The reaction between phosphate and dry soil. I. The effect of time, temperature and dryness

Phosphate (P) was added to soil in solution. The soil was air-dried or freeze-dried and then incubated at a range of temperatures for periods of up to 110 d. The rate of the continuing reaction between the P and soil was measured using the null-point method, and by measuring the amount of desorption induced by filter paper impregnated with iron oxide (Pi test). The reaction between soil and P continued in both air-dried and freeze-dried soil, albeit more slowly than in moist soil. Freezing the soil, whether moist or dry, virtually stopped the reaction. These results are consistent with the hypothesis that the continuing reaction between P and soil involves a solid-state diffusive penetration of the soil particles by the sorbed P ions. They also indicate that the common practice of storing soil air-dry, even for short periods at low temperature, will not preserve the P status of the soil as at sampling. It was estimated that for a sample of soil which remained moist at 25°C for 100d after the addition of 335 μg P g⁻¹ soil, before being sampled and stored air-dry at 4°C for 16 years, the measured P_i test value would be about 15 μg P g⁻¹. This compares with 46 μg P g⁻¹ which is the estimated Pⁱ test value measured on the same day as sampling. When samples cannot be analysed for P status immediately following sampling, they should be stored at the lowest convenient temperature, preferably below 0°C.