The residual effect of superphosphate in soils cultivated for wheat in South Australia.

Abstract

This paper presents further results of a study of phosphorus in the red-brown earths and solonized brown soils of South Australia. The investigation involved both field and pot-culture experiments in which different amounts of superphosphate were applied to soils which had received previous superphosphate applications ranging from 0 to nearly 30 cwt. per acre. Only a small part of the phosphorus applied has been removed in crops and the residual phosphorus has remained in the soil in fairly readily available inorganic forms. Where previous applications of superphosphate exceeded 15 cwt. per acre, little if any response in yield of wheat grain was obtained with current superphosphate dressings, although yields of straw were sometimes increased. The phosphorus content of wheat grown under South Australian conditions is lower than that of overseas crops and is dependent on the phosphorus status of the soil. It is only slightly increased by moderate applications of superphosphate. Evidence is presented of a relationship between response to superphosphate and the nitrogen status of the soil which suggests that no appreciable response to applied phosphate is likely with stubble-sown crops other than legumes on soils of improved phosphorus status.