Exchangeable Aluminum Evaluation in Acid Soils

One of the main factors limiting agricultural production in tropical climate regions is mainly related to the presence of exchangeable aluminum (Al) in highly weathered acid soils. Four methods of Al determination extracted with neutral 1 mol L KCl solution were evaluated: three colorimetric methods (aluminon plus ascorbic acid, and eriochrome cyanine R by FIA) and the usual titrimetric method with backtitration. Surface samples from 20 soils of different Brazilian regions, with active acidity (0.01 mol L CaCl2 pH) ranging from very high to medium (3.82 to 5.52), were used. The variance analysis revealed significant interaction among Al determination methods and soil. Mean methods comparisons within each soil (Tukey, P < 0.05) indicated that, for most of the soils, the methods differed among each other, although there were high correlations between the obtained values. Al values determined for soil samples by titration varied between 0.15 and 14.71 mmolc dm . The colorimetric methods showed higher values than the titration method, mainly for those with aluminon (up to 18.75 mmolc dm ). The Al contents of colorimetric methods correlated quadraticaly with the titration values, for the soil samples with Al > 10 mmolc dm . Among colorimetric methods, in operational terms, the eriochrome with FIA method presented analytical performance up to 50 samples per hour, easiness and sensibility for routine Al analysis in soil samples. However, due to the specificity, the titration/back-titration method should be used, despite the moroseness, when the Al ions are the objective.