Sugar-beet washing earth (1), ash from a straw combustion furnace (2), and a mixture of both those waste products (3) were added once to pots with light soil. Subsequently, lime stabilised sewage sludge from a treatment plant in Zamość, was added in the amounts equal to 2 and 5% of the soil weight. The control "0" contained no sludge. On those substrata, maize was grown in a three-year monoculture. In the beginning and at the end of the study, the level of organic carbon con tent in the soil, as well as its fraction soluble in 0.1 M Na₄P₂O₇ + 0.1 M NaOH and 0.05 M H₂SO₄ was determined. It was determined that sewage sludge had the greatest effect on the organic matter, the effect of other waste products were far less pronounced. Sludge addition increased the level of organic carbon proportionally to the amount of sludge used. The absolute content of carbon compounds soluble in pyrophosphate and sulphuric acid also increased, but the proportion of both those fractions in the total amount of organic carbon decreased. It has been established that after three years, the process of slugged organic matter transformation into the humus was already noticeable. Carbon to nitrogen ratio improved and humic acids condensation also increased, while the proportion of mobile compounds (fulvic acids and compounds extracted with H₂SO₄) decreased, which indicates organic matter stabilisation. A strong correlation between the amount of carbon fractions and maize yield, as well as many other characteristics of the soil and the plants were established.