Tamponamento de fósforo e adubação corretiva em solos da região noroeste do Rio Grande do Sul

Abstract

Highly weathered, acidic soils with low chemical fertility require corrective measures. Phosphorus concentrations in these soils are generally low due to the high adsorption capacity of oxides in the clay fraction. The study aimed to assess phosphorus buffering in soils below critical levels in the northwest region of Rio Grande do Sul, Brazil, and determine corrective doses of phosphate fertilization. The experiment took place at the soil chemistry and fertility laboratory of the Federal University of Fronteira Sul. Thirty soil samples from the northwest region were characterized and incubated for 20 days with varying P doses. At the end of the incubation period, available P values by Mehlich-1 and the phosphorus buffering capacity necessary to increase levels by 1 mg dm-3 were determined. The values of CTP (phosphorus buffering capacity) showed a significant correlation with initial P content and levels of clay, silt, and sand. Results showed that lower initial P values in the soil and higher clay content required higher doses of P2O5 to increase phosphorus levels by 1 mg dm-3. In conclusion, current soil phosphorus correction recommendations are insufficient to reach critical levels. Clay content and initial P levels should be considered in determining the corrective P dose. It was found that doses of 572.25, 227.93 and 60.23 and; 738.0, 241.0 and 68.9 kg ha-1 of P2O5 were considered necessary to bring soils in classes 1 and 2 with "very low", "low" and "medium" contents, respectively, up to the critical level.