Seasonal changes of soil carbohydrate and root weight were studied for the grey forest soil at different phases of the growth of barley. The soil structure was characterized simultaneously by soil aggregate composition, bulk density and water retention capacity. Concurrent changes of the characteristics measured were observed. Water retention capacity was inversely related to macroporosity. The role of hydrocarbons in less than 2 mm soil aggregate formation was analysed by the method of cluster processing. Surface areas of less than 3 mm aggregate fractions increased with the increase of carbohydrate content and decreased after oxidation of carbohydrates.