Availability of residual phosphorus in high phosphorus soils

Although we know soil phosphorus (P) can rapidly increase when fertilizer or manure is continually applied at rates exceeding crop removal, there is little information on the rate of P release from high P soils as a function of soil type. Surface samples (0–5 cm) were collected from five major soil types in Pennsylvania (Chester silt loam, Conotton loam, Duffield silt loam, Hagerstown silt loam, and Washington loam), covering a wide range in Mehlich-3 extractable P (180–690 mg kg−1). There was a soil dependent exponential increase in CaCl2-extractable soil P, which reflects soil solution P movement in subsurface drainage, with increasing Mehlich-3 P concentration. The kinetics of this P release as determined by sequential extraction of soil and iron (Fe)-oxide strips, varied among soils as a function of soil P concentration (R2 of 0.55–0.64). The decrease in rate of P release with successive soil extractions was less at higher initial soil P concentrations, indicative of diffusion-limited P release. The data of this study suggest that for a given soil type, soil P depletion can be estimated from Mehlich-3 extractable soil P concentration. However, data for a wider range of soils is needed to evaluate the universality and practicality of this predictive approach.