Managing salts in soil and irrigation water - GCSAA

GCM - January 2001 - Research - Managing salts in soil and irrigation waterR E S E A R C HManaging salts in soiland irrigation waterAn integrated approach helps superintendents grow healthy turf.Sowmya (Shoumo) Mitra, Ph.D.KEY POINTS■ Multiple options allow superintendentsto improve poorqualitywater or reduce itseffects on turfgrass.■ Calcium, particle aggregationand leaching can all help thesuperintendent deal with lowqualityirrigation water.■ Foliar fertilizers can replacenutrients lost to leaching orother salt-reduction strategies.The quality and quantity of irrigationwater is becoming a limiting factor inmaintaining high-quality turf for golfcourses all across the United States. Manygolf courses are experiencing salt problems.The global demand for fresh potablewater is doubling every 20 years, and golfcourse superintendents are forced to seekalternative water sources (5).Effluent water is being considered asan alternative irrigation source, and alot of golf courses in the southwesternUnited States have started using recycledwater (9). As golf course superintendentsswitch to alternative watersources, they must use an integratedapproach and proper agronomic practicesbased on individual sites.Management practices should be basedon soil and irrigation water reportsfrom a particular location.Irrigation water testsThe following items need to beexamined for salinity hazards from irrigationwater:• Sodium adsorption ratio (SAR) oradjusted SAR (adj. SAR)• Electrical conductivity (EC)• Total dissolved solids (TDS)• Amount of sodium [Na + ], calcium[Ca ++ ], magnesium [Mg ++ ],carbonate [CO 3=], and bicarbonate[HCO 3- ] ions• Residual sodium carbonate (RSC): RSCis calculated by the following equation:RSC = [CO 3=+ HCO 3- ] – [Ca+++ Mg ++ ]Note: The units must be expressed asmilliequivalents per liter. Generally, labsreport values in parts per million ormilligrams per liter. Such values mustbe divided by the equivalent weightto obtain milliequivalents per liter.Calcium sourceSoils with high amounts of sodiumwill tend to disperse soil particles and, asa result, reduce aeration and infiltrationinto the soil profile (2). High carbonatesand bicarbonates in irrigation water willreact with the available calcium and magnesiumto form insoluble salts. In orderto avoid these problems with irrigationwater and amended soils, which have ahigh exchangeable sodium percentage,we must add a soluble source of calcium.70 Golf Course Management ■ January 2001

GCM - January 2001 - Research - Managing salts in soil and irrigation waterR E S E A R C HPhotos courtesy of R.R. DuncanHigh rates of gypsum may be mixed into sodic soils to correct problems introduced by poorqualitywater.Calcium can be added in formssuch as gypsum, limestone, dolomite,lime and so on. Gypsum is the mostcommon source, because it has littleeffect on the soil pH and has high watersolubility. Gypsum is calcium sulfatedihydrate (CaSO 4 , 2H 2 O). Whengypsum comes in contact with soilsolution, it generates calcium ions,which replace sodium from the soilexchange complex.The soil exchange complex has negativecharges, which are generated mostlyby clay and organic matter. Clay mineralshave a negative charge caused bystructural substitution in their lattice(isomorphic substitution). Organicmatter, however, has negative charges onits surface as a result of dissociation ofthe carboxyl acid groups (-COOH) ofhumic and fulvic acids.The sodium sulfate (Na 2 SO 4 )formed in the soil must be removedfrom the soil profile by leaching.Addition of polyvalent (more than onecharge) cations such as calcium andmagnesium will flocculate the soil.These cations hold soil particles becausetheir charges form cationic bridges.Flocculation is a prerequisite for soilaggregation, and a well-aggregated soilforms water-stable aggregates. A wellaggregatedsoil has increased porosity,aeration and infiltration.Gypsum requirementThe gypsum required by a particulargolf course depends on the quality ofirrigation water and soil. The relativeamounts of sodium, calcium, magnesium,carbonate and bicarbonate determinethe amount of gypsum requiredto treat irrigation water.There are two ways to figurethe gypsum requirement: RSC andEaton’s gypsum requirement (EGR).The RSC method takes into accountthe amount of carbonate and bicarbonatein water compared to calciumand magnesium. It does not accountfor the sodium. Hence, it tends tounderestimate the gypsum requirement.This method is useful when theamount of carbonate and bicarbonateis high, but the amount of sodium islow compared to calcium and magnesium.The units must be in milliequivalentsper liter.Golf Course Management ■ January 2001 71

