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Soil Chemical Properties

Soil Chemical Properties. Section B Soil Fertility and Plant Nutrition. Soil Texture. The proportions of sand, silt, and clay particles in soils: Sand 2 to 0.05 mm effective diameter Silt 0.05 to 0.002 mm Clay <0.002 mm The most reactive fraction is ___________. clay. Soil Colloids.

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Soil Chemical Properties

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  1. Soil Chemical Properties Section B Soil Fertility and Plant Nutrition

  2. Soil Texture • The proportions of sand, silt, and clay particles in soils:Sand 2 to 0.05 mm effective diameterSilt 0.05 to 0.002 mmClay <0.002 mm • The most reactive fraction is ___________. clay

  3. Soil Colloids • Soil particles <0.001 mm in diameter • Are the most reactive of soil particles because of _________________ and _________________________. • Types of soil colloids: surface area electrical charge Inorganic: clay minerals, oxide minerals Organic: soil organic matter

  4. Organic Colloids • Mostly soil “humus”, the chemically resistant organic matter in soils, that results from organic matter decomposition. • Characteristics: variably charged (usually -), high cation exchange capacity (CEC)

  5. Humus Carbon Hydrogen Oxygen Nitrogen

  6. Building Blocks of layer silicates • Tetrahedral (Si+4 bonded to four O-2) • Octahedral (Al+3 bonded to six OH-) • The long chains or layers of tetrahedra and octahedra are bonded together to form layer silicates.

  7. Mineral Colloids • Layer silicate clays • 1:1 clays (Kaolinite) • 2:1 clays (Micas, Illite, Vermiculite, Montmorillonite) • 2:1:1 clays (chlorite)

  8. 1:1 Clay Mineral Layer

  9. 2:1 Clay Mineral Interlayer

  10. Layer Silicate Clays • Properties:Surface AreaChargeExpansion

  11. Layer Silicate Clays • Have a charge because of: • Isomorphous substitution“Substitution of cations of equal or lesser charge within tetrahedrons or octahedrons. This can create a negative charge deficit on the clay particle”. • pH dependent chargeH+ may attach to or detach from (depending on pH) O atoms located on the clay edges. Creates a negative or positive charge deficit.

  12. Layer Silicate Clays • Have a charge because of: • Isomorphous substitution“Substitution of cations of equal or lesser charge within tetrahedrons or octahedrons. This can create a negative charge deficit on the clay particle”. • pH dependent chargeH+ may attach to or detach from (depending on pH) O atoms located on the clay edges. Creates a negative or positive charge deficit.

  13. Hematite Fe2O3 H+ H+ H+

  14. H+ On kaolinite, most pH- dependent charge occurs on exposed octahedral Surfaces. Kaolinite

  15. H2+ H - H2+ H2+ H - H+ H - H + H+ + H+ H2+ H H H - - H2+ H - - H+ Increasing pH + charge pzc - pH

  16. Soil Colloids • Other soil minerals may occur as colloidal particles: • Fe, Al oxides - can have a pH dependent charge • Poorly crystalline clays such as allophane - also have pH dependent charge

  17. Sources of Charge on Common Soil Clays • 2:1 clays (smectites, vermiculite, etc.) • Most charge is due to isomorphous substitution (always negative) • Little pH-dependent charge • 1:1 clays (kaolinite) • Little isomorphous substitution • Most charge is due to pH-dependent charge (positive or negative)

  18. Cation Exchange • Definition: The exchange of cations adsorbed (attached) onto colloid surfaces with cations in solution. • Exchangeable cations are those attached to colloid surfaces. • Cations in solution and on colloid surfaces tend toward a state of _______________. • Exchangeable cations can be manipulated. e.g.: equilibrium

  19. Cation Exchange Capacity • CEC is: The mass of exchangeable cations that a given soil can retain per unit weight. Units are cmol(+)/kg soil or meq/100g. • Soils have CEC because of: • Soils have many more exchangeable cations than cations in solution (buffering capacity)

  20. Definitions • Atomic weight is weight in grams of 6 x 1023 atoms of a substance. One mole of substance is 6 x 1023 atoms, molecules etc. Thus, atom weight is grams/per mole. • Equivalent weight is the mass of substance that will react or displace 1 gram of H, which is 6 x 1023 charges (- or +). • Thus equivalent weight is atomic weight divided by valence.

  21. CEC • Is the quantity of negative charges per kg of soil • Expressed in units of cmol(+)/kg (i.e meq/100g) • 1 mole of (+) is 6.023 x 1023 (+) • 1 cmol of (+) is 0.01 mol (+) • 1 mol of Na+ is 23 g and contains 1 mol (+) • 1 cmol of Na+ is 0.23 g and contains 1 cmol (+) • 1 mol of Ca2+ is 40 g and contains 2 mol (+) • 1 cmol of Ca2+ is 0.40 g and contains 2 cmol (+)

  22. 2+ High CEC Low CEC 2+ 2+

  23. Strength of Adsorption • Cations attraction to clays is a function of charge and size. • Strength of attraction:Al3+ > Ca2+ > Mg2+ > K+ = NH4+ > Na+

  24. Clays and CEC • Kaolinite 2-5 cmol(+)/kg • Illite (fine mica) 15-40 cmol(+)/kg • Vermiculite 100-180 cmol(+)/kg • Montmorillonite 80-120 cmol(+)/kg • Humus 100-550 cmol(+)/kg

