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Irrigation Water Quality - Texas A&M University

Irrigation Water L-541703/02 Salinity is becoming aproblem in many areas ofTexas. As Water qualityand cropping patternschange, salinity may injurecrops and reduce to salt injuryvaries by crop. It is important that producersunderstand why and howto measure salts and how crop susceptibility to salts may well Water can be saltyIrrigation Water Quality isdetermined by the totalamounts of salts and thetypes of salts present in thewater. A salt is a combina-tion of twoelements orions. One hasa positivecharge (forexample,sodium), andthe other hasa negativecharge (suchas chloride). Water may contain a vari-ety of salts including sodi-um chloride (table salt),sodium sulfate, calciumchloride, calcium sulfate(gypsum), magnesium chlo-ride, etc.

Irrigation Water L-5417 03/02 Salinity is becoming a problem in many areas of Texas. As water quality and cropping patterns change, salinity may injure

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Transcription of Irrigation Water Quality - Texas A&M University

1 Irrigation Water L-541703/02 Salinity is becoming aproblem in many areas ofTexas. As Water qualityand cropping patternschange, salinity may injurecrops and reduce to salt injuryvaries by crop. It is important that producersunderstand why and howto measure salts and how crop susceptibility to salts may well Water can be saltyIrrigation Water Quality isdetermined by the totalamounts of salts and thetypes of salts present in thewater. A salt is a combina-tion of twoelements orions. One hasa positivecharge (forexample,sodium), andthe other hasa negativecharge (suchas chloride). Water may contain a vari-ety of salts including sodi-um chloride (table salt),sodium sulfate, calciumchloride, calcium sulfate(gypsum), magnesium chlo-ride, etc.

2 The types andamounts of salts in Water ,and thus the salinity ofthat Water , depend on Quality of well waterdepends on the composi-tion of the undergroundformations from which the Water is pumped. Whenthese are marine (ocean)formations, they usuallywill have higher salt levelsand producewater that ismore Quality ofsurface waterdepends large-ly on thesource fromirrigated land, saline seeps,oil fields, and city andindustrial wastewaters gen-erally has higher salt problems can saltywater cause?Salty Irrigation watercan cause two majorproblems in crop pro-duction salinity hazard,Mark McFarland, Robert Lemon and Charles Stichler*Critical Salt Levels for Peanuts, Cotton,Corn and Grain Sorghum*Associate Professor and Extension SoilFertility Specialist; Associate Professor andExtension Agronomist.

3 Associate Professor andExtension AgronomistTable 1 Critical Values for Salts in Irrigation Water for Major CropsMEASUREMENTPEANUTSCORNGRAIN SORGHUM COTTONS odium Adsorption Ratio (SAR)No units (just a number)1010 1010 Total Dissolved Salts (Electrical Conductivity or Total Dissolved Solids*)Micromhos per centimeter ( )2100110017005100 Microsiemens per centimeter (uS/cm)2100110017005100 Millimhos per meter (mmhos/cm) per meter (dS/m) per million (ppm)134470410883264 Milligrams per liter (mg/L) 134470410883264 Toxic Ions (Resulting in Foliar Injury)BoronParts per million (ppm) per liter (mg/L) per liter (meq/L) per million (ppm)400-500533710710 Milligrams per liter (mg/L)400-500533710710 Milliequivalents per liter (meq/L)11-14152020 SodiumParts per million (ppm)400-500533710710 Milligrams per liter (mg/L)400-500533710710 Milliequivalents per liter (meq/L)17-21233131*Different units of measurement for total soluble salts represent the same critical valueand sodium hazard.

4 Whenirrigation Water is used byplants or evaporates fromthe soil surface, salts con-tained in the Water are leftbehind and can accumulatein the soil. These salts cre-ate a salinity hazardbecause they compete withplants for Water . Even if asaline soil is Water saturat-ed, plant roots may beunable to absorb the Water ,and plants will show signsof drought stress. Foliarapplications of salty wateroften cause marginal leafburn and, in severe cases,can lead to defoliation andsignificant yield hazard is caused byhigh levels of sodium,which can be toxic toplants and damage medi-um and fine-textured the sodium level in asoil becomes high, the soilwill lose its structure,become dense and formhard crusts on the tests should be doneon Irrigation Water ?

