Managing salts in soil and irrigation water - GCSAA

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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
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