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Soil Acidity and Review of Colloid Charge

Soil Acidity and Review of Colloid Charge. Mineral Charge. “Perfect” Mineral. Si. Al. =. Positive charge. Negative charge. Al 3+ Si 4+. OH - O 2-. Tetrahedral Substitution. Al. Al 3+ for Si 4+. Aluminum contributes less (+) charge than silicon. =. Positive charge.

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Soil Acidity and Review of Colloid Charge

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  1. Soil Acidity and Review of Colloid Charge

  2. Mineral Charge

  3. “Perfect” Mineral Si Al = Positive charge Negative charge Al3+ Si4+ OH- O2-

  4. Tetrahedral Substitution Al Al3+ for Si4+ Aluminum contributes less (+) charge than silicon = Positive charge Negative charge Al3+ Si4+ OH- O2-

  5. Octahedral Substitution Mg2+ for Al3+ Magnesium contributes less (+) charge than Aluminum = Positive charge Negative charge Al3+ Si4+ OH- O2-

  6. Na+ Charge Na+ Na+ Na+ Na+ Na+ Na+

  7. O O O O H H H H H H H H Hydrated Cations + K

  8. CEC = 80-120 cmolc kg Smectites Significant substitution in the octahedra (Mg2+ for Al3+) Ca2+ Mg2+ Na+ Cations satisfying charge • Layers weakly held together by cations • Highly expansible

  9. Mg2+ Mg2+ Vermiculite Significant substitution in tetrahedra CEC = 100-180 cmolc kg Source of negative charge Is very close to the adsorbed cations Layers tightly bound Moderately expansible

  10. Soil Organic Matter and Reactivity

  11. HCl H+ + Cl- Acids give up hydrogen ions (H+) to water. Strong acids give up all their H+ ions Weak acids give up some of their H+ ions The functional groups on organic colloids are weak acids. COOH COO-+ H+ Soil Solution O-+H+ OH

  12. Dissociation of weak acid functional groups results in the development of negative charge COOH COO-+ H+ The amount of negative charge depends on how much dissociation takes place The dissociation is inhibited by H+ in soil solution Low pH = lots of H+ = less dissociation = low charge High pH = little H+ = more dissociation = high charge

  13. COOH COO-+ H+ COOH Soil Solution O- Ca2+ Mg2+ COO- Na+ K+ H+ K+ COO- Na+ COO- Mg2+ Mg2+ H+ K+ OH Na+ O- H+ Na+ K+ COO- K+ Na+ O- Na+ H+ Na+ O- Mg2+ COO-

  14. Mineral Colloids derive charge from substitution of lower-charged cations for higher charged cations in the crystal matrix during mineral formation. The result is permanent negative charge. Organic colloids derive their charge from dissociation of hydrogen ions from acidic functional groups on organic matter/humus. The result is pH-dependent charge Mineral Colloids – 0 – 180 cmol/kg Organic Colloids – 100 – 500 cmol/kg

  15. Si Al Si Soil Cation Exchange Capacity Mineral Organic + Total CEC = pH-Dependent

  16. You have 2 identical soils (same clay content, mineralogy, and O.M. content). Soil A has a pH of 6.5 Soil B has a pH of 4.0 Which soil has the higher CEC?

  17. Reactivity of Soil Horizons Organic Matter Organic Matter Organic Matter Silicate Clay Silicate Clay Silicate Clay Contribution to fertility.

  18. Soil Acidity and pH

  19. Acid Any substance which increases the Hydrogen ion concentration in solution. H+

  20. Hydrogen Ions and Organic Matter Increases dissociation of acidic functional groups Inhibits dissociation of acidic functional groups High charge on Organic Colloids Low charge on Organic Colloids Acidicbasic 0 7 14 Low H+ concentration High H+ concentration COOH COO- + H+

  21. Sources of Hydrogen Ions CO2 from microbial respiration/atmosphere CO2 dissolved in water produces carbonic acid H2CO3 CO2 + H2O H2CO3 Acid functional groups on organic matter COOH COO- + H+ Plant root exudation (release) of H+ Anthropogenic acids in rainfall Gaseous nitrogen and sulfur oxides make acid rain Hydrolysis of aluminum can produce acids

  22. What are the acids in soils?

  23. Two Ions considered Acidic in Soils H+ Aluminum Both are cationic Remember that aluminum is a common constituent of silicate clays Aluminum participates in “hydrolysis” reactions with water.

