ACID SOILS
Acid soils are those having high degree of adsorbed Aluminium and Hydrogen.
Soil acidity of two types Those are
1. Reserve acidity/Exchange acidity/potential acidity/salt replaceable acidity
2. Active acidity
Reserve acidity: Refers to acidity at exchange complex i.e. Concentration of Hydrogen and Aluminium at the exchange complex.
Strongly acid soils (pH < 5.0)
Al 3+ + H2O → Al (OH)2+ + H+
Moderately acid soils (pH 5.0-6.5)
Al (OH)2 + + H2O → Al (OH)2+ + H+
Al (OH)2 + + H2O → Al (OH)3+ + H+
Genesis of acid soils
Following are the factors responsible for formation/ genesis of acid soils.
1) Parents Materials:
Soil formed from acid parent root are usually acidic in nature is : granite, Rhyolite etc.
2) High rainfall
It is an important phenomenon in the genesis of acid soils. High rainfall leaches the bases from the soil and becomes reason for accumulation of H+ ion at exchange complex. Bases like Ca, Mg, K, Na et are removed from the soil by the water in high rainfall areas.
3) Organic Matter
Decomposition of organic matter released weak organic acids (Carbanic acid → CO2 + H2O → H2 CO3), strong organic acids. These organic acids particularly carbonic acid solublses bases which subject to leaching. Further decomposition of nitrogen containing materials release nitric acid and sulphur compounds release sulphonic acid. These inorganic acids also contributes to soil acidity.
4) Acid Forming Fertilizer
Application of acid forming fertilizers like elemental sulphur, Ammonium sulphate, Ammonium chloride etc continuously over a period to soils become a reason for the acid soil formation. Ammonical fertilizers on odixation release free hydrogen ion
NH4+ +2O2 → + NO3- + H2O + 2H+
5) Industrial Effluents
Oxides of nitrogen and sulphur are let out of te industries which are combine with rain water
Reserve acidity and activity are in equilibrium i.e. If H+ concentration in soil solution decreased. H+ from soil solution. If H+ in soil solution increased, some H+ moves to soil solid. The potential acidity is always high even more than 100 times than active acidity in soils.
Role of Al3 in soil acidity
H+ directly contributes to soil acidity, while Al3+ on hydration releases free H+ free H+ ion and thereby increase soil acidity.to form acids. These acids reach the ground and polluter the soil.
2S + 3O2 + 2 H2O→ 2 H2SO4
Effect of acidity
1. Excess hydrogen in soil solution result in injury to the growing plants.
2. In acid soils (low pH) Fe, Al and Mn are solubilised and available excessively. This causes toxicity effect on the growing plants.
3. Growth of microorganisms is restricted in acid soils particularly bacteria multiplication reduced.
4. Nutrients availability in acid soils are reduced Calcium, Magnesium are in low levels because acid soil do not have bases. Further poor decomposition of organic matter due to low microbes in acid soils leads to low availability of nitrogen and sulphur. Phosphorus is converted into Al and Fe phosphates which are insoluble hence P is deficient in acid soils. Molybdenum is another element available in low level in acid soils.
Reclamation
Acids soils can be reclaimed to normal soil by introducing suitable base cation and thereby removing excess H+ and Al3+ at exchange complex. The commonly used living materials (supply Ca to remove H+ + Al 3+) to reclaim acid soils are
a) Quick / Burnt / Oxide of lime (CaO)
b) Hydrated lime (Ca COH)2)
c) Lime / Calcite (CaCO3)
d) Dolomite [Ca Mg (CO3)2]
e) Marl/Oyster shells/ Basic slay etc)
Among the above calcite (Lime-CaCo3) and Dolomite [Ca Mg (CO3)2] are mostly used and for reclamation of acid soils.
Lime requirement
The quantity of lime required to reclaim acid soils is worked out by following standard procedures in the laboratory (Schoomaker method (Details refer practicals) Hutchinson and Macleamn method) etc.
Lime requirement of an acid soil depends on the following
1) Change in pH required: Lower the pH more lime required.
2) Buffering capacity of soils: Higher clay and organic matter in soils more will be buffering capacity of soil and more lime is required for reclamation.
3) Finess of liming material: Finer lime materials quickly solubilization and react hence less lime requirement.
4) Texture of soil: Fine textured soils require more time than coarse textured soils for reclamation.’
5) Organic matter: Higher organic matter in soils more will be buffering capacity hence higher the time requirement for reclamation.
Reactions
All liming materials on addition to soil first they converted into calcium bicarbonate in the presence of CO2 due to high partial pressure of CO2 which is hundred times more than atmosphere. Secondly they involve in exchange reaction and replaces H+ from soil colloids.
CaO + H2O+ 2CO2 → Ca (HCO3)2
Ca (OH)2 + 2 CO2 → Ca (HCO3)2
CaCO3 + H2O + CO2 → Ca(HCO3)2
H-Micelle-H + Ca(OH)2 → Micelle Ca + 2H2O
H-Micelle-H + Ca(HCO3)2 → Micelle Ca+ 2H2O + 2CO2
H-Micelle-H + CaCO3 → Micelle Ca + 2H2O + CO 2
Finess of lime:
Lime material at least 50 per cent should pass through 60 mesh sieve and coarser particles less effective than finer particles.
Acid soils in India distributed mostly in North eastern states and hilly area of other state. A special cid soil is seen in Kerala which is acid sulphate soils having low pH i.e. < 3.5 and high amount of sulphates.