EFFECT OF PREVIOUS ADDITIONS OF SUPERPHOSPHATE ON SORPTION OF PHOSPHATE

Samples of the topsoil were collected from plots of 8 field experiment s which had been fertilised with P at several rates from 8 months to m ore than 4 years previously. Phosphate sorption curves were measured a nd described by the equation: S = ac(o)(b)-q where S indicates sorptio n, c(o) observed concentration, and a, b, and q are parameters. From t he fitted curves, sorption between 0.25 and 0.35 mg P/L and sorption a t 0.3 mg P/L were calculated. Also measured were: the P retention inde x (PRI), oxalate-extractable aluminium, oxalate-extractable iron, and the pH of the soil measured in a sodium fluoride solution. The effects of the applications of P on the sorption of newly added P differed be tween sites. On 5 of the 8 sites, the slope term (a) of the fitted equ ation decreased with increasing levels of previously applied P. Conseq uently, the P buffering, measured as P sorbed between 0.25 and 0.35 mg P/L, decreased. When the decrease in P buffering was large, the indir ect measures of buffering capacity (oxalate-extractable aluminium, oxa late-extractable iron, and the pH of the soil measured in sodium fluor ide) also decreased. On the other 3 sites, the slope term did not decr ease significantly. Indeed on one site there was a small increase. On these 3 sites, the effect of the P application on the sorption curves was largely shown by increases in the intercept term (q). The indirect measures of P buffering sometimes increased with increasing applicati on of P. On all sites, both the P sorbed at 0.3 mg P/L and the values of PRI decreased with increasing P applications. This is because their values are determined by the values of both a and q. We argue that th e decreases in P buffering observed on 5 sites would increase the effe ctiveness of subsequently added fertilisers and would limit the abilit y of such soils to act as sinks for phosphate disposal. We suggest tha t the differing behaviour on the other 3 sites may have occurred becau se there was little continuing reaction between the soil and P.