Studies on the fertility of seddon soil (Kangaroo Island, South Australia). 1. The response of subterranean clover to various phosphatic fertilizers.

Seddon gravelly sandy loam from Kangaroo Island has proved to be grossly deficient in both total and available native phosphorus. Growth of subterranean clover (Trifolium subterraneum L.) in this soil is virtually nil unless phosphatic fertilizers are applied to it. Experiments with eight different phosphatic fertilizers showed that thermal phosphates of the Rhenania type were slightly superior to monocalcic phosphate as a source of plant phosphorus on Seddon soil. Both types of phosphate were equally efficient in the contrasting Urrbrae loam. Rock phosphate proved to be an extremely poor source of plant phosphorus. In the first year the availability of monocalcic phosphate applied to the Seddon soil was 20 per cent. or half that of material applied to the Urrbrae loam. The comparative residual value in the second year of applied phosphate was greater, however, in the Seddon soil than in the Urrbrae loam. Yields and phosphate availability were influenced by seasonal variation, but the phosphorus content of the plant material was nearly constant in different years for any particular yield from the same soil. The requirement for phosphorus at yields below the maximum was greater, however, in plants growing on Urrbrae loam than on Seddon soil. At maximum yields the ratio N/P205 was constant for plants from the two soils, but this did not hold where phosphorus was in minimum supply. The availability of residual phosphate from Seddon soil to which small annual additions of fertilizer had been made over 12 years was markedly lower than that from virgin soil to which phosphate had been added 12 months previously. The lower availability from the partially developed soil has been tentatively ascribed to the concurrent accumulation of organic matter. Results support the practice of making small annual applications of superphosphate to Urrbrae loam but it would appear that greater overall production could be obtained from Seddon soil by heavier initial applications. Amounts of P205 as low as 0.17-0.20 per cent. were found in the dry matter of subterranean clover growing on Seddon soil. Values below 0.3 per cent. are considered to be indicative of phosphate deficiency.