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humus 200 cmolc/kg
smectites 100 cmolc /kg
illite 25 cmolc /kg
kaolinite 10 cmolc /kg
Fe and Al oxides 5 cmolc /kg
Charge of Soil Components
5.2
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
5.3
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+ = NH4
+ > Na+
5.4
Cation Exchange
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
5.5
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)(see table 10.2)
5.8
Sodic SoilsSodic Soils
flocculationflocculation
poorpoorwaterwaterinfiltrationinfiltration
dispersiondispersion
5.9
Sodium Ion EffectSodium Ion Effect
flocculationflocculation dispersiondispersion
attraction
CaCa+2+2 && Mg Mg+2+2
repulsion
NaNa++
5.10
SAR ParameterSAR Parameter
Predict sodium effect from saturated soil Predict sodium effect from saturated soil extract or irrigation waterextract or irrigation water
SAR approximately equals ESP SAR is measured ESP is estimated
in water or extract for soil solids
Good quality irrigation water: for salt hazard = EC < 2 ds/m
for Na+ hazard = SAR < 15
5.11
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-
5.13
Human-Induced Acidity
* Chemical fertilizers
ammonium-based N materials
NH4+ (O2) HNO3
Ferrous-Fe materials
Fe+2 Fe+3 (+ 3 H2O)Fe(OH)3 + 3 H+
Elemental Sulfur
2 So + 3 O2 + 2 H2O 4 H+ + 2 SO4-2
5.14
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 (S-2) minerals
S-2 (O2, H2O) H2SO4
Human-Induced Acidity
5.15
Acid Soils: Role of Aluminum
AlAl+3+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
5.16
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
5.18
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
5.19
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
5.20