Effects of waterlogging and subsequent drainage of a pasture soil on phosphate sorption, extractable phosphate and oxalate-extractable iron

Undisturbed cores of a pasture soil from the central tablelands of New South Wales were waterlogged for periods varying from 4 to 42 days and then drained to simulate seasonal waterlogging. Phosphate sorption, Bray 1 extractable phosphate and oxalate-extractable iron levels were measured before watering, at the end of waterlogging and up to 90 days after draining. Increasing the duration of waterlogging increased the logarithms of phosphate sorption and Bray 1 extractable phosphate linearly, but the response equation of log oxalate iron was significantly quadratic. When drained, all three levels fell, but the positive sorption response to waterlogging remained 90 days after draining. These results are consistent with those from previous studies that found phosphate solubility to be associated with the reduction of iron during waterlogging and its subsequent oxidation on drying. The agronomic implication of the findings for superphosphate topdressing of seasonally waterlogged soils is discussed.