The effects of fluctuations in soil moisture content on the availability of recently applied phosphate

Abstract

Incubation for short periods at a high moisture content reduced the subsequent plant uptake of phosphorus from recently applied monocalcium phosphate in several soils. The effects on phosphorus uptake were reflected in the amounts of phosphate extracted by 0.5M sodium bicarbonate and by 0.01M calcium chloride. Phosphate availability decreased with increasing moisture content up to saturation. At saturation, availability decreased with increasing incubation period up to 4 days, but was not reversed by several weeks of subsequent incubation at 100 cm tension. The effect of waterlogging usually was greater on the air-dried soil than on soil which had undergone moist pre-incubation. The results suggest that the decrease in phosphate availability was closely associated with the reduction of iron during the anaerobic phase and its subsequent oxidation. Preincubation at 100 cm tension progressively decreased the amount of iron released, and phosphate subsequently immobilized. This appeared to be caused by oxidation of organic substrate during the aerobic phase, thus delaying the onset of anaerobiosis during waterlogging. Phosphate applied to the surface was affected by waterlogging in a similar way to phosphate mixed into the soil.