In this paper a mathematical model describing the main kinetics activated during the degradation processes of organic matter added to soil is proposed. The model analyses the evolution, with time, of the biomass, total nitrogen, inorganic nitrogen, carbon and humus, describing their interrelations. Some results have been compared to the experimental data of a degradation process, in cultivated soil, of water-soluble waste from olive processing. The model has been used to simulate kinetics resulting from perturbations caused by different CIN ratios.