The intial and residual effects of superphosphate and rock phosphate for sorghum on a lateritic red earth

The initial and residual effects of superphosphate and rock phosphate with sorghum were recorded on Tippera clay loam a lateritic red earth at Katherine, N.T. Both forms of phosphate were highly effective as initial broadcast applications and as residues. Initially, superphosphate was superior to rock phosphate on a weight per acre basis. For superphosphate the residual value left after 1 year was 50% of the initial value, and after 7 years about 8%. For rock phosphate the residual value after 7 years was about 66–70% of the initial value. The data have been extrapolated to give rough estimates of the value to complete exhaustion of the residue. These estimates suggest that for both superphosphate and rock phosphate, regular annual dressings of 1 cwt and of 2 cwt will give in the long run at least 80% and 95% respectively of the sorghum production possible with phosphate not limiting.