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 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

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)

Humus Carbon Hydrogen Oxygen Nitrogen

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.

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

1:1 Clay Mineral Layer

2:1 Clay Mineral Interlayer

Layer Silicate Clays Properties: Surface Area Charge Expansion

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 charge H+ may attach to or detach from (depending on pH) O atoms located on the clay edges. Creates a negative or positive charge deficit.

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 charge H+ may attach to or detach from (depending on pH) O atoms located on the clay edges. Creates a negative or positive charge deficit.

Hematite Fe2O3 H+ H+ H+

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

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

q Increasing pH + pzc - pH Na+ H2+ Cl- H - Cl- H2+ H2+ H Na+ Na+ Na+ charge pzc - q pH

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

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)

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

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)

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.

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 (+)

Cation Exchange Cation Clay particle Here is a schematic diagram of a negatively charged clay particle surrounded by cations. The soil liquid (soil solution) contains dissolved cations and anions. The concentration of cations is much greater close to the particle surface than in the bulk soil solution. The cations are not bonded to the clay, but just attracted to the surface. Conversely anions are repelled by negatively charged clays, so the concentration of anions is greater in the bulk soil solution than close to a clay particle.

High CEC Low CEC 2+ 2+ 2+

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

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

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

Brady and Weil, Figure 8.11

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. Measure NH4+ extracted

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+

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?

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:

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.

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

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

Buffering

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

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)

{ 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

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

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

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

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”.

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”.

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+

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. Oxidation State: A "bookkeeping" method for keeping track of electrons. The oxidation state is the difference between the charge of an atom in its current state and the charge of the neutral atom. Denoted by roman numerals, e.g. Fe(II). a. Oxidation: Loss of electrons e.g. CH2O + H2O <----> CO2 + 4e- + 4H+ where C loses electrons and goes from an oxidation state of 0 to +4 b. Reduction: Gain of electrons O2 + 4e- + 4H+ <-----> 2H2O where O gains electrons and goes from an oxidation state of 0 to -2 Oxidation and reduction can never occur separately, they must always be coupled. Something must be oxidized (e donor) and something must be reduced (e acceptor) c. Complete Oxidation-Reduction (Redox) reaction Combining the above reactions: CH2O + O2 <-----> CO2 + H2O

Redox Redox reactions have very important effects on many nutrients in soils: Oxidized Reduced NO3- NH4+, N2 Fe3+ Fe2+ Mn3+ Mn2+ SO42- H2S d. Common electron donors in soils: crop residue, SOM, reduced S, reduced Fe. e. Common electron acceptors in soils: O: 1/2O2 + 2e- + 2H+ <-----> H2O NO3: NO3- + 5e- + 6H+ <-----> 1/2N2 + 3H2O Mn: MnO2 + 2e- + 4H+ <-----> Mn2+ + 2H2O Fe: FeOOH + e- + 3H+ <-----> Fe2+ + 2H2O SO4: SO42- + 9H+ + 6e- <-----> HS- + 4H2O The other reactions will proceed more or less in sequence after O2 disappears (reduced soils). These reactions are catalyzed by bacteria that can function anaerobically (without oxygen). These are only reduction half-reactions, and do not show the electron donor. What are some possible electron donors?

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