Acidic Soils

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.

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