Avaliação de métodos de determinação da Acidez Potencial em LATOSSOLOS do Triângulo Mineiro

Abstract

The Potential Acidity (H+Al), which represents the soil's buffering capacity for acidity, is characterized by the exchangeable acidity, mainly composed of Al3+ adsorbed in the sorptive complex and extracted by ion exchange with K+ , and especially by the non-exchangeable acidity, characterized by H covalently bonded to functional groups of soil colloids, determined at a specific pH value. Accurate estimation of Potential Acidity is essential for obtaining exact levels of CEC at pH 7.0 and, consequently, the total (V) or individual base saturation, affecting the doses of soil amendments based on V. The H+Al content can be determined by incubating the soil with CaCO3, using a 0.5 mol/L Calcium Acetate extractor and SMP indices. The incubation method is not suitable for routine laboratory use due to the long time required for setup and evaluation; however, as it determines H+Al with high accuracy through a direct acid-base neutralization reaction at a given pH, it is considered a reference method. As for routine methods, scientific literature shows inaccuracy in both. For the Acetate method, the equilibrium pH of the soil/extractor is generally below 6.0, leading to an underestimation of soil acidity at pH 7.0. Meanwhile, the SMP is an indirect method based on previous regional calibration, but as soil analysis laboratories expand their activities beyond territorial limits, the estimated acidity levels vary greatly. The objective of this study was to evaluate the accuracy of the SMP and Calcium Acetate methods in OXISOLS from the Triângulo Mineiro region. Representative samples of each soil were collected, and for the 6 SMP buffer method, accuracy was also evaluated using a modified solution without potassium dichromate (a highly toxic buffer component). In the Calcium Acetate method, variations in the extractor concentration (0.5, 1.0, and 1.5 mol/L) and pH (7.0, 7.5, 8.0, and 8.5) were evaluated. To determine the reference H+Al content, 40 treatments were used (8 doses of CaCO3 and 5 soils). For the analysis of methods based on the standard SMP buffer, SMP buffer without K2Cr2O7, and standard Calcium Acetate (0.5 mol/L pH 7), 5 treatments were used (5 soils); finally, for the analysis of concentration and pH variations in the Calcium Acetate method, 36 treatments were used (3 soils, 3 buffer concentrations, and 4 pH values). The experimental design used was randomized blocks with three replications. Means were compared using the Scott-Knott test for qualitative attributes and regression analysis with breakdowns for quantitative attributes; additionally, the SISVAR and GraphPad Prism programs were used to analyze the estimated H+Al values and the equilibrium pH values obtained with pH and concentration variations in the Calcium Acetate solution. It was possible to observe a significant variation in the accuracy of the SMP equations in estimating the Potential Acidity of soils. In general, the absence of potassium dichromate in the SMP buffer resulted in more accurate H+Al values. Although some equations showed greater accuracy, the uncertainty observed in certain soils prevents them from being considered universal. Additionally, the equilibrium of the Calcium Acetate solution, after 16 h of rest, was established at a pH ranging from 6.5 to 6.8. The concentration variation of the Calcium Acetate solution more noticeably influenced the ability to extract H+Al compared to the pH variation of the solution. The Calcium Acetate solution adjusted to a concentration of 1.5 mol/L and pH 8.5 obtained H+Al levels closest to the real Potential Acidity, determined by incubation with CaCO3. Furthermore, there is a clear trend of increased H+Al extraction with the increase in buffer concentration and initial pH