GCM - January 2001 - Research - Managing salts in soil and irrigation waterR E S E A R C HRSC methodRSC method = 234 × [CO 3=+ HCO 3- ]– [Ca+++ Mg ++ ].The results are reported in pounds of100 percent grade gypsum per acre foot ofwater. If we use a lower grade of gypsum,for example, 95 percent, then we need toadd the extra 5 percent to the value.Eaton’s gypsum requirementEGR is calculated based on sodium,calcium, magnesium, carbonate andbicarbonate. EGR is calculated by thefollowing equation:Soil testThe major sources of salt problemscan be traced to irrigation water, culturalpractices (mainly fertilization) andsalt water intrusion. Hence, when weplan to treat the problem, we have totreat the irrigation water and alsoaddress the salt problem in the soil.The following items need to beobserved for soil salinity hazards:• Sodium absorption ratio (SAR)• Electrical conductivity (EC)• Exchangeable sodium percentage(ESP)ESP is calculated by the following equation:EGR = 234 × [a + b + c], wherea = 0.43 × Na + - (Ca ++ + Mg ++ )b = 0.7 × CO 3=+ HCO 3-c = 0.3.The results are reported in poundsof 100 percent grade gypsum per acrefoot of water. If we use a lower grade ofgypsum, for example 97 percent, thenwe need to add the extra 3 percent tothe value.ESP ( percent) = [Exchangeable sodium/Soil cation exchange capacity] × 100.The units for the amount of sodiumshould be in milliequivalents per liter.Acid injectionMany golf course superintendentshave started using acid-injection systems,but they need to be careful inselecting the right tool to countersodium and bicarbonate (HCO - 3 ) hazards.Acid-injection systems include theuse of sulfurous generators, sulfuricacid (H 2 SO 4 ), urea plus sulfuric acid(monocarbamide dihydrogensulfate) orphosphoric acid (H 3 PO 4 ) (1). Sulfurousgenerators use elemental sulfur and oxidizeit to SO 3 by burning in the presenceof oxygen. The SO 3 gas reacts withwater to form sulfuric acid (H 2 SO 4 ).Sulfuric acid (H 2 SO 4 ) reacts withbicarbonates (HCO - 3 ) to form carbondioxide (CO 2 ), and hence removes thebicarbonates, which would have reactedwith Ca and Mg to form insoluble precipitates(3).H 2 SO 4 + HCO 3- → HSO4 - + H2 O + CO 2Seashore paspalum is the best choice for sites with poor-quality water in warm climates.Sulfur is added through burning sothat it will react with the lime alreadypresent in the soil to form gypsum(CaSO 4 ). However, gypsum will notform if excess lime is not present. Even72 Golf Course Management ■ January 2001

GCM - January 2001 - Research - Managing salts in soil and irrigation waterwhen gypsum is added to the soil, thegypsum may revert to insoluble lime(CaCO 3 ) in the presence of excessHCO 3- and CO3 = (3). Hence, in orderto use sulfur to form gypsum, it is necessarytoremove excessive HCO 3- andCO 3=. Superintendents should be verycareful when they use sulfur burners,because, under anaerobic conditions,reduced forms of sulfur may lead toblack-layer problems later on. Therefore,sulfur products should be used inareas that are well aerated and have noinfiltration problems.Another reason superintendents useacid-injection systems is to reduce soilpH. Unfortunately soil pH cannot bechanged easily by such acid applications,because soil has a huge bufferingcapacity. Reducing soil pH is very difficultor even impossible in highlybuffered, calcareous soils (3). It is muchmore practical and also less expensive toreduce soil pH by spreading sulfur overdry soil as an amendment.A few superintendents use acid injectionin order to prevent the precipitationof calcium and magnesium carbonatesin irrigation plumbing, but thisproblem can be solved with muchcheaper materials. A weak organic acidand a weak organic base combinationwill take care of the problem, and thesetypes of materials are readily availableand relatively inexpensive.on threshold EC values, but final selectionshould be made after checking thegrowth vs. EC level, threshold EC valuesfor 50 percent growth reduction ofshoot