Distribution of Different Forms of Phosphorus in Surface Soils of Rice Growing Areas of Red and Laterite Zone of West Bengal

Phosphorus in soil is present in both organic and inorganic forms. In general, inorganic P is the predominant form of soil P, constituting 20 to 80% of the total P in the surface layer (Tomar 2003). It is the inorganic fraction, which is more intimately related to phosphate nutrition to plants in agricultural soils. Phosphorus transformation in soils involves complex microbiological, chemical and biological processes. Plant availability of inorganic P can be limited by the formation of sparingly soluble calcium phosphate in alkaline and calcareous soils; by adsorption onto Fe and Al oxides in acid soils and by formation of Fe and Al phosphate complexes with humic acids (Gerke 1992). The nature and distribution of different forms of P have provided useful information for assessing the available P status of soil and for estimating the degree of chemical weathering of the soil, P deficiency, etc. Estimation of available P indicates only the amount of P present in soil solution and soil surface which is available to plants but it does not indicate about the relative contribution of different fractions of P towards available P. Thus understanding of the relationship between various forms of P, their interactions in soil and various factors influencing P availability to plants is essential for efficient P management in soil.