The objective of this study was the evaluation of herbage production and botanical composition of a native grassland, ("flechilla?'), of the Salado River Basin fertilized, during 3 years, with 0, 381, or 762 kg&a-'*yr-' of ammonium sulphate alone or combined with 0 or 208 kg-ha-'*yr-' of triple superphosphate. Annual dry matter accumulation of herbage, (ADMA), was restricted by a negative water-balance in the soil during the first experimental year. A linear response to ammonium sulphate rate, irrespective of triple superphosphate, was observed. In the following years the relationship between ADMA and ammonium sulphate became progressively quadratic, and it was manifest firstly in the plots fertilized with superphosphate. The response to superphosphate seemed to be due to an environmentally stimulated growth demand more than to a phosphorus deficiency in the soil. Ammonium sulphate promoted the growth of gramhtoids and decreased that of legumes and forbs. Superphosphate increased the proportion of legumes and ameliorated the detrimental effect of ammonium sulphate, but only few species reflected the effect of fertilization. Because (a) the sward reacted remarkably to the addition of a nitrogenous fertilizer, and (b) its main legume species, Medicogo polymorpha, (annual), was scarce, it is suggested that the sward's herbage production could be substantially increased by its enrichment with perennial legumes provided that their growth and expansion were assisted by periodic additions of a phosphorous fertilizer. Authors are research scientists, Centro de Ecofisiologia Vegetal (FECIC-CONICET-Fund. M. LILLO), Serrano 661, 1414 Capital Federal, R. Argentina. Authors wish to acknowledge Dr. Ratil Arraras Vergara for having permitted the conduction of the present trial in his farm "El Tr.&ito"and Ings. Agrs. E. Myers, R. Mendoza, a@ Mariana Kade for their invaluable help in the collection of data. Manuscript received April 18, 1980. JOURNAL OF RANGE MANAGEMENT 35(l). January 1982 Thurston, J.M. 1969. The effect of liming and fertilizers on the botanical composition of permanent grassland, and on yield of hay. In: T.H. Rorison (ed.), Ecological Aspects of the Mineral Nutrition of Plants, p, 3-10. Blackwell Scientific Publications, Oxford. Vervoorst, F. 1967. "La Vegetaci6n de la Repiblica Argentina. 7. Las Communidades Vegetales de la Depresi6n de1 Salado. (Pcia. de Buenos Soils". 6th Edition. MacMillan, New York. 567 p. Aires)". Serie Fitogeogritica. INTA. Buenos Aires. 259 p.