The nitrogen status of red and yellow earths in the semiarid tropics as influenced by Caribbean stylo (Stylosanthes hamata) grown at various rates of applied phosphorus

Abstract

Changes in the nitrogen status of a red and a yellow earth in the semi-arid tropics of central north Queensland were examined following five years' growth of Stylosanthes hamata pasture under a range of phosphorus inputs. The results highlight the importance of leaching of nitrate-N in these environments. This could be accounted for satisfactorily by a simple water and solute exchange model. Small increases in organic carbon and total nitrogen in soil were restricted to the early years of the pasture phase but were not significantly related to the phosphorus treatments. The nitrogen supply to a test crop of millet, both in the field and for surface 0-10 cm samples cropped in the glasshouse, was enhanced by previous growth of legume, and the magnitude of the effect depended upon the yields of legume grown as influenced by its phosphorus nutrition. In terms of freshly applied ammonium nitrate, the contribution from the legume measured in the field was equivalent to 33 kg N ha-1 for an annual production of 1 t ha-1 of legume dry matter on the red earth, and similarly 38 kg N ha-1 on the yellow earth. Of several methods of soil analysis studied, mineral-N extracted with 1 M KCl, either at room temperature or at boiling point, were the best single predictors of the nitrogen supply as measured by the nitrogen uptake of millet. Evidence is provided to show that forms other than mineral-N also contributed to the nitrogen supply, but none of the methods of soil analysis used was able to quantify this nitrogen arising from mineralisation.