Soil Chemical Properties Section B Soil Fertility and Plant Nutrition.

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  • Slide 1
  • Soil Chemical Properties Section B Soil Fertility and Plant Nutrition
  • Slide 2
  • Soil Texture The proportions of sand, silt, and clay particles in soils: Sand 2 to 0.05 mm effective diameter Silt0.05 to 0.002 mm Clay
  • Strength of Adsorption Cations attraction to clays is a function of charge and size. Strength of attraction: Al 3+ > Ca 2+ > Mg 2+ > K + = NH 4 + > Na +
  • Slide 29
  • Clays and CEC Kaolinite2-5 cmol(+)/kg Illite (fine mica)15-40 cmol(+)/kg Vermiculite100-180 cmol(+)/kg Montmorillonite80-120 cmol(+)/kg Humus100-550 cmol(+)/kg
  • Slide 30
  • 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
  • Slide 31
  • Brady and Weil, Figure 8.11
  • Slide 32
  • Measuring CEC CEC is commonly measured in laboratories by: 1.Saturating soil cation exchange sites with a cation (e.g. NH 4 + ) 2.Extracting the soil with another cation to remove the NH 4 + 3. Measure NH 4 + extracted
  • Slide 33
  • Exchangeable Cations The exchangeable cations have very important influences on soil properties: Ca 2+ 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 _____. Al 3+ Na +
  • Slide 34
  • 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?
  • Slide 35
  • Anion Exchange Capacity Some soil colloids can have a positive charge, leading to an anion exchange capacity. This is due only to a pH dependent charge, not isomorphous substitution. Most important where oxide (Al, Fe) minerals are abundant in soils:
  • Slide 36
  • 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.
  • Slide 37
  • Titration CurveWeak Acid pH Base added Buffering Acid Alkaline
  • Slide 38
  • Buffering in Solutions Acetic Acid in water: HC 2 H 3 O 2 H + + C 2 H 3 O 2 - K eq 10 -5 Add a base: NaOH + H + Na + + H 2 O
  • Slide 39
  • Buffering
  • Slide 40
  • Buffering Capacity 10 gallon fuel tank 30 gallon fuel tank Highly buffered What about fertilization? Poorly buffered soils: 1.Store limited amounts of available nutrients 2.Should be fertilized more often 3.Should be fertilized with lesser amounts Poorly buffered
  • Slide 41
  • 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)
  • Slide 42
  • Buffering Capacity Solution Concentration Amt. Of exch. Or mineral nutrient Highly Buffered Poorly Buffered Affects how frequently some soil amendments, fertilizers need to be added, and how much. { x 1 y x 2 { { { y
  • Slide 43
  • Potassium Buffering Capacity K in soil solution mmol/L Exchangeable K mmol/kg From Barber, 1984 p.37
  • Slide 44
  • 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
  • Slide 45
  • Redox Reaction 2FeO + 2H 2 O 2FeOOH + 2H + + 2e- (oxidation ) O 2 + 2H + + 2e- H 2 O (reduction) _________________________________ 2FeO + 1/2O 2 + H 2 O 2FeOOH (oxidation-reduction [redox]) Represents Fe oxidation in an aerobic soil environment
  • Slide 46
  • Redox Reactions (1) Typical redox reaction in an aerobic soil: CH 2 O + O 2 CO 2 + H 2 O Represents the decomposition of organic matter in soils. C in CH 2 O is oxidized in the reaction, O in O 2 is reduced in the reaction. The O 2 is called the electron acceptor.
  • Slide 47
  • Redox Reactions (2) If a soil becomes anaerobic because of waterlogging, O 2 is not present, so another electron acceptor is needed: 3 CH 2 O + 2 NO 3 - 3 CO 2 + N 2 + 2 H 2 O +2H + Represents the decomposition of organic matter in an anaerobic soil. C in CH 2 O is oxidized in the reaction, N in NO 3 - is reduced in the reaction. The NO 3 - is called the electron acceptor.
  • Slide 48
  • 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, NH 4 +, S, Fe 2+ Common e- acceptors in soils: O 2, NO 3 -, Fe 3+, SO 4 2-, Mn 4+
  • Slide 49
  • 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.
  • Slide 50
  • Redox Redox reactions have very important effects on many nutrients in soils: OxidizedReduced NO 3 - NH 4 +, N 2 Fe 3+ Fe 2+ Mn 3+ Mn 2+ SO 4 2- H 2 S
  • Slide 51
  • Soil Redox Potential Aerobic soils have sufficient supplies of O 2, 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 O 2. 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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