Biological processes affecting soil fertility and productivity in land ecosystems are mainly based on transformations of organic matter, both native one and that introduced with postharvest plant residues and animal decomposition. Therefore, the objective of the study, was to determine fundamental physico-chemical parameters of humic acids developed over the decomposition of plant residues in soil, both under aerobic and anaerobic conditions. The investigations were carried out in pots under laboratory conditions. The soil originated from a field not used for agricultural production. The samples were taken from three depths 0–10 cm, 10–20 cm and 20–30 cm and then separated to two parts, A and B. The A samples were supplemented with plant material at the ratio 50:1, mixed thoroughly and incubated. After that the samples were assayed for basic physico-chemical parameters. Humic acids isolated from those samples were assayed for elemental composition, susceptibility to oxidation with 1.5% H2O2 and optical properties of 0.02% solutions in 0.1 mol NaOH at the ranges: 400 nm, 464 nm, 600 nm and 664 nm.