ABSTRACT High acidity and low soil P availability in the soils from the Pampa natural grasslands generally limit the forage growth and economic return. Thus, to increase the economic return and avoid environmental degradation and biodiversity loss due to replacement by intensive cropping systems, it is essential to improve the soil P availability. This study aimed to diagnose the nutritional status of the pluri-specific natural grasslands amended with different history of P sources. Additionally, we also try to indicate the most appropriate range of soil P availability for maximizing forage growth. The experiment started in 1997 in an area of Pampa natural grasslands in the state of Rio Grande do Sul, Brazil. The treatments evaluated the application of Gafsa hyperphosphate (HP) and triple superphosphate (SP) for four years (1997, 1998, 2002, and 2010). Soil and aboveground biomass sampling were performed five times from October 2010 to March 2011 to access aboveground biomass production, botanical composition, soil available P, and N and P nutrition index. Greater P availability in the soil amended with SP produced more forage and resulted in a higher P accumulation than in the soil amended with HP. The ideal range of soil P availability in the soils of natural grasslands ranged from 14 to 20 mg kg˗1, varying for different plant species. The species Paspalum plicatulum and Aristida laevis produce less aboveground biomass and accumulate less P in their tissue than Dichantelium sabulorum and Eustachys uliginosa, and P. notatum. Therefore, areas of natural grasslands dominated by D. sabulorum, E. uliginosa and P. notatum demand higher soil P availability to maintain high forage production. Soil P fertilization of pluri-specific natural grasslands in the Brazilian Pampa biome must consider the dominant forage species in the area and the soil P availability.