PREDICTING INCREASES IN EXTRACTABLE PHOSPHORUS FROM FERTILIZING SOILSOF VARYING CLAY CONTENT

The change in extractable P due to fertilization should be quantified for reliable P recommendations, but this effect is dependent on the P buffering capacity of the soil. To determine this change, rates of fer tilizer P were applied in six field experiments on Ultisols that varie d in clay content. Mehlich-3 extractable P (M3P) was measured annually , and an initial increase in M3P due to fertilization followed by an e xponential decrease in M3P due to sorption were modeled. The change in extractable P after 1 yr at each site was calculated from the model. These data were combined with those from previous studies on two Ultis ols in North Carolina and five Oxisols in Brazil to broaden the range in clay content observed. The increase in M3P per unit of applied P af ter 1 yr was 0.7 for soils with only 10% clay, and it decreased expone ntially to <0.2 for soils with >50% clay. The results of this study we re similar to conclusions from a P incubation study extracted with Bra y-1 in Natal, South Africa, on 54 soils from five U.S. taxonomic order s, including Ultisols and Oxisols. It appears there is a broad range o f cultivated soils for which knowledge of the clay content would allow more reliable predictions of fertilizer P to keep extractable P at a level sufficient for optimum crop yields.