The aim of research is to study the direction of humification processes – dehumification during prolonged irrigation on various types of soils. Objects of research are soils of key areas in various agro-climatic zones from gray forest (Ryazan region) to dark chestnut (Saratov region). Investigations on the effect of prolonged irrigation on humus content and its qualitative composition were carried out. As a result it was found that in gray forest soils with prolonged irrigation with fresh water the total humus content practically does not change and the total absence of anaerobic conditions in the vegetative fall of these soils even under irrigation leads to formation of a high-quality humus with a predominance of humic acids. The total humus content in typical chernozem of the forest-steppe zone increased by 0.08 %, and in ordinary chernozem – by 0.13 % in comparison with the plots without irrigation. In both soils by irrigation there has been a tendency to decrease the content of humic acids, but they remain humate by the type of humus. In comparison with non-irrigated plots in ordinary chernozem of the steppe zone with fresh water irrigation, the humus content is less by 0.53 %, and in irrigation with low-mineralized water by 0.93 %. The process of dehumification is also observed in the southern chernozems with solonets complex by irrigation with unfavorable water composition, which is also facilitated by the natural negative properties of these soils, first of all, alkalinity and low calcium content in the soil absorbing complex. In the dark chestnut soils, manure application over three rotations of a six-field grain-forage crop rotation with an average annual dose of 10 tons per hectare and mineral fertilizers of at least 150 kilograms per hectare, as well as a decrease in the water load on soils ensures sustainable increase of humus content in comparison with non-irrigated analogues by 18 %, and humus remains a humate one.
Key words: soil types, humification, dehumification, humus, humic acids, fulvic acids, irrigation.
