We incubated samples of five unfertilized Argentinian soils with phosphate at 4, 25, 40, and 60°C and measured the change in the null-point phosphate concentration through time. There was an initial decrease in this phosphate concentration, but, in some cases, it was followed by an increase in concentration. The increases were most marked after long periods or high temperatures and especially at low levels of added phosphate. The increases in concentration appeared to be due to release of phosphate already present in the soil. The amount released increased with both time and temperature of incubation. Functions to describe this release were included in equations to describe the continuing reaction between added phosphate and the soil. This produced a close description of the changes in concentration through time. We argue that it is consistent with mechanistic models of phosphate reaction with soils to subdivide phosphate added at different times into separate “pulses.” The net outcome is then the algebraic sum of the trend for each “pulse.”.