Estudos de calibração para determinação da acidez potencial e necessidade de calagem pelo método pH SMP de solos da região oriental do Paraguai

Abstract

Potential acidity is important to determine the liming requirement using the base saturation method. However, determination with calcium acetate at pH 7.0 is time consuming and hinders routine analyses. Shoemaker, MacLean, and Prats showed that the pH measured with the pH SMP buffer, which is an abbreviation of their surnames, has a good correlation with the potential acidity determined by calcium acetate. In Paraguay, a number of laboratories estimate potential acidity using calcium acetate at pH 7.0 method while others use the pH SMP buffer, even though with equations developed in Brazil. Because of the great variability of existing soils, regional studies are required for a suitable estimation. The objectives of this study were to create a calibration curve using the pH SMP buffer to determine the potential acidity of soils of the eastern region of Paraguay, at two sampling depths; to evaluate the method of incubation with increasing CaCO3 rates in five soils with different granulometry; and to compare it with other methods of liming requirement (LR). Soils with different chemical and physical characteristics were sampled at two depths (0-10 and 0-20 cm), totaling 202 samples. The analyses were carried out in the laboratory of the Sudamerica Research and Development Center, municipality of Minga Guazú, department of Alto Paraná, Paraguay. A strong correlation was found between the potential acidity measured with calcium acetate at pH 7.0 and the potential acidity estimated with pH SMP buffer, using the equation H+Al (cmolc dm-3) = 91.549 – 23.077pH SMP + 1.457 pH SMP2 (R2 = 0.9581). The calculation of liming requirement using the Minas Gerais method resulted in values very close to those obtained with the SMP index by incubation with CaCO3 to reach pH (H2O) 5.5, whereas the other methods underestimate or overestimate LR, which may result in values above or below the ideal.