Acidic Soils
Soils having PH less than 6 are categorised as acid soils. These soils can be defined as those having PH less than 5.5 in 1:1 soil water extract. More precisely a soil which is acid in reaction throughout the root zone are called as acid soils. Such types of soils have more H+ concentration than OH- in the soil solution.
A. Why Soils are Acidic:
There are various factors which influence formation of acid soils.
1. Panant Material:
Rocks like granite and rhyolite are acid rocks and on disintegration and decomposition of these rocks results in accumulation of acid soil material.
2. Climate:
In heavy rainfall areas continuous leaching process removes most of CaCO3 and gypsum from soil giving rise to increased soil acidity.
3. Organic Matter:
The carboxylic, enolic and phenolic groups present in organic matter debris dissociate releasing H+ thus contributes towards soil acidity.
4. Root Biomass and Soil Organisms:
Respiration by plant roots and soil micro organisms are major contributing factors to soil acidity . CO2 liberated during respiration reacts with water to produce carbolic acid which in turn breaks down to release H+.
CO2 + H2O—-> H2CO3 + H+
On the other hand when plants exchange cations with external medium they give an equivalent number of H+ and thus contribute to soil acidity.
Soil Acidity Reactions:
Contribution of H+ in formation of acid soils.
1. CO2 from decomposition organic matter
CO2 + HOH à HCO3 + H+
2. Ammonia fertilizers are oxidized by the bacteria to form
NH4 + 2O2 à NO3 + H2O + 2H+
3. Aluminium ion in soil solution
Al ( H2O)6 +++ à Al ( H2O)5 OH++ + H+ ( PH bout 4.0)
Al ( H2O)5 OH)++ à Al ( H2O)4 ( OH) + H+ ( PH at 5.0)
4. Sulphur : as an ingredient in some fungicides and fertilizers.