Caracterização analítica da acidez em organossolos

Abstract

The Histosols have a small geographic extension in the Brazilian territory, but they have high agricultural usage, mainly for the subsistence farming, and have a great environmental importance. The high levels of organic matter in these soils play a fundamental part in all soil properties. In which refers to chemical soil properties, the pH and acidity of organic soils is related to the presence of organic compounds, to exchangeable hydrogen and aluminum, to iron sulfites and other oxidize able sulphur compounds, more than to the nature of the mineral soil fraction. Due to the dominance of low soil pH values on most Histosols, the Al hydrolysis is inferred as the main cause of exchangeable soil acidity and a high concentration of soluble Al in the soil solution. However, as a result of the strong interaction between the organic matter and the elements sorbed, the analytical routine methods in soil fertility may not properly evaluate the soil acidity in Histosols. The objectives of this study were: a) to evaluate analytical methods to measure acidity in soils with high level of organic matter; b) to test extracting solutions for methods to measure monomeric and total Al; c) to compare spectrophotometric methods to quantify Al; and d) to correlate chemical attributes of Histosols with the soil acidity parameters. The study was developed with twelve soil profiles from different Brazilian states; eleven profiles were classified as Histosols and one as an Inceptisol with high organic carbon content. To compare the soil acidity relationship to organic matter attributes and soil formation environment, the profiles were grouped in soils from coastal and river plains, and soils from high altitude mountainous areas and flat highlands. The soil pH was measured by different procedures: with water, KCl, CaCl2, and SMP soil:solution. The content of Al was measured using two extracting solutions, with copper chloride 0.5 mol L-1 and potassium chloride 1 mol L-1 (Al_K), and three methods for Al quantification; they were: titration with NaOH 0,025 mol L-1 (_Na), inductively connected plasma (_PL), and molecular spectrometry with xilenol orange (_AX) and pyrocatecol violet. The results showed that, for Histosols, the pH measured in water, KCl, and SMP solutions might be equally used to evaluate soil acidity. A high correlation between soil pH measured by SMP and potential acidity was also observed. The KCl solution was efficient to extract Al from soil solution, and the Al weakly associated to organic matter. The CuCl2 solution extracted much more Al when compared with the KCl. The amount of Al quantified by the PL and extracted by CuCl2 varied from 80.59 to 777.04mmolc dm-3 while the amount extracted by KCl varied from 1.8 to 102.33mmolc dm-3. The Al quantified by the PL showed high correlation with the Al measured by the AX. The comparison of correspondence between the methods Al_K_PL and Al_K_AX showed a high correlation (r = 0.99***), and the dispersion of the data is small, thus the methods are equivalent. The same did not happen when compared the methods Al_K_PL e Al_K_Na. The pyrocatecol violet method was not efficient quantifying the monomeric and the total Al. The levels of Al quantified by this method where weakly consistent with the values found for the other procedures. The differences between the two groups of profiles, as related to soil acidity and Al content, and their relationship to organic matter fractions, indicate that the separation of Histosols according to landscape position and soil formation environment is important to recommend practices for management of soil acidity, as well as soil fertility evaluation of these soils.