A simple equation for calculating the residual effect of phosphorus fertilizers

A simple equation to calculate the residual effect of fertilizer phosphorus is derived from a model distinguishing only two soil phosphorus pools. With time intervals of one year the model calculates the phosphorus transfers between the pools, the uptake of phosphorus by the crop and the resulting pool sizes. Most input data required to operate the model can be obtained from ordinary one-season phosphorus fertilizer trials.For easily soluble phosphorus fertilizers the residual effect can also be calculated with a formula: Rt = (0.8 – R1)^t-1* R1 where Rt and R1 are the recovery fractions in year t and year 1, respectively. During the first five years after fertilizer application the residual effects, as calculated with the equation, are almost equal to those obtained with the model.