Lecture 5 Exchange Reactions Cation exchange Salt/Sodium Affected Soils Acid Soils
General Classes (layer build-up) of Phyllosilicate Minerals: Layer Type Charge Trioctahedral Dioctahedral 1 octahedra 0 brucite, Mg(OH) 2 gibbsite, Al(OH) 3 1 tet. : 1 oct. 0 serpentine, Mg 3 Si 2 O 5 (OH) 4 kaolinite, Al 2 Si 2 O 5 (OH) 4 2 tet. : 1 oct. 0 talc, Mg 3 Si 4 O 10 (OH) 2 pyrophyllite, Al 2 Si 4 O 10 (OH) 2 2 tet: 1 oct. 1 phlogopite muscovite KMg 3 (AlSi 3 O 10 )(OH) 2 KAl 2 (AlSi 3 O 10 )(OH) 2 1 biotite KFe 3 (AlSi 3 O 10 )(OH) 2 0.6-0.8 illite (hydrous mica) K(Na,Ca) Al 1.3 Fe 0.4 Mn 0.2 Si 3.4 Al 0.6 O 10 (OH) 2 0.6-0.9 vermiculite 0.25-0.6 smectite The layer charge per formula unit
5.2 Charge of Soil Components humus 200 cmol c /kg smectites 100 cmol c /kg illite 25 cmol c /kg kaolinite 10 cmol c /kg Fe and Al oxides 5 cmol c /kg
5.3 Origin of Charge cmol / Kg Colloid Negative charge Positive charge % constant % variable Humus 200 0 10 90 Vermiculite 120 0 95 5 Smectite 100 0 95 5 Illite 40 0 80 20 Kaolinite 12 4 5 95 Fe & Al Oxides 5 5 0 100
5.4 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 +
Cation Exchange 5.5 Exchange process Ca +2 -colloid + 2 H + 2 H + -colloid + Ca +2 = H + replaces Ca +2 adsorbed to soil colloids Ca-x + 2 H + 2 H-x + Ca +2 x = the soil solid phase Ca (ad) + 2 H + 2 H (ad) + Ca +2 X (ad) = "adsorbed" cation X
Saline-Sodic Soils 5.6 before rainfall
Our fields! Time for Rice? Oh No... 5.7 after rainfall
5.8 Saline Soils EC > 4 ds/m = osmotic stress * salt sensitive plants (EC = 2 ds/m) bean, onion, potato, raspberry, carrot, dogwood, larch, linden, peach, rose, tomato * salt tolerant plants (EC = 10 ds/m) sugarbeets,, barley, cotton, rosemary, wheat grass, wild rye (see table 10.2-13 th ed. or 10.3 12 th ed.)
5.9 Sodic Soils poor water infiltration flocculation dispersion
Sodium Ion Effect flocculation dispersion attraction Ca +2 & Mg Mg +2 repulsion Na +
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: for salt hazard = EC < 2 ds/m for Na + hazard = SAR < 15
Acid Soils
Sources of Acidity 5.13 Water: H 2 O H + + OH - CO 2 from soil respiration CO 2 + H 2 O H 2 CO 3 H + + HCO 3 - carbonic acid Organic acids from O.M. decomposition RH R - + H + Oxidation of S and N S H 2 SO 4 2 H + + SO 4-2 NH 3 HNO 3 H + + NO 3 -
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
Human-Induced Acidity 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
5.16 Acid Soils: Role of Aluminum Al +3 Al(OH) +2 Al(OH) + 2 Al(OH) 3 strongly moderately alkaline acid soils acid soils soils
Phases of Soil Acidity bound acidity exchangeable acidity soluble acidity As acidity is removed from or added to soil solution maintain equilibrium within system must change all forms to change ph
Liming Materials 5.19 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
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
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 3. Soil ph increases 4. Al solubility decreases Al +3 + 3 OH - Al(OH) 3 soluble (toxic) insoluble (not toxic)
Acid Soil Properties
Wetland (Hydric) Soils and Redox Conditions
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 2-4 )
Energy Yields *ph 7 Donor CH 2 O Acceptor O 2 Eh (mv)* 700 Condition CO 2 H 2 O CH 2 O NO 3 - oxic CO 2 N 2 CH 2 O MnO 2 CO 2 Mn 2+ 400 CH 2 O Fe(OH) 3 suboxic CO 2 Fe 2+ CH 2 O SO 4 2-100 CO 2 H 2 S anoxic
Redoximorphic Features - Soil colors - Color Distribution
Soil Colors Aerobic Environments Yellow -> Orange -> Red Black (veneer) Dark Brown (disseminated) Fe(III) minerals Mn(IV) minerals Organic Matter Anaerobic Environments Gray -> Green -> Black Dark Brown (disseminated) Fe(II) minerals Organic Matter
Redoximorphic Features Gleyed colors Root linings Histic Horizons Iron masses Redox depletions Mottling Nodules Rotten Eggs
Plant Effects on Redox Conditions
Plaque Formation on Plant Roots O 2(g) Fe III (OH) 3 deposit O 2 Fe 2+ H 2 O Fe(OH) 3