  24. Hydrolysis of Water (ionization of water) H2O OH- + H+ H+ is the acid OH- is the base Bases are the opposite of acids In pure water, the amounts of H+ and OH- are equal

  25. Aluminum Aluminum has the ability to break water apart and react with the resulting OH- H2O OH- + H+

  26. Hydrogen and Aluminum The only soil ions considered to be acidic Hydolysis by Aluminum H2O OH- + H+ Al3+ + H20 = Al(OH)2+ + H+ Al(OH)2+ + H20 = Al(OH)2+ + H+ Al(OH)2+ + H20 = Al(OH)o3 + H+

  27. Strongly Acid Soils (pH < 5) Aluminum is soluble => Al3+ or Al(OH)2+ Both are exchangeable and strongly adsorbed They are also in equilibrium with aluminum in solution The aluminum cations in solution can hydrolyze => H+ Moderately Acid Soils (pH 5 – 6.5) Aluminum exists as Al(OH)2+ and Al(OH)2+ Both are exchangeable and can be strongly adsorbed They are also in equilibrium with aluminum in solution. The solution aluminum cations hydrolyze => H+

  28. Types of Soil Acidity

  29. Active Acidity Acidity associated with soil solution H+ ions

  30. Active Acidity Acidity associated with the soil solution Typically a 1:1 or 2:1 extract 10 g soil and 10 mL water 10 g soil and 20 mL water Acidicbasic 0 7 14

  31. 5.6 pH meter Acidicbasic 0 7 14

  32. Active Acidity pH Range Plant Alfalfa Sweet Clover Beets Cauliflower Spinach Peas Carrots Cotton Wheat Tomatoes Potatoes Blueberries Azaleas 6.0 – 8.0 5.5 - 8.0 5.3 - 7.5 5.0 – 7.2 4.5 – 5.5 < 5 Sometimes soil pH must be adjusted to accommodate plants

  33. Exchangeable Acidity

  34. Exchangeable Acidity Acidity associated with cation exchange sites on mineral or organic colloids. Al+3 H+

  35. Types of Acidity Al+3 Na+ H+ Na+ H+ H+ H+ Ca2+ K+ H+ H+ Ca2+ Ca2+ Al+3 Clay minerals/Organic matter Exchangeable Active Acidity H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ H+ Soil Solution

  36. Measurement of Exchangeable Acidity

  37. Initial Equilibrium Al+3 Na+ H+ Na+ H+ H+ H+ Ca2+ K+ H+ H+ Ca2+ Ca2+ Al+3 Clay minerals/Organic matter Exchangeable Active H+ Ca+2 Ba+2 Na+ H+ H+ H+ Na+ Ca+2 Mg+2 H+ Ca+2 K+ Ca+2 Ca+2 H+ Mg+2 Cu+2 H+ Soil Solution

  38. Flood System with Ba2+ Exchangeable Ba+2 Ba+2 Ba+2 Al+3 Na+ H+ Na+ H+ Ba+2 Ba+2 Ba+2 H+ H+ Ca2+ K+ H+ Ba+2 Ba+2 Ba+2 Ba+2 H+ Ca2+ Ca2+ Al+3 Ba+2 Ba+2 Clay minerals/Organic matter Soil Solution

  39. Total Acidity Ba+2 Ba+2 Ba+2 Ba+2 Ba+ Ba+2 Ba+2 Ba2+ B+2 H+ Ba+2 Ba2+ Ba2+ Ba+2 Clay minerals/Organic matter Active and Exchangeable Ca2+ H+ H+ H+ H+ Na+ K+ H+ H+ H+ H+ K+ K+ H+ H+ H+ H+ Ca2+ H+ H+ Ba+2 Soil Solution

  40. Exchangeable acidity can be many times greater than active acidity. Managing soil acidity must account for exchangeable acidity as well as active acidity.

  41. Next: Acidity and Fertility

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