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Foi estudada a capacidade de troca de cations (C.T.C.), da matéria orgânica e da fração mineral, em 103 amostras de 22 perfis de solos do Estado de São Paulo. A C.T.C. da matéria orgânica variou de 190 a 400 e.mg por 100 g, em amostras superficiais, e de 160 a 490 em subsuperficiais. Verificou-se estreita correlação entre a C.T.C. da matéria orgânica e o pH dos solos. A C.T.C. da fração mineral no horizonte B variou entre 2,7 e 20,0 e.mg. Em média, a contribuição da matéria orgânica à C.T.C. foi de 74% para amostras superficiais. Atingiu a 58% para os horizontes A2 e A3, a 47% para o B1 e a 35% para B2 e B22. Data of cation-exchange capacity (C.E.C.) were obtained for 22 soil profiles of the State of São Paulo. C.E.C. of organic matter varied from 190 to 400 me. per 100 g in the samples from surface horizons, and from 160 to 490 in those from other horizons. A highly significant relationship was obtained between organic C.E.C. and soil pH, for the soil samples from surface horizons (r++= 0.81) and also for the horizons collected just below the surface horizon (r++ = 0.76). Clay C.E.C. varied between 2.7 and 20.0 me. per 100 g in 21 of the soil profiles studied. It was not possible to relate the clay C.E.C. values to the mineralogical composition, as determined by X-ray diffraction. As average, the contribution of organic matter to the C.E.C. in 21 soil profiles was: 74 per cent for surface samples; 58 per cent for the A2 and A3 horizons; 47 per cent for the B1 horizon; and 35 per cent for the B2 horizon

A capacidade de troca de catíons das frações orgânica e mineral em solos