Does long contact with the soil improve the efficiency of rock phosphate? Results of isotopic studies

The effect of incubation on the fate of phosphorus in four phosphatic fertilizers (diammonium phosphate and three rock phosphates) applied to four weakly acid to acid soils was studied. Percent utilisation of fertilizer P by the crop was measured by isotopic labelling and the level and quality of available soil P following addition of fertilizer was measured by the isotopic dilution kinetics method. Percent utilisation of fertilizer P decreased as time of contact between fertilizer and soil increased. The quantity of available soil P increased immediately after applying fertilizer but then decreased. The efficiency of P from rock phosphate was not increased by application long before sowing the crop. From practical viewpoint it is important to apply P fertilizer as near as possible to the time of planting in order to reduce the negative effects of P fixation by the soil.