P-FRACTIONATION IN SODIUM-BICARBONATE EXTRACTS OF ANDIC SOILS

The hypothesis of stabilization of organic phosphate by Al(Fe) in andic soils was investigated. P fractionation in soil bicarbonate extracts was studied in relation to soluble C (DOG), Al and Fe. Phosphatase-hydrolysable P was determined in order to assess the bioavailable P forms in the extractable organo-mineral compounds. The centrifugation temperature of the bicarbonate extracts had an important influence on the extracted compounds, especially DOC and P; the results pointed to the existence of colloidal minerals together with Al(Fe)-humus complexes. The results questioned the conventional classification of P-i and P-o by fractionation in this type of soils, P-o being underestimated in the acid solution. Phosphatase-hydrolysable P (P-en) ranged from 33 to 93% of P-o and decreased with decreasing centrifugation temperature. The results have been interpreted in terms of soluble ternary complexes of organic or inorganic P with Al(Fe)-humus complexes.