With the development of soil classification to the fourth category level of the Brazilian Soil Classification System - SiBCS, it seeks the development and standardization of proposals for the classification of soils in 5 th and 6 th categorical levels (family and series). In this sense, this paper aims to available the viability of the humic fractions for the classification of diagnostic horizons in the 5th or 6th categorical levels (family and series) of SiBCS. For the development of the proposal a total of 169 diagnostic horizons distributed among Folistic Epipedon (O hístico), Histic Epipedon (H hístico), Mollic Epipedon (A chernozêmico), Umbric Epipedon (A húmico) and Spodic Horizon (B espódico) were used. These horizons were analyzed for chemical and physical composition. Also quantified the levels of organic carbon fractions: fulvic acids (C-FAF), humic acids (C-HAF) and humin (C-HUM) and alkaline extract (C-AE). Relations were calculated C-HAF/C-FAF, C-AE/C-HUM (AE = C-FAF + C-HAF) and %FAF, % FAH and %HUM. Proposals for classes completed for each diagnostic horizons in the 5th or 6th categorical levels are related to the C-FAH, C-HUM and C-HAF/C-FAF and C-AE/C-HUM ratios, identified from significant correlations with the chemical and physical attributes. For O and H Epipedons presents classes based on the C-HAF/C-FAF ratio. The Mollic and Umbric Epipedons have been two classes for each horizon, based on the C-HUM and C-HAF/C-FAF ratio for the first and the C-HAF and C-HUM, for the second. For Spodic Horizon, are presented three classes, from the C-HAF and C-HAF/C-FAF and C-AE/C-HUM ratios.
SiBCS; organic matter and classification classes
