Irrigation Water

L-541703/02

Salinity is becoming aproblem in many areas of

Texas. As water qualityand cropping patterns

change, salinity may injurecrops and reduce yield.

Susceptibility to salt injuryvaries by crop. It is

important that producersunderstand why and how

to measure salts and how crop susceptibility

to salts may differ.

Why well water can be salty

Irrigation water quality isdetermined by the totalamounts of salts and the

types 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. The types andamounts of salts in water,and thus the salinity ofthat water, depend on thesource.

The 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 levels

and producewater that ismore salty.The quality ofsurface waterdepends large-ly on thesource ofrunoff.Drainagewater from

irrigated land, saline seeps,oil fields, and city andindustrial wastewaters gen-erally has higher salt levels.

What 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; Associate Professor andExtension Agronomist

Table 1 Critical Values for Salts in Irrigation Water for Major CropsMEASUREMENT PEANUTS CORN GRAIN SORGHUM COTTON

Sodium Adsorption Ratio (SAR)No units (just a number) 10 10 10 10

Total Dissolved Salts (Electrical Conductivity or Total Dissolved Solids*)Micromhos per centimeter (umhos.cm) 2100 1100 1700 5100Microsiemens per centimeter (uS/cm) 2100 1100 1700 5100Millimhos per meter (mmhos/cm) 2.1 1.1 1.7 5.1Decisiemens per meter (dS/m) 2.1 1.1 1.7 5.1Parts per million (ppm) 1344 704 1088 3264Milligrams per liter (mg/L) 1344 704 1088 3264

Toxic Ions (Resulting in Foliar Injury)

BoronParts per million (ppm) 0.75 2.0 3.0 3.0Milligrams per liter (mg/L) 0.075 2.0 3.0 3.0Milliequivalents per liter (meq/L) 0.075 0.2 0.3 0.3

ChlorideParts per million (ppm) 400-500 533 710 710Milligrams per liter (mg/L) 400-500 533 710 710Milliequivalents per liter (meq/L) 11-14 15 20 20

SodiumParts per million (ppm) 400-500 533 710 710Milligrams per liter (mg/L) 400-500 533 710 710Milliequivalents per liter (meq/L) 17-21 23 31 31

*Different units of measurement for total soluble salts represent the same critical value

and sodium hazard. 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 hazard

because 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 water

often cause marginal leafburn and, in severe cases,can lead to defoliation andsignificant yield loss.Sodium hazard is caused byhigh levels of sodium,which can be toxic toplants and damage medi-

um and fine-textured soils.When the sodium level in asoil becomes high, the soilwill lose its structure,become dense and formhard crusts on the surface.

What tests should be doneon irrigation water?

To evaluate a salt haz-ard, a water sampleshould be analyzed

for three major factors:

• Total soluble salts.

• Sodium hazard (SAR).

• Toxic ions.

Total soluble salts meas-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 prob-lem.

Sodium hazard is based ona calculation of the sodiumadsorption ratio (SAR). 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 ions include ele-ments like chloride, sulfate,sodium and boron.Sometimes, 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.

What are the critical levels?

Agricultural crops dif-fer greatly in theirability to tolerate

salts. 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 and

each type of salt test orproblem. One of the mostconfusing factors is that therecan be many different unitsof measurement for the sametest. That 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. 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 crops.

Management factors

Irrigation water with asalt level near the criti-cal value is referred to as

“marginal” quality water.In some cases, marginalquality water can be used

Water analyses can be accurate only if the sample is takencorrectly. Please use the following guidelines when collecting

a well water sample for irrigation water quality analysis:

Containers

Samples should be collected in a clean, plastic bottle with a screw cap.Wash bottles thoroughly before taking samples to eliminate any contami-nation. An 8-ounce plastic, disposable baby bottle is the best kind of

container 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.

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 mixture.

Testing should also be done on irrigation water from ponds, reservoirs,streams or other surface water sources. Samples can be obtained by collectingwater from a faucet near the pumping station after operating for 20 minutesor longer. For irrigation water sources where no pump is present, obtain sam-ples by attaching a clean bottle to a pole or extension and collecting and mix-ing several samples into a “composite,” which is sent to the laboratory.

Package and mail all samples to the laboratory as soon as possible to preventchemical changes in the water during storage. Keep good records of the dateand location of each sample. This can best be done by keeping a copy of theLaboratory Information Sheet that must be submitted with each sample.

In most cases, a Routine Irrigation Water Analysis is the most appropriate testto request for irrigation water. Regardless of the laboratory selected, be cer-tain that the analysis includes the three major factors—total soluble salts, sodi-um hazard (SAR) and individual potentially toxic ions. For special cases or ifuncertain, contact your County Extension Office for information.

For additional information, see our website at http://soilcrop.tamu.edu.

HOW TO GET A WATER TESTto produce a crop, recog-nizing that some loss inyield (10 percent to 75 per-cent) may occur. Plants cancontinue to grow in thepresence of low salts, butthe yield potential will notbe maximized. Plantsgrown in salty soils or irri-gated with salty water arealways in a drought-stressed condition.

Management systems formarginal quality watermust be carefully designed.Major factors that must beconsidered include soiltype, internal drainage, irri-gation system and methods(rates, frequency) and crop-ping systems. Growersshould consult an experi-enced agronomist or irriga-tion specialist for assistancein planning a managementstrategy for using marginalquality irrigation water.

Educational programs of Texas Cooperative Extensionare open to all people without regard to race, color,sex, disability, religion, age or national origin.

Issued in furtherance of Cooperative Extension Workin Agriculture and Home Economics, Acts of Congressof May 8, 1914, as amended, and June 30, 1914, incooperation with the United States Department ofAgriculture. Chester P. Fehlis, Deputy Director, TexasCooperative Extension, The Texas A&M UniversitySystem.

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