Mehlich-3 phosphorus buffer coefficients

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 mu 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.