Exchange Reactions Cation exchange Acid Soils Salt/Sodium Affected Soils Lecture 5

ò humus  200 cmol c /kg ò smectites  100 cmol c /kg ò illite  25 cmol c /kg ò kaolinite  10 cmol c /kg ò Fe and Al oxides  5 cmol c /kg Charge of Soil Components 5.2

Origin of Charge 5.3

Adsorbed cations (a) arid region soils = "basic" cations Ca +2, Mg +2, K +, Na + (b) humid region soils = “acidic” cations as well Ca +2, Mg +2, H + and Al +3 (c) strength of adsorption Al +3 > Ca +2 = Mg +2 > K + = NH 4 + > Na + 5.4

Cation Exchange Exchange process Ca +2 -colloid + 2 H +  2 H + -colloid + Ca +2 = H + replaces Ca +2 adsorbed to soil colloids Ca-x + 2 H +  2 H-x + Ca +2 x = the soil solid phase Ca (ad) + 2 H +  2 H (ad) + Ca +2 X (ad) = "adsorbed" cation X 5.5

before rainfall Saline-Sodic Soils 5.6

after rainfall Our fields! Oh No... Trout Farm? 5.7

Saline Soils EC > 4 ds/m = osmotic stress * salt sensitive plants (EC = 2 ds/m) 3 bean, onion, potato, raspberry, carrot, dogwood, larch, linden, peach, rose, tomato 3 bean, onion, potato, raspberry, carrot, dogwood, larch, linden, peach, rose, tomato * salt tolerant plants (EC = 10 ds/m) 3 sugarbeets, barley, cotton, rosemary, 3 sugarbeets, barley, cotton, rosemary, wheat grass, wild rye wheat grass, wild rye (see table 10.2) (see table 10.2) 5.8

Sodic Soils flocculation poorwaterinfiltration dispersion 5.9

Sodium Ion Effect flocculationdispersion  attraction  Ca +2 & Mg +2  repulsion  Na

SAR Parameter Predict sodium effect from saturated soil extract or irrigation water SAR approximately equals ESP SAR is measured ë ë ë ESP is estimated in water or extract for soil solids Good quality irrigation water: 4 for salt hazard = EC < 2 ds/m 4 for Na + hazard = SAR <

Acid Soils

Y Sources of Acidity á Water: H 2 O  H + + OH - á CO 2 from soil respiration CO 2 + H 2 O  H 2 CO 3  H + + HCO 3 - carbonic acid á Organic acids from O.M. decomposition RH  R - + H + á Oxidation of S and N S  H 2 SO 4  2 H + + SO 4 -2 NH 3  HNO 3  H + + NO

Human-Induced Acidity * Chemical fertilizers  ammonium-based N materials NH 4 +  (O 2 )  HNO 3  Ferrous-Fe materials Fe +2  Fe +3  (+ 3 H 2 O)  Fe(OH) H +  Elemental Sulfur 2 S o + 3 O H 2 O  4 H SO

Acid Rain : N and S gases emitted from combustion processes SO 2  (O 2, H 2 O)  H 2 SO 4 NO x  (O 2, H 2 O)  HNO 3 mining wastes, wetland drainage - oxidation of sulfide (S -2 ) minerals S -2  (O 2, H 2 O)  H 2 SO 4 Human-Induced Acidity 5.15

Acid Soils : Role of Aluminum Al +3 Al(OH) +2 Al(OH) 2 + Al(OH) 3 Al +3  Al(OH) +2  Al(OH) 2 +  Al(OH) 3 | strongly | moderately | alkaline |  strongly  |  moderately  |  alkaline acid soils acid soils soils acid soils acid soils soils 5.16

Acid Soil Properties 5.17

Phases of Soil Acidity bound acidity exchangeable acidity soluble acidity As acidity is removed from or added to soil solution maintain equilibrium within system Ø maintain equilibrium within system must change all forms to change pH Ø must change all forms to change pH 5.18

Liming Materials Carbonate forms (a) "limestone" deposits and industrial byproducts (b) calcite = (CaCO 3 ) = calcium carbonate and dolomite = CaMg(CO 3 ) 2 (c) dolomitic limestone maintains Ca:Mg balance 5.19

Liming Materials (cont’d) Oxide and Hydroxide forms (a) oxides formed by heating limestones CaCO 3  (heat)  CaO + CO 2 calcite gas burned lime or quicklime (b) add water to oxides to form hydroxides CaO + H 2 O  Ca(OH) 2 hydrated lime 5.20

Lime Reactions in Soil 1. Neutralize acidity 2 H-X + CaCO 3  Ca-X + H 2 CO 3 + H 2 O 2. Base Saturation increases 3. Soil pH increases 4. Al solubility decreases Al OH -  Al(OH) 3 soluble insoluble (toxic) (not toxic) 5.21