Lecture 5, Soil Chemistry

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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, Mg3Si2O5(OH)4
kaolinite, Al2Si2O5(OH)4
2 tet. : 1 oct.
0
talc, Mg3Si4O10(OH)2
2 tet: 1 oct.
1
phlogopite
KMg3(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
pyrophyllite, Al2Si4O10(OH)2
muscovite
KAl2(AlSi3O10)(OH)2
0.6-0.9
vermiculite
0.25-0.6
smectite
† The layer charge per formula unit
Charge of Soil Components
• Humus  200 cmolc/kg
• Smectite/Vermiculite  100 cmolc /kg
• Illite  25 cmolc /kg
• Kaolinite  10 cmolc /kg
• Fe and Al oxides  5 cmolc /kg
Origin of Charge
cmol / Kg
N e g a tive
ch a rg e
200
P o sitive
ch a rg e
0
% co n sta n t
10
% va ria b le
90
V e rm icu lite
120
0
95
5
S m e ctite
100
0
95
5
Illite
40
0
80
20
K a o lin ite
12
4
5
95
5
5
0
100
C o llo id
Humus
F e & A l O xid e s
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
Dispersion
Ca-x + 2 Na+  2 Na-X + Ca2+
X = the soil solid phase
Saline-Sodic Soils
before rainfall
after rainfall
Saline Soils
EC > 4 ds/m = osmotic stress
* salt sensitive plants (EC = 2 ds/m)
3
bean, onion, potato, raspberry, carrot,
dogwood, larch, linden, peach, rose, tomato
* salt tolerant plants (EC = 10 ds/m)
3 sugarbeets, barley, cotton, rosemary,
wheat grass, wild rye
(see table 10.2 - 13th ed. or 10.3 – 12th ed.)
Sodic Soils
(ESP > 15)
poor
water
infiltration
flocculation
dispersion
Sodium Ion Effect
flocculation
 attraction 
Ca2+ & Mg2+
dispersion
 repulsion 
Na+
SAR Parameter
Predict sodium effect from saturated soil
extract or irrigation water
SAR is measured
in water or extract
ESP/ESR is estimated
for soil solids
ESR = 0.015(SAR) - 0.01
Good quality irrigation water:
4 for salt hazard = EC < 2 ds/m
4 for Na+ hazard = SAR < 15
Acid Soils
Sources of Acidity
 Water: H2O  H+ + OH CO2 from soil respiration
CO2 + H2O  H2CO3  H+ + HCO3carbonic acid
 Organic acids from O.M. decomposition
RH  R- + H+
 Oxidation of S and N
S H2SO4 2 H+ + SO42NH3 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-
Human-Induced Acidity

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
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
Acid Soils: Role of Aluminum
Al3+ Al(OH)2+  Al(OH)2+  Al(OH)3
| strongly | moderately | alkaline
acid soils
acid soils
soils
Acid Soils: Role of Aluminum
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
Changes in Al Speciation
Why [Al3+] ~ [H+] in Acid Soils
Clay Interlayer
-
- - -
- -
Soil Solution
H+
Al+3 Al(OH)+2  Al(OH)2+  Al(OH)3
-
- - -
- -
pH 4
H+
pH 6
Why Not Iron?
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
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 Soil
1. 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
(toxic)
insoluble
(not toxic)
Acid Soil Properties
Wetland (Hydric) Soils
and
Redox Conditions
Anaerobic Organisms
Food Source
Electron Acceptor
• Organic carbon*
• Nitrate (NO3-)
• Ammonium Ion (NH4+)
• Manganese (Mn4+)
• Ferrous Iron (Fe2+)
•Ferric Iron (Fe3+)
• Hydrogen Sulfide (H2S)
• Sulfate (SO42-)
*pH 7
Energy Yields
Donor
CH2O
Acceptor
O2
CO2
H2O
CH2O
NO3-
CO2
CH2O
CO2
CH2O
Condition
700
oxic
N2
MnO2
Mn2+
400
Fe(OH)3
CO2
Fe2+
CH2O
SO42-
CO2
Eh (mV)*
H2S
suboxic
100
anoxic
Redoximorphic Features
- Soil colors
- Color Distribution
Soil Colors
Aerobic Environments
Yellow -> Orange -> Red
Fe(III) minerals
Black (veneer)
Mn(IV) minerals
Dark Brown (disseminated)
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
O2(g)
FeIII(OH)3
deposit
O2
H2O
Fe2+
Fe(OH)3

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