The effects of superphosphate on the growth and development of Townsville stylo (Stylosanthes humilis) in pure ungrazed swards at Katherine, NT

Abstract

The effects of four levels of superphosphate (0, 112, 224, and 448 lb an acre) on the growth and development of ungrazed Townsville stylo (Stylosanthes humilis) in pure swards were examined on virgin Tippera clay loam at Katherine, N.T. Yields and distribution of dry matter, nitrogen, and phosphorus, leaf area, and growth rate, were measured every two weeks. Leaf relative water content and soil water extraction were measured at the end of the wet season. Yield responses to superphosphate were curvilinear and fitted regressions of the form y = a nx + bx + c at all harvests. Superphosphate encouraged root exploitation of the soil volume, but phosphorus deficient swards were more resistant to drought. The maximum recovery of phosphorus was 20, 14, and 14 per cent respectively when 112, 224, and 448 lb an acre superphosphate was applied. Loss of 31 per cent dry matter, 34 per cent nitrogen, and 44 per cent phosphorus in the eight weeks following maximum yield was concluded to be due to translocation to the root system. Even with 448 lb an acre superphosphate the phosphorus content of standing material was below the the critical percentage quoted in the literature as indicating phosphorus sufficiency.