Response of subterranean clover to rock phosphates as affected by particle size and depth of mixing in the soil

Abstract

Samples of Christmas Island and Florida rock phosphates with different particle size ranges were applied to a phosphorus-deficient acid sandy soil. The rock phosphates were compared with monocalcium phosphate at various levels of addition of phosphorus. The fertilizers were mixed uniformly with either the top 1, 5, or 10 cm of the soil. Dry matter yield and phosphorus uptake in the tops of subterranean clover (Trifolium subterraneum) and the distribution (by weight) of the roots in the soil were determined. Monocalcium phosphate produced higher dry matter yields (tops and roots) than rock phosphates, even when the latter were applied in amounts containing four times the amount of phosphorus added as monocalcium phosphate. The concentration and uptake of phosphorus in the tops was also higher when monocalcium phosphate was applied, although the difference in concentration decreased as the plants matured. Dry matter yield and phosphorus concentration in the clover tops were increased by increasing the fineness of grinding of rock phosphate and by increasing the depth of mixing with the soil. Increasing the depth of mixing also produced a greater weight and more even distribution of roots. The yield of tops and roots was unaffected by the depth to which monocalcium phosphate was mixed, although shallow placement concentrated the distribution of the roots near the soil surface. Florida rock phosphate was a better source of phosphorus for subterranean clover than Christmas Island rock when the samples were coarsely ground, but both rocks were equally effective when ground to give 70 per cent < 100 mesh B.S.S.