Pools and solubility of soil phosphorus as affected by liming in long-term agricultural field experiments

Published laboratory studies suggest minimum solubility of soil phosphorus (P) in the pH range traditionally considered as the target for lime applications on agricultural land. However, the potential significance of these results for the timescale relevant in practical agriculture remains to demonstrate. We revisited four long-term (50-76 years) agricultural lime experiments located on Quaternary deposits with topsoil texture ranging from silt loam to silty clay and initial pH between 5.0 and 6.0. Crop uptake data and desorption experiments in the laboratory indicated that liming enhanced the solubility of P added with fertiliser in the decades following the application of lime. An empirical mass-balance model demonstrated that changes in acetate-lactate extractable P ('P-AL'), previously shown to be related to reactive phosphate adsorbed on soil mineral surfaces, was proportional to field P balances (fertiliser P - harvested P) and inversely proportional to the net proton load to the soil. Extraction data suggest that liming enhanced the incorporation of silica in non-crystalline sesquioxides, which in turn inhibited the conversion of fertiliser P into less extractable forms by migration of P to inside the oxides. Liming induced a lowering of the C/N ratio in one of the experiments, suggesting stimulated decomposition of soil organic matter. However, lime applications did not lead to a decline in the pool of soil organic P, as measured by the combustion method. Mass-balance considerations showed that changes in organic-matter input due to liming were unlikely to result in observable changes in the pool of soil organic P.