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

Humus  200 cmol c /kg Smectite/Vermiculite  100 cmol c /kg Illite  25 cmol c /kg Kaolinite  10 cmol c /kg Fe and Al oxides  5 cmol c /kg Charge of Soil Components

Origin of Charge cmol / Kg

Ion Adsorption Surface charge neutralized by ions from the soil solution

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 +

Cation Exchange Exchange process Ca 2+ -colloid + 2 Na +  2 Na + -colloid + Ca 2+ = Na + replaces Ca +2 adsorbed to soil colloids Ca-x + 2 Na +  2 Na-X + Ca 2+ X = the soil solid phase Dispersion

before rainfall Saline-Sodic Soils

after rainfall

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 th ed. or 10.3 – 12 th ed.) (see table th ed. or 10.3 – 12 th ed.)

Sodic Soils (ESP > 15) flocculation poorwaterinfiltration dispersion

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

SAR Parameter Predict sodium effect from saturated soil extract or irrigation water SAR is measured ESP/ESR is estimated in water or extract for soil solids ESR = 0.015(SAR) Good quality irrigation water: 4 for salt hazard = EC < 2 ds/m 4 for Na + hazard = SAR < 15

Acid Soils

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 3 -

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 4 2-

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

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

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

Al 3+ + H 2 O  Al(OH) H + K = Al(OH) H 2 O  Al(OH) 2+ + H + K = Al(OH) 2+ + H 2 O  Al(OH) 3 o + H + K = Al(OH) 3 o + H 2 O  Al(OH) H + K = 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 Changes in Al Speciation Clay Interlayer Soil Solution pH 4 pH 6 H+H+ H+H+ Why [Al 3+ ] ~ [H + ] in Acid Soils

Fe 3+ + H 2 O Fe(OH) H + K = Fe(OH) H 2 O Fe(OH) 2+ + H + K = Fe(OH) 2+ + H 2 O Fe(OH) 3 o + H + K = Fe(OH) 3 o + H 2 O Fe(OH) H + K = Why Not Iron?

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

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

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 BS = (CEC – [Al 3+ ][H + ]) / (CEC) * 100 BS = {[Na]+[K]+[Ca]+[Mg]}/CEC * Soil pH increases 4. Al solubility decreases Al OH -  Al(OH) 3 soluble insoluble (toxic) (not toxic)

Acid Soil Properties

Wetland (Hydric) Soils and Redox Conditions

Anaerobic Organisms Food Source Organic carbon* Ammonium Ion (NH 4 + ) Ferrous Iron (Fe 2+ ) Hydrogen Sulfide (H 2 S) Electron Acceptor Nitrate (NO 3 - ) Manganese (Mn 4+ ) Ferric Iron (Fe 3+ ) Sulfate (SO 4 2- )

CH 2 O CO 2 O2O2 H2OH2O NO 3 - CH 2 O CO 2 N2N2 Fe(OH) 3 CH 2 O CO 2 Fe 2+ CH 2 O CO 2 SO 4 2- H2SH2S Energy Yields Donor Acceptor Eh (mV)*Condition oxic suboxic anoxic *pH 7 MnO 2 CH 2 O CO 2 Mn 2+

Redoximorphic Features - Soil colors - Color Distribution

Soil Colors Yellow -> Orange -> RedFe(III) minerals Black (veneer)Mn(IV) minerals Dark Brown (disseminated)Organic Matter Aerobic Environments Gray -> Green -> BlackFe(II) minerals Dark Brown (disseminated)Organic Matter Anaerobic Environments

Iron masses Redox depletions Root linings Mottling Nodules Gleyed colors Redoximorphic Features Histic Horizons “Rotten Eggs”

Plant Effects on Redox Conditions

Fe III (OH) 3 deposit O 2(g) Plaque Formation on Plant Roots Fe(OH) 3 O2O2 H2OH2O Fe 2+