Description

Abstract: Phosphorus (P) fertilizer recommendations can be improved if the amount of P required to raise the soil test to a desired value is known. The objective of this study was to determine the increase in soil test P per unit of applied P, the P buffer coefficient, in the laboratory by various means and to compare those values with P buffer coefficients from field data. Phosphorus was applied to soils with predominately kaolinitic mineralogy. The mixtures were then incubated for either 180 and 120 d with numerous drying cycles, or for 7 d and 16 h with either one drying cycle or maintained moist. The samples were extracted with the Mehlich?3 solution and the change in extractable P per unit of applied P was calculated and termed the Mehlich?3 P buffer coefficient (M3PBC). The M3PBC varied widely among soils, but was linear within the range of 0 to 300 ?g P cm?3 applied for each. The M3PBC decreased with an increase in time, markedly between 16 h and 7 d, then more slowly from 7 d to 180 d. Drying also reduced the M3PBC, especially on the coarser textured soils. Mehlich?3 P buffer coefficients from ten field studies were related to the laboratory M3PBC values from dried samples. The prediction of Field M3PBC from the 180?d M3PBC was linear, while those for the 7?d and 16?h M3PBC were quadratic. As time is critical in routine analysis, the 16?h, dried method was selected as most practical. A prediction equation was calculated to estimate the Field M3PBC from the results of the 16?h, dried method.