EFFECT OF PREVIOUS ADDITIONS OF PHOSPHATE ON PHOSPHATE ADSORPTION BY SOILS

Phosphate, at several levels of application, was incubated with four soils for 12 months at 25°C or was left in contact with soil in the field for three years. The capacity of the soils to adsorb further phosphate and the proportion of the phosphate remaining isotopically exchangeable were then measured. Previous additions of phosphate reduced the capacity of the soils to adsorb further phosphate. Not only were the adsorption curves displaced but they were also of lower slope; that is, the buffering capacity for phosphate was decreased. Only a small proportion of the previously added phosphate remained isotopically exchangeable within 24 hr. The results indicated that some of the phosphate had been converted into a form which was occupying phosphate adsorption sites, blocking them from further reaction, and thus reducing the buffering capacity for phosphate. The reduction was not linearly related to level of application. Low levels of application produced a proportionately larger effect than high levels.