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The objective of this study was the evaluation of herbage production and botanical composition of a native grassland, ("flechillar"), of the Salado River Basin fertilized, during 3 years, with 0, 381, or 762 kg ha-1 yr-1 of ammonium sulphate alone or combined with 0 or 208 kg ha-1 yr-1 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 graminoids 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, Medicago 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. This material was digitized as part of a cooperative project between the Society for Range Management and the University of Arizona Libraries. The Journal of Range Management archives are made available by the Society for Range Management and the University of Arizona Libraries. Contact lbry-journals@email.arizona.edu for further information. Migrated from OJS platform August 2020