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

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

5.3 Origin of Charge

Adsorbed cations (a) arid region soils = "basic" cations 5.4 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+ = NH4+ > Na+

Cation Exchange Exchange process 5.5 Cation Exchange Exchange process Ca+2-colloid + 2 H+ ¬¾® 2 H+-colloid + Ca+2 = H+ replaces Ca+2 adsorbed to soil colloids Ca-x + 2 H+ ¬¾® 2 H-x + Ca+2 x = the soil solid phase Ca(ad) + 2 H+ ¬¾® 2 H(ad) + Ca+2 X(ad) = "adsorbed" cation X

5.6 Saline-Sodic Soils before rainfall

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

Saline Soils EC > 4 ds/m = osmotic stress 5.8 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 * salt tolerant plants (EC = 10 ds/m) 3 sugarbeets, barley, cotton, rosemary, wheat grass, wild rye (see table 10.2)

5.9 Sodic Soils poor water infiltration dispersion flocculation

Sodium Ion Effect 5.10 flocculation dispersion è attraction ç ç repulsion è Na+ Ca+2 & Mg+2

5.11 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 < 15

Acid Soils

Y Sources of Acidity á Water: H2O ¬® H+ + OH- 5.13 Y Sources of Acidity á Water: H2O ¬® H+ + OH- á CO2 from soil respiration CO2 + H2O ¬® H2CO3 ¬® H+ + HCO3- carbonic acid á Organic acids from O.M. decomposition RH ¬® R- + H+ á Oxidation of S and N S ¾® H2SO4 ¾® 2 H+ + SO4-2 NH3 ¾® HNO3 ¾® H+ + NO3-

Human-Induced Acidity 5.14 Human-Induced Acidity * Chemical fertilizers ü ammonium-based N materials NH4+ ¾(O2)® HNO3 ü Ferrous-Fe materials Fe+2 ® Fe+3 ¬(+ 3 H2O)® Fe(OH)3 + 3 H+ ü Elemental Sulfur 2 So + 3 O2 + 2 H2O ¾® 4 H+ + 2 SO4-2

Human-Induced Acidity 5.15 Human-Induced Acidity ü Acid Rain: N and S gases emitted from combustion processes SO2 ¾(O2, H2O)® H2SO4 NOx ¾(O2, H2O)® HNO3 ü mining wastes, wetland drainage - oxidation of sulfide (S-2) minerals S-2 ¾(O2, H2O)® H2SO4

Acid Soils: Role of Aluminum 5.16 Acid Soils: Role of Aluminum Al+3 ¬® Al(OH)+2 ¬® Al(OH)2+ ¬® Al(OH)3 |¬ strongly ¾®|¬ moderately ¾¾¾®|¬ alkaline acid soils acid soils soils

Acid Soil Properties 5.17

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

Liming Materials Carbonate forms (a) "limestone" deposits and 5.19 Liming Materials Carbonate forms (a) "limestone" deposits and industrial byproducts (b) calcite = (CaCO3) = calcium carbonate and dolomite = CaMg(CO3)2 (c) dolomitic limestone maintains Ca:Mg balance

Liming Materials (cont’d) 5.20 Liming Materials (cont’d) Oxide and Hydroxide forms (a) oxides formed by heating limestones CaCO3 ¾(heat)® CaO + CO2 calcite gas burned lime or quicklime (b) add water to oxides to form hydroxides CaO + H2O ¾® Ca(OH)2 hydrated lime

Lime Reactions in Soil 1. Neutralize acidity 5.21 Lime Reactions in Soil 1. Neutralize acidity 2 H-X + CaCO3 ¾® Ca-X + H2CO3 + H2O 2. Base Saturation increases 3. Soil pH increases 4. Al solubility decreases Al+3 + 3 OH- ¾® Al(OH)3 soluble insoluble (toxic) (not toxic)