Transformation of Applied Water Soluble Phosphate in Latosolic Lowland Rice Soils

The transformation of applied water soluble phosphates in six latosolic lowland rice soils was studied in relation to their chemical and physico-chemical properties. Added phosphorus remaining in solution in equilibrium with the soils declined sharply, the rate being comparatively lower in soils poor in free iron oxide and native iron phosphate contents. Phosphorus extractable according to the Bray No. 1 method remained all along very low in all the soils. Soils showing greater transformation of phosphorus into aluminium phosphate recorded higher values of phosphorus extracted by Bray's reagent. Transformation into aluminium and iron phosphate appeared to be directly related to the quantity of these inorganic forms of phosphorus already present in the soils, proportionate to the total amount of inorganic soil phosphorus Transformation into calcium phosphate was low in all the soils, the average ranging from 6 to 12per cent of the added amount in the different soils. 10 to 30 per cent of the cdded amount was transformed into reductant-soluble iron phosphate. Transformation into occluded iron and aluminium phosphate was negligibly low in all soils.

Organic matter addition resulted in a higher percentage of added phosphorus remaining in soluble form during the initial period of incubation, but it distinctly lowered the amount of phosphorus extractable by Bray No. 1 reagent and the amount of aluminium phosphate in all the soils It also lowered the transformation of added inorganic phosphcrus into iron and reductant-soluble iron phosphate in some soils. Organic matter did not show any significant influence on the transformation into calcium phosphate.