The experiments were done on a productive clay loam Gleyic Cambisol used for agricultural production and were designed to estimate the effects of various cover crops – red clover (Trifolium pratense L.) and white clover (Trifolium repens L.) mixture with Italian ryegrass (Lolium multiflorum Lamk.) and white mustard (Sinapis alba L.) biomass and wheat (Triticum aestivum L.) straw incorporated into the soil on the composition of humus. In the first year, incorporation of only the cover crops’ biomass or together with straw increased the content of mobile humic acids (HA 1) by 10.7–28.0% compared with that before the trial was established.. Conditionally stable humic acids fractions (HA 2, HA 3) formed more intensively in the second year of the effects of the measures applied. Having incorporated cover crops’ biomass together with straw, the fraction of humic acids HA 3 tended to increase or was the same as that before the trial establishment. An increase in the content of humic acids, compared with the levels before the trial establishment determined positive changes in the one of the main indicators of humus quality – humification rate; they were the most distinct having incorporated red clover phytomass together with straw. The incorporation of mineral nitrogen fertilizer N 45 together with straw increased the soil organic matter mineralization rate and determined a reduction in humic acids content.