Phosphorus extractability in surface soil samples as affected by mixing with subsoil

Samples taken from the plow layer (Ap horizon)and subsoil (B horizon)of six cultivated soil profiles were analyzed as original samples and as mixtures containing 25% or 50% material from the B horizon. Acid ammonium acetate extractable phosphorus, degree of phosphorus saturation (DPS), and a phosphorus Q/I-plot were determined for each sample and mixture to evaluate the effect of bulking of dissimilar materials on results and to assess the possibilities of reducing P solubility in P-enriched surface soils. The results obtained for the mixtures were compared with mass-weighed average results of the original samples. Measured values of DPS corresponded well and those of acetate-extractable P reasonably well to the estimated values, and the results were linearly correlated with the mass fraction of horizon B material in the mixed samples (r2>0.85).Water-extractable P behaved dissimilarly; the equilibrium P concentration (EPC) estimated from the Q/I-plots decreased dramatically when the fraction of highly sorptive horizon B material increased in the mixture. The marked effect of subsoil material on EPC values may provide a technique to reduce potential losses of soluble P by deep tillage.;