Sulphur-nitrogen relationships in the tropical legume Stylosanthes humilis

The effects of age and sulphur fertilization on various sulphur fractions in the legume Stylosanthes humilis were examined. Plant material was obtained in two consecutive seasons from nodulated plants growing in the field and from a glasshouse experiment. A very close relationship between the concentration of total nitrogen (Nt), that of total sulphur (St), and age was found. In young material Nt increased at first then reached a plateau with increases in St (brought about by additions of sulphur). As the plants aged, however, both Nt and St declined and the relation between them became more nearly linear. Nt and St were also closely related in seed from these plants but the St/Nt ratio was much lower than in the vegetative material. A moderate deficiency of sulphur had little effect on the ratio of alcohol-insoluble to total nitrogen. In contrast with published work on other species it was found that sulphur applications increased the ratio of sulphur to nitrogen in the alcohol-insoluble (protein) fraction.