Residual effects of phosphate fertilizers in relation to phosphate sorptivities of 27 soils

Three different parameters of the long term effects of phosphate fertilizers on perennial clover-based pastures were measured over 3–4 years in 27 experiments on acidic soils in an elevated region of eastern Australia. Recovery of fertilizer P was the difference between herbage P uptake in the presence of fertilizer and uptake in its absence, expressed as a % of the amount of P applied in the first year. Residual value was the size of the response to fertilizer P, applied in the first year, expressed as a percentage of the response to freshly applied P in the second and third years. Effectiveness was the product of the values of the Mitscherlich curvature and response parameters for each response curve, and residual effectiveness was relative to initial effectiveness. Soils varied widely in their P sorptivities, and represented Alfisols, Entisols, and Ultisols of basaltic, granitic and sedimentary origin. Mean P recoveries of 29% in the first year and 49% over 3 years, residual values of 84% in the second year and 60% in the third, and residual effectiveness of 77% in the second year and 47% in the third were high by most standards. Increasing P sorptivity tended to increase fertilizer effectiveness in the first year and residual value in the second year, but it depressed P recovery in the first year and residual effectiveness in later years. The long term effect of increasing P sorptivity on cumulative P recoveries tended to be negative at low to medium rates of fertilizer application and positive at high rates of application. There was a much smaller decline in residual values and effectiveness over the 3 or 4 years than there was in P recovery, and this was attributed to the beneficial effects of P on soil N fertility, via clover N fixation, and the subsequent growth of grasses in the phosphated treatments.