Soil phosphorus fractions in solution: influence of fertiliser and manure, filtration and method of determination.

This study investigated the forms of soil P released to solution, accuracy of their determination, and influence of colloids on P sorption/desorption dynamics. A Hagerstown silt loam, amended with dairy and poultry manure or superphosphate at five rates (0, 25, 50, 100, and 200 kg P ha(-1)), was extracted at two soil:solution ratios (1:5 and 1:100) and filtered at three pore sizes (0.8, 0.45, and 0.22 microm). Results showed that relative to the proportion of dissolved organic P (DOP, determined as the difference between total dissolved P [TDP] and P detected by ion chromatography), DRP increased with amendment rate. Relative to Mehlich-3 extractable P, DRP exhibited a power relationship with a much greater potential for soil P release at concentrations in excess of ca. 50 mg Mehlich-3 P kg(-1). Concentrations of DRP, determined by the acid molybdate method, were on average 12.5% greater than P detected by ion chromatography indicating P was solubilised during colorimetric determination. A linear relationship was found between total Al and DRP, which could indicate acid mediated hydrolysis of A1-humic-P substances, although acid mediated desorption of P from colloids cannot be discounted. No difference in solubilised P was found between solutions filtered at 0.22 and 0.45 microm, but was found between 0.8 microm and smaller filter sizes. Organic P extracted from manured soils was more recalcitrant than that extracted from soils amended with superphosphate, the later attributed to its accumulation in more labile pools. The sorption/desorption of P by colloids in solution were greatly affected by the rate of amendment and the soil:solution extraction ratio. More P was sorbed by superphosphate solutions compared to dairy manure amended soil solutions and was attributed to the saturation of colloidal P sorption sites by organic matter. In order to minimise the effects of colloids on P dynamics and the potential for hydrolysis in solution, filtration to at least 0.45 microm is required. However, soils with a lesser aggregate stability may require additional filtration.