Effect of pH and prior treatment with phosphate on the rate and amount of reaction of soils with phosphate

The rate at which phosphate continues to react with soil is an important determinant of its effectiveness as a fertiliser. Yet it has seldom been measured and we do not know which soil properties affect its value. Here, the effects of electric charge are investigated. The charge of a low phosphorus status soil was changed both by changing the soil pH by incubation with acid or alkali for 2 days at 60 degrees C, and also by adding phosphate and incubating for 30 days at 80 degrees C. The amount and rate of reaction of further additions of phosphate were then measured after periods of 1, 3, 10 and 30 days of incubation at 25 degrees C. Prior reaction with phosphate greatly decreased both the amount and the rate of sorption of subsequent additions. We think this is one factor causing differences between soils in the rate of reaction. Changing the pH had smaller effects on the amount and rate of reaction. We think the effects on rate of reaction were caused by changing the pH rather than the pH itself. The effects of changed pH and of phosphate treatment were closely described by a mechanistic model, with the effects on amount of sorption described by varying the electric potential and the effects on rate of reaction described by varying the rate of diffusion. The two methods of changing the charge have differing effects on both the amount and rate of reaction and we discuss the reasons for these differences. Highlights The rate at which phosphate continues to react with soil is an important determinant of its effectiveness as a fertiliser. Loading with phosphate greatly decreased both the amount and the rate of sorption of subsequent additions. Changing the pH had smaller effects. Results were closely described by a mechanistic model. Phosphate status is an important cause of differences between soils in the rate of reaction.