Phosphorus (P) fertilizer recommendations can be improved if the amoun
t of P required to raise the soil test to a desired value is known. Th
e 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 ka
olinitic mineralogy. The mixtures were then incubated for either 180 a
nd 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 app
lied P was calculated and termed the Mehlich-3 P buffer coefficient (M
3PBC). 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 slo
wly from 7 d to 180 d. Drying also reduced the M3PBC, especially on th
e coarser textured soils. Mehlich-3 P buffer coefficients from ten fie
ld studies were related to the laboratory M3PBC values from dried samp
les. The prediction of Field M3PBC from the 180-d M3PBC was linear, wh
ile those for the 7-d and 16-h M3PBC were quadratic. As time is critic
al in routine analysis, the 16-h, dried method was selected as most pr
actical. A prediction equation was calculated to estimate the Field M3
PBC from the results of the 16-h, dried method.