Nature of Soil Acidity in Some Soils of Manipur

In the Entisols, Inceptisols and Ultisols of Manipur, the total potential acidity is high. Exchange acidity contributes only 2.2 to 21.5 per cent whereas pH-dependent acidity is 78.5 to 97.8 per cent of total potential acidity. All types of soil acidity are relatively high in the hill soils than in the alluvial soils due to variations in physicochemical properties of the soils. High amount of ammonium acetate (pH 4.8) extractable Al indicates the presence of appreciable quantity of soluble Al(OH)₃ and hydroxy Al polymers in soils. The major controlling soil factor on exchange acidity is exchangeable Al³⁺ and pH-dependent acidity is influenced by organic matter, non-exchangeable Al and free iron oxides. The exchangeable Al³⁺ and per cent aluminium saturation increases with decreasing soil pH and it tends to be zero at pH 5.6. The management of such acid soils includes liming to neutralize exchangeable Al³⁺ to the tolerance level of crops.