonlycalculated using the followingrelationship:

ECiwLF = (1)

ECe

where

LF = leaching fraction- the fraction ofapplied irrigationwater that mustbe leachedthrough the rootzone

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

ECe = the electric con-ductivity of thesoil in the rootzone

Equation (1) can be used to deter-mine the leaching fraction neces-sary to maintain the root zone at atargeted salinity level. If theamount of water available forleaching is fixed, then the equationcan be used to calculate the salini-ty level that will be maintained inthe root zone with that amount ofleaching. Please note that equation(1) simplifies a complicated soilwater process. ECe should bechecked periodically and theamount of leaching adjustedaccordingly.

Based on this equation, Table 13lists the amount of leaching need-ed for different classes of irriga-tion waters to maintain the soilsalinity in the root zone at adesired level. However, additionalwater must be supplied because ofthe inefficiencies of irrigation sys-tems (Table 14), as well as toremove the existing salts in thesoil.

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

Yield potential, ECiw

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 different crops.(continued)

Yield potential, ECiw

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 5.6

Grape 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.

Subsurface Drainage

Very saline, shallow water tablesoccur in many areas of Texas.Shallow water tables complicatesalinity management since watermay actually move upward intothe root zone, carrying with it dis-solved salts. Water is then extract-ed by crops and evaporation, leav-ing behind the salts.

Shallow water tables also con-tribute to the salinity problem byrestricting the downward leachingof salts through the soil profile.Installation of a subsurfacedrainage system is about the onlysolution available for this situa-tion. The original clay tiles havebeen replaced by plastic tubing.Modern drainage tubes are cov-ered by a “sock” made of fabricto prevent clogging of the smallopenings in the plastic tubing.

A schematic of a subsurfacedrainage system is shown inFigure 2. The design parametersare the distance between drains(L) and the elevation of the drains(d) above the underlying impervi-ous or restricting layer. Properspacing and depth maintain thewater level at an optimum level,shown here as the distance mabove the drain tubes. The USDANatural Resources ConservationService (NRCS) has developeddrainage design guidelines thatare used throughout the UnitedStates. A drainage computermodel developed by WayneSkaggs at North Carolina StateUniversity, DRAINMOD, is alsowidely used throughout the worldfor subsurface drainage design.

Seed Placement

Obtaining a satisfactory stand isoften a problem when furrow irri-gating with saline water. Growerssometimes compensate for poorgermination by planting two orthree times as much seed as nor-mally would be required.

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However, planting procedures canbe adjusted to lower the salinity inthe soil around the germinatingseeds. Good salinity control isoften achieved with a combinationof suitable practices, bed shapesand irrigation water management.

In furrow-irrigated soils, plantingseeds in the center of a single-row,raised bed places the seeds exactlywhere salts are expected to con-centrate (Figure 3). This situationcan be avoided using “salt ridges.”With a double-row raised plantingbed, the seeds are placed near theshoulders and away from the areaof greatest salt accumulation.Alternate-furrow irrigation mayhelp in some cases. If alternatefurrows are irrigated, salts oftencan be moved beyond the singleseed row to the non-irrigated sideof the planting bed. Salts will stillaccumulate, but accumulation atthe center of the bed will bereduced.

With either single- or double-rowplantings, increasing the depth ofthe water in the furrow canimprove germination in saline

soils. Another practice is to usesloping beds, with the seeds plant-ed on the sloping side just abovethe water line (Fig. 3b). Seed andplant placement is also importantwith the use of drip irrigation.Typical wetting patterns of dripemitters and micro-sprinklers areshown in Figure 4. Salts tend tomove out and upward, and willaccumulate in the areas shown.

Other SalinityManagementTechniques

Techniques for controlling salinitythat require relatively minorchanges are more frequent irriga-tions, selection of more salt-toler-ant crops, additional leaching, pre-plant irrigation, bed forming andseed placement. Alternatives thatrequire significant changes inmanagement are changing the irri-gation method, altering the watersupply, land-leveling, modifyingthe soil profile, and installing sub-surface drainage.

Residue ManagementThe common saying “salt lovesbare soils” refers to the fact thatexposed soils have higher evapora-tion rates than those covered byresidues. Residues left on the soilsurface reduce evaporation. Thus,less salts will accumulate and rain-fall will be more effective in pro-viding for leaching.

More FrequentIrrigationsSalt concentrations increase in thesoil as water is extracted by thecrop. Typically, salt concentrationsare lowest following an irrigationand higher just before the next irri-gation. Increasing irrigation fre-quency maintains a more constantmoisture content in the soil. Thus,more of the salts are then kept insolution which aids the leachingprocess. Surge flow irrigation isoften effective at reducing theminimum depth of irrigation thatcan be applied with furrow irriga-tion systems. Thus, a larger num-ber of irrigations are possibleusing the same amount of water.

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 withsalty 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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With proper placement, drip irriga-tion is very effective at flushingsalts, and water can be appliedalmost continuously. Center pivotsequipped with LEPA water appli-cators offer similar efficiencies andcontrol as drip irrigation at lessthan half the cost. Both sprinklerand drip provide more control andflexibility in scheduling irrigationthan furrow systems.

