The residual effectiveness of phosphorus for stylosanthes pastures on red and yellow earths in the semi-arid tropics

Abstract

A mathematical description is presented of the effects through time of phosphorus fertilizer application on the growth of Stylosanthes hamata cv. Verano and on changes in extractable soil phosphorus on a red and a yellow earth in the semi-arid tropics. For the five years of observations, the models used provided reasonable descriptions of both aspects. It was not possible, however, to clearly demonstrate which of two functions tested better described the decreasing effectiveness of phosphorus with time. The simpler exponential function provided as good a fit to the yield data as did the function proposed by Barrow, whilst for the extractable phosphorus data the latter did explain a significantly higher proportion of the variance, but its coefficients could not be estimated with great precision. The results show that the phosphorus requirements of such pastures on these soils are modest. Extractable soil phosporus levels of approximately 12 �g g-1 in either 0.005 M H2SO4 or 0.5 M NaHCO3 are indicated as being adequate to ensure near-maximum yields of the legume. Long-term maintenance fertilizer needs as predicted by the models are discussed.