PHOSPHATE BEHAVIOR IN SUBMERGED CALCAREOUS SOILS

A laboratory study was conducted to investigate the influence of submergence on the solubility relationships of native and applied P in four calcareous soils from the Nile delta. Surface soil samples were submerged in the laboratory with and without added P under O-free conditions for 0.16, 38, 70, and 170 days. Ion activities were calculated from solution analyses by correcting for hydrolysis, inorganic and organic complexation, and activity coefficients. Submergence resulted in a decrease in soil redox potential that was proportional to the soil dissolved organic C. Treatments without added P differed in pH behavior from those with added P, but both tended to approach neutrality after 170 days of submergence. For the reduced soils, Mn concentration was consistently lower in the presence than in the absence of added P. The P sorption capacity of the soils increased with time of submergence and was highly correlated with the oxalate-extractable Fe. Without added P, soil solution P appeared to increase with reduction. With added P, the soil solutions remained saturated with respect to DCPD throughout the 170 days of submergence. The DCPD may coexist with Mn (III)-phosphate at high redox and with vivianite at low redox.