This article throws light upon the five main factors responsible for the development of acid soil. The factors are: 1. Leaching due to Heavy Rainfall 2. Microbiological Factors 3. Origin of Acid from Acid Parent Rock Material 4. Removal of Bases by Crops 5. Continuous Application of Acid-Forming Fertilizers.
Factor # 1. Leaching due to Heavy Rainfall:
The magnitude of leaching is the main factor for the development of acid soil. The rainfall carries lime and other bases (Ca++,Mg++ etc.) downward beyond the reach of plant roots. When all the soluble bases are lost, the H ions of the carbonic and other acids developed in the soil replace the basic cations of the colloidal complex.
As the soil gets gradually depleted of its exchangeable bases through constant leaching, it gets de-saturated and becomes increasingly acid. Thus, the regions having an annual rainfall of 100 cm (40 inches) or more (humid region) have a high percentage of acid soils.
CO2 + H2O = H2 CO3 (Carbonic acid)
H2CO3+ Ca CO3 = Ca (HCO3)2â†“Leachable
Factor # 2. Microbiological Factors:
Several microorganisms are responsible for many processes such as decomposition of organic residues and nitrification. As a result of microbial activity, acids are constantly being formed. If these acids are not neutralized by bases (Ca++, Mg++ etc.) then cause of soil solution to be acid.
Factor # 3. Origin of Acid from Acid Parent Rock Material:
Some acid soil have developed from parent rock materials which are of acid nature such as granite. Here also, the leaching effect is predominating is creating acidity.
Factor # 4. Removal of Bases by Crops:
Some crops like sugar beet absorb bases from the soil. Due to removal of bases from the soil, acid soil develop.
Factor # 5. Continuous Application of Acid-Forming Fertilizers:
The use of ammonium sulphate and ammonium nitrate increase soil acidity. When nitrogenous fertilizers of ammonical nature added to soil, they are nitrified and nitric acid is liberated. The acid so developed reacts with calcium and other bases to form calcium and other nitrates. Being soluble, they are either removed by crops or lost in drainage. At the same time, a part of the NH4+ ions are adsorbed by the soil colloidal material.
The ammonium ions so adsorbed replace calcium and other cations, which are leached down. The ammonium ions of the colloidal material get nitrified and form nitric acid. The residual anions give rise to acids like sulphuric and hydrochloric which further deplete the soil solution and the colloidal complex of their bases.
On these accounts, the clay gets de-saturated, H ion concentration increases and the soil pH is lowered, the reactions involved may be expressed as under: