Loss of availability of phosphate in New Zealand soils

Abstract

SUMMARYPhosphate sorption was measured by the method of Barrow (1980) using a laboratory incubation procedure for up to 60 d on four soils which had different mineralogies but medium to high phosphate retention. All the soils had slow reactions where phosphate sorption continued, but at a decreasing rate, with time. The rate of decrease in the slow reactions was similar on all the soils. Phosphate became less available to plants during the slow reactions, and results of a pot trial with white clover showed that, on all the soils, phosphate incubated with the soils for 218 d was about 65% as effective as phosphate incubated for 10d.When 700 mg P kg−1 was added to allophanic soils (Andisols), about 100 mg kg−1 was strongly adsorbed, about 200 mg kg−1 became unavailable in about 200 days and the remainder was weakly adsorbed. A similar result was obtained on Waiarikiki soil (Inceptisol), which contained ferrihydrite and Al‐humus as the predominant reactive species. On the Kerikeri soil (Oxisol) about 150 mg P kg−1 became unavailable with time as a result of reactions with geothite, hematite and Al‐humus.The phosphate uptake by the microbial biomass was similar to the uptake by the clover, and immobilization of phosphate in the biomass can contribute to the loss of availability of phosphate in soils.