Predicting soil phosphorus buffer coefficients using potential sorption site density and soil aggregation

The phosphorus (P) buffer coefficient, a ratio of the increase in extractab le P to the amount of applied fertilizer P, is a source of considerable unc ertainty in determining the amount of fertilizer needed to meet crop P requ irements. The use of clay as a predictor of the P buffer coefficient has be en suggested for soils of similar mineralogy, However, it has not been sati sfactory for soils with a wide range of soil mineralogies but relatively hi gh clay content. The objective of this study was to improve the prediction of buffer coefficients using soil characteristics associated with the proce ss of P sorption, such as mineralogy, surface area, and aggregation. The so il P sorption site density, estimated from detailed clay mineralogy, and re active mass, the fraction of the total soil mass in the surface aggregates where newly added P can be sorbed, were used to predict the buffer coeffici ent. The P buffer coefficients of 10 soils with a wide range in P sorption were estimated by Mehlich 3, modified Truog, and 0.5 M NaHCO3 extractants f or incubation periods of 32 and 180 d. The inclusion of P sorption site den sity and reactive mass substantially improved predicting the P buffer coeff icients when compared with the P buffer coefficients predicted by only soil flay content. Statistical models showed that the P buffer coefficients wer e negatively correlated with both log of the P sorption site density and re active mass. Thus, soils with fewer P sorption sites, lower reactive mass, and larger aggregate size will tend to have higher buffer coefficients, ind icating that a greater portion of the added P remains plant available.