The information obtained through the humic fractions distribution can be used in brazilian soil studies as a diagnostic attribute or differential characteristic and therefore, contribute to the evolution of Brazilian System of Soil Classification (SiBCS). The objective of this study was to quantify the organic carbon in humic fractions of surface and subsurface diagnostic soil horizons and propose the use in the brazilian system of soil classification. A total of 169 diagnostic horizons distributed between Folistic Epipedon (O hístico), Histic Epipedon (H hístico), Mollic Epipedon (A chernozêmico), Umbric Epipedon (A húmico) and Spodic Horizon (B espódico) were used for this study. These horizons were evaluated for chemical and physical composition. The organic carbon tenors in the fractions: fulvic acids (C-FAF), humic acids (C-HAF) and humin (C-HUM) were quantified. C-HAF/C-FAF, C-AE/C-HUM and C-AE/COT (C-AE = C-FAF+C-HAF) ratio and the percentage of each fraction in the total organic carbon (TOC), %FAF, %HAF, %HUM, %AE (Alkaline Extract ) were calculated. The Folistic and Histic Epipedon presented predominance of C-HUM, folowed by C-HAF, the Mollic Epipedon presented absolutily dominance of C-HUM, the Umbric Epipedon presented predominance of C-HUM, followed by C-HAF and on Spodic Horizon were observed more expressive of C-HAF or C-FAF. The information of the distribution of humic fractions suggests the use of the following differential characteristics for the families and series levels of SiBCS: stable organic matter (surface mineral horizons): C-AE/C-HUM (< 0.5); iluvial organic matter (subsurface mineral horizons): C AE/C HUM (> 2.0) and the potential for leaching (system or soil): C-HAF/C-FAF and C AE/C HUM (> 1.0).
quantitative fractionating; characterization and soil classification