5 To evaluate a salt haz-ard, a Water sampleshould be analyzedfor three major factors: Total soluble salts. Sodium hazard (SAR). Toxic soluble saltsmeas-ures the salinity hazard byestimating the combinedeffects of all the differentsalts that may be in thewater. It is measured as theelectrical conductivity (EC)of the Water . Salty watercarries an electrical currentbetter than pure Water , andEC rises as the amount ofsalt increases. Many peoplemake the mistake of testingonly for chlorides, butchlorides are only one partof the salts and do notdetermine the entire hazardis based ona calculation of the sodiumadsorption ratio (SAR).

6 Thismeasurement determines ifsodium levels are highenough to damage the soilor if the concentration isgreat enough to reduceplant growth. Sometimes afactor called the exchange-able sodium percentage(ESP) may be listed or dis-cussed on a Water test;however, this is actually ameasurement of soil salini-ty, not Water Quality . Toxic ionsinclude ele-ments like chloride, sulfate,sodium and , even thoughthe salt level is not exces-sive, one or more of theseelements may become toxicto plants. Many plants areparticularly sensitive toboron. In general, it is bestto request a Water analysisthat lists the concentra-tions of all major cations(calcium , magnesium,sodium, potassium) andanions (chloride, sulfate,nitrate, boron) so that thelevels of all elements canbe evaluated.

7 What are the critical levels?Agricultural crops dif-fer greatly in theirability to toleratesalts. Some crops have spe-cial methods for managinghigh salt levels inside theplant that allow them tocontinue to grow and pro-duce. In most cases, criticallevels have been estab-lished for each crop andeach type of salt test orproblem. One of the mostconfusing factors is that therecan be many different unitsof measurement for the is, the numbershave the same relativemeaning, but the units ofmeasurement used toexpress the value are differ-ent (much like saying 12inches or 1 foot).The Texas CooperativeExtension Soil, Water andForage Testing Laboratoryuses standard units ofmicromhos per centimeter(umhos/cm) for total solu-ble salts and parts per mil-lion (ppm) for individualions.

8 Other laboratoriesmay use different units ofmeasure that can be calcu-lated by making simpleconversions. Table 1 liststhe different tests and cor-responding critical valuesfor different units of meas-urement. These values rep-resent the maximum saltlevel in Irrigation waterthat can be used withoutreducing crop yield. Keepin mind that these valuesare estimates. Actual cropresponse may vary depend-ing on soil type, rainfall, Irrigation frequency andweather conditions. Notecotton s ability to toleratehigher levels of salt thanother common Texas factorsIrrigation Water with asalt level near the criti-cal value is referred to as marginal Quality some cases, marginalquality Water can be usedWater analyses can be accurate only if the sample is takencorrectly.

9 Please use the following guidelines when collectinga well Water sample for Irrigation Water Quality analysis:ContainersSamples should be collected in a clean, plastic bottle with a screw bottles thoroughly before taking samples to eliminate any contami-nation. An 8-ounce plastic, disposable baby bottle is the best kind ofcontainer to use. Rinse the container several times with the Water to be testedbefore collecting the final sample. Always clearly identify each container witha specific sample identification (well site). When mailing samples, place thebottles in a box or pack them with a soft packing material (newspaper or sty-rofoam) to prevent crushing.

10 Collecting the Water sampleWhen testing well Water , allow the pump to operate for at least 20 minutesbefore taking the sample to be sure the Water is representative of what isbeing tested. Take the Water sample at the pump so that residues from thelines do not contaminate the sample. If two or more wells supply an irrigationsystem, one sample may be taken from the system after pumping (flushing) forat least one hour. However, if a Water test indicates a problem, all wells sup-plying the system will need to be tested individually to determine the source ofthe problem. Sometimes one poor Quality well can dramatically reduce thequality of a should also be done on Irrigation Water from ponds, reservoirs,streams or other surface Water sources.


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