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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, Mg3Si2O5(OH)4 kaolinite, Al2Si2O5(OH)4
2 tet. : 1 oct. 0 talc, Mg3Si4O10(OH)2 pyrophyllite, Al2Si4O10(OH)2
2 tet: 1 oct. 1 phlogopite muscovite KMg3(AlSi3O10)(OH)2 KAl2(AlSi3O10)(OH)2 1 biotite KFe3(AlSi3O10)(OH)2 0.6-0.8 illite (hydrous mica) K(Na,Ca) Al1.3Fe0.4Mn0.2Si3.4Al0.6O10(OH)2
0.6-0.9 vermiculite 0.25-0.6 smectite † The layer charge per formula unit
• Humus 200 cmolc/kg
• Smectite/Vermiculite 100 cmolc /kg
• Illite 25 cmolc /kg
• Kaolinite 10 cmolc /kg
• Fe and Al oxides 5 cmolc /kg
Charge of Soil Components
ColloidNegativecharge
Positivecharge % constant % variable
Humus 200 0 10 90Vermiculite 120 0 95 5Smectite 100 0 95 5Illite 40 0 80 20Kaolinite 12 4 5 95Fe & Al Oxides 5 5 0 100
Origin of Charge
cmol / Kg
-
-
-
-
-
-
-
-
-
-
-
+
+
+
+
+
+
+
+
+
Ion Adsorption
Surface charge neutralized by ions from the soil solution
Adsorbed Cations
(a) arid region soils = "basic" cations Ca2+, Mg2+, K+, Na+
(b) humid region soils
= “acidic” cations as well Ca2+, Mg2+, H+ and Al3+
(c) strength of adsorption Al3+> Ca2+ = Mg2+ > K+ = NH4
+ > Na+
Cation Exchange
Exchange process
Ca2+-colloid + 2 Na+ 2 Na+-colloid + Ca2+
= Na+ replaces Ca+2 adsorbed to soil colloids
Ca-x + 2 Na+ 2 Na-X + Ca2+
X = the soil solid phase
Dispersion
Saline Soils Saline Soils
EC > 4 ds/m = osmotic stress
* salt sensitive plants (EC = 2 ds/m)* salt sensitive plants (EC = 2 ds/m)
bean, onion, potato, raspberry, carrot, bean, onion, potato, raspberry, carrot, dogwood, larch, linden, peach, rose, tomatodogwood, larch, linden, peach, rose, tomato
* salt tolerant plants (EC = 10 ds/m)* salt tolerant plants (EC = 10 ds/m)
sugarbeets, barley, cotton, rosemary,sugarbeets, barley, cotton, rosemary,
wheat grass, wild ryewheat grass, wild rye
(see table 10.2 - 13(see table 10.2 - 13thth ed. or 10.3 – 12 ed. or 10.3 – 12thth ed.) ed.)
Sodic Soils (ESP > 15)Sodic Soils (ESP > 15)
flocculationflocculation
poorpoorwaterwaterinfiltrationinfiltration
dispersiondispersion
Sodium Ion EffectSodium Ion Effect
flocculationflocculation dispersiondispersion
attraction
CaCa2+2+ && Mg Mg2+2+
repulsion
NaNa++
SAR ParameterSAR Parameter
Predict sodium effect from saturated soil Predict sodium effect from saturated soil extract or irrigation waterextract 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
Sources of Acidity Water: H2O H+ + OH-
CO2 from soil respiration CO2 + H2O H2CO3 H+ + HCO3
-
carbonic acid
Organic acids from O.M. decomposition RH R- + H+
Oxidation of S and N S H2SO4 2 H+ + SO4
2-
NH3 HNO3 H+ + NO3-
Human-Induced Acidity
* Chemical fertilizers
ammonium-based N materials
NH4+ (O2) HNO3
Ferrous-Fe materials
Fe2+ Fe3+ (+ 3 H2O)Fe(OH)3 + 3 H+
Elemental Sulfur
2 So + 3 O2 + 2 H2O 4 H+ + 2 SO42-
