Nature of acidity in some soils of Sikkim

Forty five surface soil samples for three soil acidity groups, namely (i) strongly acidic soil (pH <5.0) -Group A, (ii) moderately acidic soil (pH 5.0 -5.5) -Group B and (iii) mildly acidic (pH 5.6 -6.5) -Group C were collected from farmers'field of south Sikkim district of Sikkim state to study the nature of the acidity and their relationship with physical and chemical properties as well as lime requirement. Electrostatically bound H⁺ contributed 50 to 70% of exchange acidity for three soil groups. Electrostatically bound H⁺, extractable acidity and pH dependent acidity were the highest in soils of strongly acidic soil group (Group A), while other forms of acidity were maximum in soils of moderately acidic soil group (Group B). pH dependent acidity constituted 85.3, 81.8 and 75.8% of total potential acidity, while hydrolytic acidity constituted 71.6, 74.8 and 62.7% of total acidity for the soils of group A, B, C, respectively. Different forms of acidity were significantly related among themselves as well as with physical and chemical properties like pH, silt, clay, exchange Al and extractable Al content. Among the soil properties variation in the amount of exchange Al and extractable Al, pH_Ca and organic carbon were responsible for the variation in different forms of soil acidity. Lime requirement values were mostly influenced by pH_Ca and organic carbon content of the soils.