and root. Seashore paspalum hashigh salinity tolerance, can toleratewaterlogging and has a low soil-oxygenrequirement (4). These attributes makeseashore paspalum an ideal turfgrasscultivar for warm-climate golf coursesfacing severe salinity problems.LeachingLeaching of excess salts is the mostimportant step in managing salts.Without leaching, the replaced sodiumand the excess salts beyond the rootzone plague turf. It is difficult to leachpush-up greens or other parts of thegolf course that have finer soil texture.Leaching is more of an art, which asuperintendent learns through experienceand knowledge about the soil texture,the amount of water required orthe time needed for a particular green.If we try to understand the movementof water through a soil profile, wewill find that, according to the laws ofhydraulic conductivity, a particular soilhorizon will release water as soon as itreaches field capacity. Water movesAtomic weightsR E S E A R C HSelection of turfgrassProper turfgrass selection is animportant management tool when salinityproblems are anticipated. Differentcultivars have differential responses tosalinity levels. For example, commoncentipedegrass (Eremochloa ophiuroides)is sensitive to salt levels, whereasseashore paspalum (Paspalum vaginatum)can tolerate higher salinity levels(14 decisiemens per meter) (1). Salt-sensitiveplants (glycophytes) should bereplaced with halophytes. When selectinga turfgrass species, it is advisable tocheck the growth versus EC curves forthat particular species. Turfgrass selectionfor salinity tolerance is often basedFor some calculations of water quality, units must be expressed as millequivalentsper liter. Generally, labs report values in parts per million or milligramsper liter. Such values need to be divided by the equivalent weight toobtain millequivalents per liter.Ions Atomic/molecular Equivalentweight (grams) Valence weight (meq/liter)Ca ++ 40 2 20Mg ++ 24 2 12Na + 23 1 23HCO 3- 61 1 61CO 3=60 2 30Golf Course Management ■ January 2001 73

GCM - January 2001 - Research - Managing salts in soil and irrigation waterR E S E A R C Hthrough the soil profile in concentriccircles. Within a particular soil horizon,water moves laterally (instead of immediatelymoving downward) until thehorizon reaches field capacity. For thisreason, deep, infrequent irrigation ispreferred over light, frequent irrigation.For a particular golf course, it isimportant to determine the amount ofwater required to flush the greens. Thefraction of water that infiltrates the soilsurface and passes through the rootzone is called the leaching fraction (LF).For practical reasons, the LF is calculatedas:LF = EC W / EC DW,where EC W is the electrical conductivity(EC) of the irrigation water, and EC DWis the EC of the drainage water. Hence,the salt content of drainage water can beadjusted by adjusting the LF.The leaching requirement (LR) isused to represent the minimum amountof water that must pass through theroot zone to control salts to an acceptablelevel (2). The LR is expressed interms of the percentage of water thatneeds to be applied to meet the evapotranspiration(ET) needs of a particularturfgrass.The oldest method of calculation (7)uses the ratio of EC of irrigation water(EC W ) to the salinity tolerance of the turfgrassbeing grown (EC TS ) to determinethe amount of water that must be addedto leach excess salts from the root zone:LR = [EC W / EC TS ] × 100,where the EC TS is the threshold valuethat a particular turfgrass can tolerate.For example, if we are irrigating agolf course with mainly Kentucky bluegrass(Poa pratensis) with an EC TS valueof 3.0 decisiemens per meter from awater source having an EC W value of1.5 decisiemens per meter, the leachingrequirement can be calculated as:LR = [1.5 / 3.0] × 100 = 50%In other words, growing Kentuckybluegrass with this irrigation waterrequires an additional 50 percent irrigationSalt toleranceTurfgrass species vary in their tolerance of saline conditions. Greater electrical conductivity (EC) tolerance indicatesgreater salt tolerance (4).EC range causingCommon name Scientific name Threshold EC range 50% growth reductionAnnual bluegrass Poa annua 0-3 —Rough bluegrass Poa trivialis 0-3 —Kentucky bluegrass Poa pratensis 0-6 3-30Centipedegrass Eremochloa ophiuroides 0-3 8-9Common bermudagrass Cynodon dactylon 0-12 12-32Tall fescue Festuca arundinacea 5-10 8-12Perennial ryegrass Lolium perenne 3-10 8-10St. Augustinegrass Stenotaphrum secundatum 0-18 22-44Seashore paspalum Paspalum vaginatum 0-20 18-49Creeping bentgrass Agrostis palustris 0-10 8Zoysiagrass Zoysia species 0-11 4-4074 Golf Course Management ■ January 2001