Acid Rain: N and S gases emitted from combustion processes
SO2 (O2, H2O) H2SO4
NOx (O2, H2O) HNO3
mining wastes, wetland drainage - oxidation of sulfide (S2-) minerals
S2- (O2, H2O) H2SO4
Human-Induced Acidity
Phases of Soil Acidity
bound acidityexchangeable
aciditysoluble acidity
As acidity is removed from or added to soil solution As acidity is removed from or added to soil solution maintain equilibrium within systemmaintain equilibrium within system must change all forms to change pHmust change all forms to change pH
Acid Soils: Role of Aluminum
AlAl3+3+ Al(OH)Al(OH)2+2+ Al(OH) Al(OH)22++ Al(OH) Al(OH)33
|| strongly strongly || moderately moderately || alkaline alkaline acid soils acid soilsacid soils acid soils soils soils
Al3+ + H2O Al(OH)2+ + H+ K = 10-4.93
Al(OH)2+ + H2O Al(OH)2+ + H+ K = 10-4.97
Al(OH)2+ + H2O Al(OH)3o + H+ K = 10-5.7
Al(OH)3o + H2O Al(OH)4
- + H+ K = 10-7.4
Acid Soils: Role of Aluminum
AlAl+3+3 Al(OH)Al(OH)+2+2 Al(OH) Al(OH)22++ Al(OH) Al(OH)33
Changes in Al Speciation
- - - - - -
- - - - - -
Clay Interlayer Soil Solution
pH 4 pH 6
H+
H+
Why [Al3+] ~ [H+] in Acid Soils
Fe3+ + H2O <--> Fe(OH)2+ + H+ K = 10-2.19
Fe(OH)2+ + H2O <--> Fe(OH)2+ + H+ K = 10-3.5
Fe(OH)2+ + H2O <--> Fe(OH)3o + H+ K = 10-7.4
Fe(OH)3o + H2O <--> Fe(OH)4
- + H+ K = 10-8.5
Why Not Iron?
Liming Materials
Carbonate forms(a) "limestone" deposits and
industrial byproducts
(b) calcite = (CaCO3) = calcium carbonate and
dolomite = CaMg(CO3)2
(c) dolomitic limestone maintains Ca:Mg balance
Liming Materials (cont’d)
Oxide and Hydroxide forms(a) oxides formed by heating limestones
CaCO3 (heat) CaO + CO2
calcite gas
burned lime or quicklime
(b) add water to oxides to form hydroxides CaO + H2O Ca(OH)2
hydrated lime
Lime Reactions in Soil1. Neutralize acidity
2 H-X + CaCO3 Ca-X + H2CO3 + H2O
2. Base Saturation increases
BS = (CEC – [Al3+][H+]) / (CEC) * 100
BS = {[Na]+[K]+[Ca]+[Mg]}/CEC *100
3. Soil pH increases
4. Al solubility decreases Al+3 + 3 OH- Al(OH)3
soluble insoluble (toxic) (not toxic)
Anaerobic Organisms
Food Source
• Organic carbon*
• Ammonium Ion (NH4+)
• Ferrous Iron (Fe2+)
• Hydrogen Sulfide (H2S)
Electron Acceptor
• Nitrate (NO3-)
• Manganese (Mn4+)
•Ferric Iron (Fe3+)
• Sulfate (SO42-)
CH2O
CO2
O2
H2O
NO3-CH2O
CO2 N2
Fe(OH)3CH2O
CO2 Fe2+
CH2O
CO2
SO42-
H2S
Energy YieldsDonor Acceptor
700
400
100
Eh (mV)* Condition
oxic
suboxic
anoxic
*pH 7
MnO2CH2O
CO2 Mn2+
Soil Colors
Yellow -> Orange -> Red Fe(III) minerals
Black (veneer) Mn(IV) minerals
Dark Brown (disseminated) Organic Matter
Aerobic Environments
Gray -> Green -> Black Fe(II) minerals
Dark Brown (disseminated) Organic Matter
Anaerobic Environments
Iron massesRedox depletions
Root linings
MottlingNodules
Gleyed colors
Redoximorphic Features
Histic HorizonsHistic Horizons
“Rotten Eggs”