Evaluation of Fertilizer Phosphate Residues by Plant Uptake and Extractable Phosphorus

Residual phosphate from fertilizer P or manure seemed to accumulate mainly as octocalcium phosphate (OCP) in 23 alkaline and calcareous soils of eastern Colorado. Soil solution composition data were corrected for six ion pairs in calculating the calcium phosphate potential. These soils contained 37 to 162 mg kg −1 NaHCO 3 ‐soluble P. Phosphorus removed by cropping varied between 58 and 275 mg kg −1 with five to eight crops before P deficiency reappeared. Phosphorus uptake was highly correlated with available P before cropping as measured by NaHCO 3 ‐soluble P (four successive extracts), resin‐extractable P, and labile P by 32 P isotopic dilution. Plants absorbed more P at equal initial concentrations of soluble P (in 0.01 M CaCl 2 ) from soils containing higher clay contents. All the OCP dissolved during cropping. This material seems to have a very high availability coefficient for plants. Accumulation of fertilizer P residues in this form would appear to be beneficial since this P is potentially all available to crops. Therefore, methods for estimating the amount of OCP in soils should be useful.