GCM - January 2001 - Research - Managing salts in soil and irrigation waterwater to leach salts beyond the root zone.MicroorganismsProper leaching requires improvingthe soil structure. Structure buildinginvolves two different but complementaryprocesses. The coherence of particlesin the structural units or aggregatesrequires the presence of cementing substancesand the mechanical forces thatdisintegrate and mold the soil mass intostable aggregates (7).Microorganisms can bind soil particlesby adhesion of extracellular polymersand enzymes and by charge interactions(6). Several researchers haveinvestigated the role of microorganismsin enhancing soil aggregation. Somemicroorganisms secrete polysaccharides,which help in soil aggregation,and during the past 40 years, study ofmicrobial polysaccharides has dominatedthe field (7). Microbial polysaccharidesmay be divided into threegroups according to their morphologicallocalization:• Extracellular surface polysaccharideslocated outside the cell wall and frequentlytermed “capsular polysaccharides”• Cell-wall polysaccharides• Somatic or intracellular polysaccharideslocated inside the cytoplasmicmembrane (11)Researchers have reported that certainspecies of Bacillus, Leuconostoc andMycor secrete polysaccharides that areinvolved in binding soil particles intowater-stable aggregates (7, 8). Microbialpolysaccharides provide the glue thatcements soil particles together, whereasfungal mycelia physically encompass(like a rope) the particles to increasetheir physical association.Soil management is one of the mostimportant factors influencing the structureof soils. The interaction of managementwith soil biochemistry and soilmicrobiology determines the extent ofsoil aggregation. Soil mean aggregate sizehas been correlated with soil monosaccharidecontent, total soil protein contentand extractable humic substances (10).Hence, the addition of monosaccharidesand protein is recommended.In conclusion, a golf course superintendentshould develop an integratedsalt-management program in order togrow healthy turf including the followingstrategies:• Using or adding a soluble source ofcalcium in soil• Increasing soil aggregation toimprove aeration and infiltration• Leaching excess salts and displacedsodium from the root zone• Applying foliar nutrients to replenishthe leached-out nutrients ■AcknowledgmentHelp from John M. Doyle is highly appreciated.Literature cited1. Carrow, R.N., and R.R. Duncan. 1998. Saltaffectedturfgrass sites: Assessment and management,Ann Arbor Press, Chelsea, Mich.2. Carrow, R.N., R.R. Duncan and M. Huck.1999. Treating the cause, not symptoms.USGA Green Section Record 37(6):11-15.3. Christians, N. 1999. Why inject acid intoirrigation water? Golf Course Management67(6):52-56.4. Duncan, R.R., and R.N. Carrow. 2000. Soonon golf courses: New seashore paspalums.Golf Course Management 68(5):65-67.5. Duncan, R.R., R.N. Carrow and M. Huck.2000. Effective use of seawater irrigation onturfgrass. USGA Green Section Record38(1):11-17.6. Fletcher, M., M. Latham, J. Lynch and P.R.Rutter. 1980. The characteristics of interfacesand their role in microbial attachment.p. 67-78. In: R.C.W. Berkley, J.M. Lynch, J.Melling, P.R. Rutter and B. Vincent (eds.),Microbial adhesion to surfaces. EllisHorwood Limited, Chichester, England.7. Griffiths, E. 1965. Micro-organisms and soilstructure. Biological Review 40:129-142.8. Harris, R.F., C. Chesters, O.N. Allen and O.J.Attoe. 1964. Mechanisms involved in soilaggregate stabilization by fungi and bacteria.Soil Science Society Proceedings 24:529-532.9. Huck, M., R.N. Carrow and R.R. Duncan.2000. Effluent water: Nightmare or dreamcome true? USGA Green Section Record38(2):15-29.10. Martens, D.A. 2000. Management and cropresidue influence soil aggregate stability.Journal of Environmental Quality 29:723-727.11. Stacey, M., and S.A. Barker. 1960.Polysaccharides of micro-organisms.Clarendon Press, Oxford.Sowmya (Shoumo) Mitra, Ph.D., is product managerfor Eco Soil Systems Inc.R E S E A R C HGolf Course Management ■ January 2001 75