Preplant IrrigationSalts often accumulate near thesoil surface during fallow periods,particularly when water tables arehigh or when off-season rainfall isbelow normal. Under these condi-tions, seed germination andseedling growth can be seriouslyreduced unless the soil is leachedbefore planting.

Changing SurfaceIrrigation MethodSurface irrigation methods, such asflood, basin, furrow and border areusually not sufficiently flexible topermit changes in frequency ofirrigation or depth of water appliedper irrigation. For example, withfurrow irrigation it may not bepossible to reduce the depth ofwater applied below 3-4 inches.As a result, irrigating more fre-quently might improve water avail-ability to the crop but might alsowaste water. Converting to surgeflow irrigation may be the solutionfor many furrow systems.Otherwise a sprinkler or drip irri-gation system may be required.

Chemical AmendmentsIn sodic soils (or sodium affectedsoils), sodium ions have becomeattached to and adsorbed onto thesoil particles. This causes a break-down in soil structure and resultsin soil sealing or “cementing,”making it difficult for water toinfiltrate. Chemical amendmentsare used in order to help facilitatethe displacement of these sodiumions. Amendments are composed

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

References

Ayres, R.S. and D.W. Westcot.1976. Water Quality forAgriculture. Irrigation andDrainage Paper No. 29. Foodand Agriculture Organizationof the United Nations. Rome.

Cuena, R.H. 1989. IrrigationSystem Design. Prentice Hall,Englewood Cliffs, NJ. 552pp.

Hoffman, G.S., R.S. Ayers, E.J.Doering and B.L. McNeal.1980. Salinity in IrrigatedAgriculture. In: Design andOperation of Farm IrrigationSystems. M.E. Jensen, Editor.ASAE Monograph No. 3. St.Joseph, MI. 829pp.

James, D.W., R.J. Hanks and J.H.Jurinak. 1982. ModernIrrigated Soils. John Wileyand Sons, NY.

Jensen, M.E. (Editor). 1980.Design and Operation ofFarm Irrigation Systems.American Society ofAgricultural Engineers, St.Joseph MI. 829pp.

Longenecker, D.E. and P.J.Lyerly. 1974. B-876 Controlof Soluble Salts in Farmingand Gardening. TexasAgricultural ExperimentStation, Texas A&MUniversity System, CollegeStation. June. 36pp.

Pair, C.H. (editor). 1983.Irrigation. The IrrigationAssoc., Arlington, VA. 680pp.

Rowe, D.R. and I.M. Abdel-Magid. 1995. Handbook ofWastewater Reclamation andReuse. CRC Press, Inc.550pp.

Stewart, B.A. and D.R. Nielsen.1990. Irrigation ofAgricultural Crops. AmericanSociety of Agronomy.1,218pp.

Tanji, K.K. 1990. AgriculturalSalinity Assessment andManagement. AmericanSociety of Civil Engineers.

14

Table 9. Chloride tolerance of agricultural crops. Listed in orderof 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 andcultural 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.

of sulphur in its elemental form orrelated compounds such as sulfu-ric acid and gypsum. Gypsum alsocontains calcium which is animportant element in correctingthese conditions. Some chemicalamendments render the naturalcalcium in the soil more soluble.As a result, calcium replaces theadsorbed sodium which helpsrestore the infiltration capacity ofthe soil. Polymers are also begin-ning to be used for treating sodicsoils.

It is important to note that use ofamendments does not eliminatethe need for leaching. Excesswater must still be applied to leachout the displaced sodium.Chemical amendments are onlyeffective on sodium-affected soils.Amend-ments are ineffective forsaline soil conditions and oftenwill increase the existing salinityproblem. Table 15 lists the mostcommon amendments. The irriga-tion books listed under the

References section present equa-tions that are used to determinethe amount of amendments neededbased on soil analysis results.

Pipe Water DeliverySystems StabilizeSalinityAs illustrated in Fig. 1, any openwater is subject to evaporationwhich leads to higher salt concen-trations in the water. Evaporationrates from water surfaces oftenexceed 0.25 inch a day duringsummer in Texas. Thus, the salini-ty content of irrigation water willincrease during the entire timewater is transported through irriga-tion canals or stored in reservoirs.Replacing irrigation ditches withpipe systems will help stabilizesalinity levels. In addition, pipesystems, including gated pipe andlay-flat tubing, reduce water lostto canal seepage and increase theamount of water available forleaching.

15

Manuals and Reports onEngineering Practice Number71. 619pp.

van der Leeden, F., F.L. Troise andD.K. Todd. 1990. The WaterEncyclopedia. LewisPublishers. 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 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 moretolerant; 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 beanOnionTurnipCabbageLettuceCarrot

(2.0 mg/L of Boron)

Table 11. Relative susceptibility of crops to foliar injury fromsaline 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.

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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 anddrainage 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.

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Table 15. Various amendments for reclaiming sodic soil and amountequivalent to gypsum.

Amendment Physical description Amount equivalent100% 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.System Overall 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.