Soils 205-90
Lecture 10- Soil Reaction
Videos Pages in Text.
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15,16 |
363-393 |
Soil reaction refers to:
A. Acid Soil Properties
1. Strongly acid soils (pH<5.0)
(a) Al+3 becomes soluble
(b) Al+3 undergoes hydrolysis
(c) Al+3 and Al(OH)+2 are adsorbed to soil colloids
(d) exchangeable H+ at strong acid sites
2. Moderately acid soils (5.0<pH<6.5)
(a) more bases (Ca+2 and Mg+2)
(b) further Al hydrolysis
Al(OH)+2 + H2O ¬¾® Al(OH)2+ + H+
Al(OH)2+ + H2O ¬¾¾® Al(OH)3 + H+
insoluble gibbsite
(c) Al-OH compounds polymerize on surfaces
(d) polymers of Al(OH)2+ and insoluble Al(OH)3
form on clay surfaces
(e) pH-dependent sites release H+
2 H-x + Ca+2 ¬¾® Ca-x + 2 H+
3. Neutral to alkaline soils (pH>6.5)
(a) bases (Ca+2 and Mg+2) dominate exchanger
(b) Al is insoluble = Al(OH)3 minerals such as gibbsite
Al(OH)2+ + OH- ¬¾® Al(OH)3
(c) increased negative charge as pH increases
4. Summary
B. Classification of Soil Acidity
1. Active acidity
w H+ in soil solution
w obtained by pH measurement
w small, but important
2. Exchangeable acidity
w adsorbed H and Al ions
w salt-replaceable with KCl, NaCl
w 1000’s x active acidity in strongly acid soils
> 100 x in moderately acid
3. Residual acidity
w "bound" to colloids
w not salt-replaceable - must increase pH
w very large with variable-charge colloids (organic, kaolinite, oxides)
w source of pH dependent charge
w can be neutralized by liming materials
w pH does not reflect total soil acidity
C. Base Saturation Percentage (BSP)
1. Relative amount of acid or basic cations in soil
2. Definition of BSP
% base saturation = (sum of exchangeable bases/cation exchange capacity) x 100
å exchangeable bases = CEC - exchangeable acidity
% bases + % acid = 100 %
example: CEC = 12 cmolc/kg and exchangeable acidity = 3 cmolc/kg
BSP = {(12 - 3)/12} x 100 = 75 %
or (3/12) x 100 = 25 % acidity
100 - 25 = 75 % bases
3. Decreased BSP = more acid soil (lower pH)
Note: at neutral pH values, BSP = 100 %
D. Buffering
1. Resistance to pH change
bound acidity exchangeable acidity active acidity
As acidity is removed from or added to soil solution
- maintain equilibrium within system
- must change all forms to change pH
2. Buffer Capacity of soils
high CEC = high buffering
E. Acid Soil Formation
1. Leach bases from soil
2. Replace exchangeable bases with acid
- sources of acidity
x Water: H2O ¬¾® H+ + OH-
x CO2 from soil respiration
CO2 + H2O ¬¾® H2CO3 ¬¾® H+ + HCO3-
carbonic acid
x Organic acids from duff or O.M. decomposition
RH ¬¾® R- + H+
x Oxidation of S and N
S ¾® H2SO4 ¾® 2 H+ + SO4-2
NH3 ¾® HNO3 ¾® H+ + NO3-
3. As H+ goes on clay = Al+3 is solubilized and hydrolyzed
F. Human-Induced Changes
1. Chemical fertilizers
ü ammonium-based N materials
NH4+ ¾(O2)® HNO3
2. Acid rain
ü N and S gases emitted from combustion processes
3. Waste disposal
ü sewage sludge
- organic acids from decomposition of organics
- inorganic acids from oxidation of inorganics
ü mining wastes
- oxidation of sulfide (S-2) minerals
S-2 ¾(O2, H2O)® H2SO4
4. Wetland drainage
ü coastal acid sulfate soils
- iron sulfide (FeS) and elemental S (So)
FeS and So ¾(O2, H2O)® H2SO4
