Humic substances (HS) represent soil organic matter fractions resulting from intermediate processes of microbiological decomposition of organic matter present in an ecosystem. Humus represents organic matter that has undergone a microbial decomposition process and has the characteristic of being resistant to further microbial attack. Among the HS, humic acids (HA) and fulvic acids (FA) are the most important because they act effectively in the environment. The objective of this work is to determine the chemical composition of the HA in a topossequence on Algodoal Island/Maiandeua-Pa. In the soil samples, Eutrofic Fulvic Neosol, Distrofic Fulvic Neosol, Distrofic Yellow Argillaceous and Spodosol, collected at depths of 0-5cm, 5-10cm and 10-20cm, the determination of organic carbon (OC), by the Walkley-Black method, of the organic carbon (AF) fraction was performed, since these soils present a large amount of this substance, while the values of the AH fraction found were negligible. The CO contents determined, of each soil profile, are distributed as follows along the topossequence: P6 (3%); P5A (3.07%); P5 (3.40%); P4A (3.11 %) and P4 (4.05%). The results of profiles P4 and P4A, when compared to the total organic carbon, of the same profiles, determined by Ribeiro (2000), show that most or almost all organic carbon is supplied by SH. Thus, it could be verified that other carbon suppliers to the soil, such as microbial biomass, labile carbon, etc., are contributing little while the FA fraction is being the main supplier. The CO values determined were low. The results obtained allow us to conclude that the low values of CO, of the AF fraction, can be related to a precarious microbial activity as a result of the quality of the organic matter supplied to the soil, the vegetation that predominates in the region is the Restinga and "leaves" are the most supplied material to the soil, and the lack of mineral nutrients available for this activity to continue.
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