Nutrient problems in sown pasture on an acid soil. 3. Thermal phosphates

Abstract

Two thermal silico-phosphates, Rhenania phoshate and Mitsui phosphate, were compared with superphosphate, in the presence and absence of lime, as phosphate fertilizers for an infertile acid soil. The field evidence suggested that silico-phosphates, up to rates supplying 193 kg P ha-1, can be as effective as the lime-superphosphate combination. The combination, supplying 386 kg P ha-1 yielded 4.9 and 8.0 t dry matter ha-1 in the first and second years respectively. Rhenania and Mitsui phosphates, each supplying 193 kg P ha-1, respectively yielded 3.1 and 2.8 t ha-1 in the first year and 4.4 and 5.2 t ha-1 in the second year. There was no evidence that thermal phosphates were superior to the limesuperphosphate combination. All three phosphates had a good residual value in the field. Lime-superphosphate, lime-silicate, and Rhenania phosphate dressings increased soil pH and reduced titratable soil acidity, which in this case was principally exchangeable aluminium. Supplying calcium and raising soil pH appeared to be more important effects of these fertilizers than the effect of their silicate content, though there was some evidence of a positive interaction between the effects of calcium and pH, and the effect of the silicate. Plant growth increased the titratable acidity in proportion to the growth made and the evidence was consistent with the formation of an aluminium-phosphorus complex providing an important source of phosphorus. The role of such a complex in the heavy phosphate applications required and the good residual value of these dressings are-discussed.