Particles of single superphosphate were incubated at a range of levels with a soil for periods ranging up to 217 days and at temperatures which ranged from 10°C to 35°C. In a separate study, particles of superphosphate were incubated at a range of water contents. The effectiveness of the phosphate for plant growth was then compared with that of a control treatment in the glasshouse. The index used to measure relative effectiveness was the ratio of the exponents in an exponential equation used to describe the plant response. Effectiveness of phosphate did not decrease in a treatment which was incubated air-dry after an initial wetting to near field capacity. Effectiveness decreased in a treatment incubated at just below permanent wilting point but the decrease was not as large as in the treatments incubated at a higher water content. The decrease in effectiveness was greatly affected by the temperature of incubation. It was possible to describe the effects of time and temperature by the empirical rate equation: (Equation presented) where α is the proportion of the phosphate converted into an ineffective form. The rate constant k was found to be proportional to exp (−E/RT) where E is analogous to the energy of activation and T is the temperature in °K. It was found that the value of E was 19,600 calories per mole. This indicates that there was a more than threefold increase in the rate of change for each 10°C increase in temperature. The value of n was about 3.