The change of cultivation systems from conventional to no-tillage, the change in soil sampling depth and the higher crop yields over time can influence the critical P content, fertility ranges and fertilizer doses recommended for crops. This study calibrated the soil P tests, Mehlich-1, Mehlich-3 and anion-exchange (AER) resin for soyabean, wheat and maize plants cultivated under no-tillage system, and to estimate the P fertilizer amounts for a maximum economic yield. Soil samples and yield results from several experiments under no-till and different P doses, conducted by institutions of education, research and extension of the state of Rio Grande do Sul, Brazil, were used. The calibration curves were fitted using non-linear model functions, and the levels of soil fertility and fertilizer doses for a maximum economic yield were inferred. Results indicated that the average increases in maize, wheat and soyabean yields were 47.1, 12.4 and 7.2 kg/ha per kg of P 2O 5, respectively; the determination coefficients between the relative yield and soil P concentrations were higher when the soils were separated by texture classes and were highest in the 0-10 cm than the 0-20 cm layer; the critical values of P, based on Mehlich-1 were 7.5, 15.0 and 21.0 mg/kg in the 0-20 cm layer, and of 16.0, 28.0 and 40.0 mg/kg in the 0-10 cm layer in the clay classes 1, 2 and 3, respectively. Phosphorus fertility ranges were wider by Mehlich-3 and AER as compared to Mehlich-1. The quantities of P fertilizer were higher for soyabean and maize when compared to the currently recommended quantities. The critical P concentrations are higher in soils under no-tillage with soyabean, wheat and maize in the 0-20 as well as the 0-10 cm layer. The fertility ranges by resin and Mehlich-3 were wider than by Mehlich-1. The fertilizer P quantities are higher for soyabean and maize grown under no-tillage.