  25. Clays and CEC • What will be the CEC of a clay loam soil with 30% kaolinite clay? • 5 cmol(+)/kg clay x 30 kg clay/100 kg soil = ____ cmol(+)/kg soil • What will be the CEC of a clay loam soil with 30% montmorollonite clay? • 90 cmol(+)/kg clay x 30 kg clay/100 kg soil = ____ cmol(+)/kg soil 1.5 27.0

  26. Brady and Weil, Figure 8.11

  27. Measuring CEC • CEC is commonly measured in laboratories by:1. Saturating soil cation exchange sites with a cation (e.g. NH4+)2. Extracting the soil with another cation to remove the NH4+3. MeasureNH4+ extracted

  28. Exchangeable Cations • The exchangeable cations have very important influences on soil properties: • Ca2+ is the dominant exchangeable cation in most soils. • Soils become acidic when they contain significant amounts of exchangeable _______ . • Soils have poor structure when they contain significant amounts of exchangeable _____ . Al3+ Na+

  29. Weathering and Soil Minerals • Soil mineralogy depends on: • Parent material • Weathering • Soils that are not highly weathered will tend to contain smectite and illite (mica) colloids in the clay fraction. • Soils that are highly weathered will tend to contain kaolinite and oxide colloids in the clay fraction • How does this affect soil CEC?

  30. Buffering Capacity • Definition: • The soil solids control or “buffer” the composition of the soil solution. • Caused by dissolution of minerals, adsorption/desorption of exchangeable cations. The resistance of the soil solution to a change in composition.

  31. Titration Curve—Weak Acid “ Alkaline pH “Buffering” Acid Base added

  32. Buffering in Solutions • Acetic Acid in water:HC2H3O2 H+ + C2H3O2-Keq ≈ 10-5 • Add a base:NaOH + H+ Na+ + H2O

  33. Buffering

  34. Buffering Capacity Poorly buffered 10 gallon fuel tank What about fertilization? Highly buffered • Poorly buffered soils: • Store limited amounts of available nutrients • Should be fertilized more often • Should be fertilized with lesser amounts 30 gallon fuel tank

  35. Buffering Capacity • The amount of buffering capacity is: • Proportional to minerals present (e.g. soils high in K-feldspars will be highly buffered with respect to K). • Proportional to amount of exchangeable cations (e.g. soils high in exchangeable Ca will be highly buffered with respect to Ca) • Typically, highly-weathered soils are less well-buffered with respect to nutrients than are lightly-weathered soils (more CEC, more primary minerals)

  36. Buffering Capacity Affects how frequently some soil amendments, fertilizers need to be added, and how much. Highly Buffered Amt. Of exch. Or mineral nutrient { ∆y Poorly Buffered { ∆y { { ∆x2 ∆x1 Solution Concentration

  37. Potassium Buffering Capacity Exchangeable K mmol/kg From Barber, 1984 p.37 K in soil solution mmol/L

  38. Oxidation-Reduction (Redox) • Involves exchange of electrons between chemical species. • In soils, redox reactions often are catalyzed by ____________________. • Oxidation is _______________________. • Reduction is _______________________. • Oxidation and reduction always occur together. microorganisms a loss of electrons a gain of electrons

  39. Redox Reaction 2FeO + 2H2O  2FeOOH + 2H+ + 2e-(oxidation ) ½ O2 + 2H+ + 2e-  H2O (reduction) _________________________________ 2FeO + 1/2O2 + H2O  2FeOOH (oxidation-reduction [redox]) Represents Fe oxidation in an aerobic soil environment

  40. Redox Reactions (1) • Typical redox reaction in an aerobic soil:CH2O + ½ O2 CO2 + H2O • Represents the decomposition of organic matter in soils. C in CH2O is oxidized in the reaction, O in O2 is reduced in the reaction. The O2 is called the “electron acceptor”.

  41. Redox Reactions (2) • If a soil becomes anaerobic because of waterlogging, O2 is not present, so another electron acceptor is needed:3 CH2O + 2 NO3- 3 CO2 + N2 + 2 H2O +2H+ • Represents the decomposition of organic matter in an anaerobic soil. C in CH2O is oxidized in the reaction, N in NO3- is reduced in the reaction. The NO3- is called the “electron acceptor”.

  42. Redox • Organisms gain energy by oxidizing compounds (e- donors). They have to dispose of the electrons using other compounds (e- acceptors). • Common e- donors in soils: • Organic matter, NH4+ , S, Fe2+ • Common e- acceptors in soils: • O2, NO3-, Fe3+, SO42-, Mn4+

  43. Oxidation State • The oxidation state is the difference between the charge of an atom in its current state and the charge of the neutral atom. Is equal to the number of electrons gained or lost. • In redox reactions, electron gain and loss must be balanced.

  44. Redox • Redox reactions have very important effects on many nutrients in soils:OxidizedReduced NO3- NH4+, N2Fe3+ Fe2+ Mn3+ Mn2+ SO42- H2S

  45. Soil Redox Potential • Aerobic soils have sufficient supplies of O2, which is the primary e- acceptor. Inorganic N, Mn, Fe, and S tend to be present in their oxidized forms. • Anaerobic soils have little or no O2. An anaerobic condition may be caused by _________________. In this case, N, Mn, Fe, and S may be used as e- acceptors. • N and S availability to plants decrease when reduced, availability of Fe and Mn increase when reduced. flooded soil

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