RETENTION OF ORTHO- AND POLYPHOSPHATES IN SOME QUEBEC SOILS AS AFFECTED BY ADDED ORGANIC RESIDUES AND CALCIUM CARBONATE

Abstract

Retention of orthophosphate (OP), pyrophosphate (PP) and PolyN-P (an OP–PP mixture) was studied on surface samples of Uplands, Chicot and Rideau soils, and a subsoil sample of Uplands soil. Surface samples retained more P as PP and PolyN-P than as OP, in contrast to the Uplands subsoil which retained more P as OP than as PP and PolyN-P. Results from surface soils appeared to be related to the greater ability of the polyphosphates, compared to OP, to solubilize soil organic matter. This may have had the effect of exposing more mineral surfaces with greater capacity for P retention. Sorption of OP estimated by the Freundlich intercept was correlated significantly with the extractable Al and Fe contents of the soils. No linear correlations were obtained between polyphosphate adsorption and extractable Al and Fe contents, but some significant curvilinear relationships were observed. The results indicate that PP and PolyN-P may be retained to a lesser degree than OP in soils dominated by sesquioxides of Fe and Al. Application of farmyard manure (FYM) and alfalfa residues reduced the retention of ortho- and polyphosphate in the surface soils. The magnitude of the effect was greater with FYM than with alfalfa residues. The results suggest that P uptake from added polyphosphate could be enhanced by added organic residues. CaCO3 effects were studied on Uplands topsoil and subsoil only. Added CaCO3 reduced PP retention but had no effect on OP retention in Uplands topsoil. In the Uplands subsoil, added CaCO3 consistently increased PP and PolyN-P retention but reduced OP retention. Key words: Organic residues, pH, clay, extractable Fe and Al, Langmuir and Freundlich adsorption isotherms