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

3. 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

4. Origin of Charge cmol / Kg

5. ---------------------- + + + + + + + + + Ion Adsorption Surface charge neutralized by ions from the soil solution

6. 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 +

7. 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

8. before rainfall Saline-Sodic Soils

9. after rainfall

10. 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 - 13 th ed. or 10.3 – 12 th ed.) (see table 10.2 - 13 th ed. or 10.3 – 12 th ed.)

11. Sodic Soils (ESP > 15) flocculation poorwaterinfiltration dispersion

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

13. 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) - 0.01 Good quality irrigation water: 4 for salt hazard = EC < 2 ds/m 4 for Na + hazard = SAR < 15

14. Acid Soils

16. 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 -

17. 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) 3 + 3 H +  Elemental Sulfur 2 S o + 3 O 2 + 2 H 2 O  4 H + + 2 SO 4 2-

18. 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

19. 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

20. 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

21. Al 3+ + H 2 O  Al(OH) 2 + + H + K = 10 -4.93 Al(OH) 2 + + H 2 O  Al(OH) 2+ + H + K = 10 -4.97 Al(OH) 2+ + H 2 O  Al(OH) 3 o + H + K = 10 -5.7 Al(OH) 3 o + H 2 O  Al(OH) 4 - + H + K = 10 -7.4 Acid Soils : Role of Aluminum

22. 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

23. Fe 3+ + H 2 O Fe(OH) 2 + + H + K = 10 -2.19 Fe(OH) 2 + + H 2 O Fe(OH) 2+ + H + K = 10 -3.5 Fe(OH) 2+ + H 2 O Fe(OH) 3 o + H + K = 10 -7.4 Fe(OH) 3 o + H 2 O Fe(OH) 4 - + H + K = 10 -8.5 Why Not Iron?

24. 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

25. 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

26. 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 *100 3. Soil pH increases 4. Al solubility decreases Al +3 + 3 OH -  Al(OH) 3 soluble insoluble (toxic) (not toxic)

27. Acid Soil Properties

28. Wetland (Hydric) Soils and Redox Conditions

29. 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- )

30. 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 700 400 100 Eh (mV)*Condition oxic suboxic anoxic *pH 7 MnO 2 CH 2 O CO 2 Mn 2+

31. Redoximorphic Features - Soil colors - Color Distribution

32. 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

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

34. Plant Effects on Redox Conditions

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