Soil & Tillage Research, 26 (1993) 227-236 227 Elsevier Science Publishers B.V., Amsterdam Tillage and crop rotation interactions on humic substances of a Typic Haplorthox from southern Brazil P.L.O. de A. Machado and M.H. Gerzabek Austrian Research Centre Seibersdorf, Department for Agriculture and Biotechnology, A-2444 Seibersdorf, Austria (Accepted 3 December 1992) ABSTRACT The effects of soil tillage and crop rotation on humic substances of an Oxisol from southern Brazil were studied. Soils were sampled from conventional (CT) and no-tillage (NT) systems applied to soybean-wheat (S-W) and soybean-lupine-maize-wheat (S-L-M-W) rotations. Two additional samples from soils with secondary forest and eucalyptus were collected. Humic substances (HS) were extracted by chelating ion exchange resin and water. Grey humic acids (GHA), brown humic acids (BHA) and fulvic acids (FA) were analysed chromatographically on controlled pore glass. As expected, cultivation and tillage led to a decrease in soil organic carbon content. This diminution was somewhat lower in conventional tillage with S-L-M-W, compared with S-W rotation. The total amount of extractable HS was higher for forest soils. Cultivated soils with both CT and NT showed higher contents of GHA than BHA and FA. Crop rotation had an important influence on the quality of the organic matter as the S-W rotation with both CT and NT showed the highest GHA contents. Soils with eucalyptus exhibited the highest amount of extractable humic substances among the in- vestigated soils. INTRODUCTION Oxisols are widely distributed over Brazil and many of them are acidic with low base saturation (Dematt~, 1981; Resende et al., 1988). The role of soil organic matter in alleviating nutrient and stress problems in tropical soils is well known (Morais et al., 1976; Sanchez, 1976 ), but compared with soils of temperate regions, fewer investigations exist about the composition and Correspondence to: P.L.O. de A. Machado, Austrian Research Centre Seibersdorf, Department for Agriculture and Biotechnology, A-2444 Seibersdorf, Austria. © 1993 Elsevier Science Publishers B.V. All rights reserved 0167-1987/93/$06.00