Phosphorus Diffusion in Soils

Abstract

AbstractThe validity of activity measurements as estimates of phosphorus diffusion from P32‐tagged phosphate sources was investigated. Significant changes in the apparent specific activity of diffusing phosphorus were found; these could be accounted for by assuming exchange between diffusing P32 and native soil P31. Despite such changes in apparent specific activity, the overall picture of phosphorus diffusion obtained from activity measurements was not substantially different from that obtained by total phosphorus analysis. One‐dimensional diffusion of tagged phosphates from Ca(H2PO4)2 and KH2PO4 sources was studied in 10 water‐saturated soils using x‐ray film and Geiger tubes to estimate the resulting distribution of diffused phosphorus. The x‐ray film revealed many local areas of P32 concentrations 0.5 to 2 millimeters in diameter. Large‐scale irregularities in the curves relating phosphorus concentration in the soil column to distance from the source were found in three cases. In both Miami silt loam and Muscatine silt loam with Ca(H2PO4)2 as source, peaks of phosphorus concentration were found at 4‐cm. intervals. With Miami silt loam and KH2PO4 as source, one peak of phosphorus concentration was found about 1 cm. from the source. Distribution of diffused calcium, as measured by extraction with 0.5 N acetic acid, was similar to distribution of diffused phosphorus in Muscatine and Miami silt loam columns treated with Ca(H2PO4)2.