Origin of Acidity
Sources:
a. Organic matter
b. Aluminosilicate clays
c. Hydrous oxides
d. Soluble salts
a. Organic matter:
Decomposition:
Humic residues with acidic carboxylic and phenolic groups.
Higher CO2 partial pressure.
CO2 + H2O ↔ H+ + HCO3- ↔ H+ + CO32-
Under humid climatic conditions:
Carbonic acid is responsible for the loss of Ca and Mg salts.
Ca40 Ca38
Al20+ 5H2CO3 → Al20 + 2Ca(HCO3)2 +
M(HCO3)
+ -
H20 ( 5H + 5HCO3 )H25 (Subject to loss in M19
M20
During Decomposition:
Several organic acids are produced.
Organic acids contribute to the total acidity and upon dissociation produce
active acidity.
Humus also reacts with Fe and Al ions and forms complexes which may
undergo hydrolysis to produce H ions.
b. Aluminosilicate clays:
Aluminosilicate clays can hold H+ and Al3+ ions on permanent and pH
dependent charges.
Due to acidity some aluminium from the clay mineral lattice will go to
solution.
Strongly Acid Soils pH 4.5-5
Aluminium becomes soluble and is present in the form of aluminium or
aluminiumhydroxy cations. These become adsorbed even in preference to
hydrogen by the permanent electrostatic changes of clay minerals.
Al ↔ Al 3+
(Adsorbed Aluminium) (Soil Solution Aluminium)
Hydrolysis of Al3+:
Al3+ + H2O → Al(OH)2+ + H+
The hydrogen ions thus released give a very low pH value in the soil solution and
are perhaps the major source of H+ in very acid soils.
The adsorbed hydrogen is a second source of hydrogen ions in very acid soils.
H ↔ H+
(Adsorbed Hydrogen)(Soil Solution Hydrogen)
Thus it can be seen that the effect of both adsorbed hydrogen andaluminium is to
increase the H+ concentration in the soil solution.
Moderately acid soils:
Soils have somewhat higher percentage base saturation and pH values. The
aluminium can no longer exist as Al3+ but has been converted to
aluminiumhydroxy ions by reactions such as
Al3++OH- →Al(OH)2+
Al(OH)2+ + OH-→ Al(OH)2+
Some of the aluminiumhydroxy ions are adsorbed and act as exchangeable
cations and exist in equilibrium with solution hydroxy ions. In the soil
solution, they are able to produce hydrogen ions by the following hydrolysis
reaction.
Al(OH)2+ + HOH → Al(OH)2+ + H+
Al(OH)++ HOH → Al(OH)3 + H+
Adsorbed H+ also contribute to soil acidity.
In addition with the rise in pH, some hydrogen ions which have been held
tenaciously through covalent bonding by the organic matter and clay are
subject to release. ( pH dependent charge)
H +Ca2+ → Ca+2H+
H
Bound H Exchangeable Soil solution
(Not dissociated) calcium hydrogen
Neutral to alkaline soils:
Adsorbed H+ is released through exchange reaction with baseforming
cations.
c. Hydrous oxides :
Amorphous particles or crystalline colloidal material such as gibbsite.
As coatings on other mineral particles.
As interlayers between crystal lattice structures.
Under acid condition the iron and aluminium from these oxides may be
brought into solution and undergo stepwise hydrolysis with the release of
hydrogen ions.
d. Soluble salts:
1. NH4 containing salts / Mineralization of nitrogen containing organic
compounds / Urea
NH4+ +3/2O2 ↔ NO2- +2H+ + H2O + energy
NO2-+1/2O2 ↔ NO3- +energy
2. Sulphur compounds:
Thiobacillus thioparus
5Na2S2O3 + 4O2 + H2O → 5Na2SO4 + H2SO4 +4S
Na2S4O6 +Na2CO3 + 1/2O2→ 5Na2SO4 + 2S + CO2
Thiobacillus thiooxidans
S + 1 1/2O2 + H2O→ H2SO4
Thiobacillus denitrificans
5S + 6KNO3 + 2H2O→K2SO4 + 4KHSO4 + 3N2
3. Absorption of basic portion of the salts.
2FeS2 + 7O2 + 2H2O ↔ 4SO42- + 4H+ + 2Fe2+
2Fe2+ + 1/2 O2 + 5H2O ↔ 2Fe(OH)3 + 4H+
