The aim of research was to determine the changes of physical properties of ordinary chernozem under rainfed and irrigated conditions (for sprinkling and drip irrigation). The study was conducted in central irrigated zone of Rostov region. It has been found out that by agricultural use under irrigated and rainfed conditions the microstructure of ordinary chernozem remains practically unchanged compared with the virgin soils. The dispersion factor of investigated soils ranged from 2.6 to 6.8. Such values enable to evaluate its microstructure properties as high ones. The data on the index of soil instability indicate the weakening of humus influence on soil aggregation in a line: virgin soils > rainfed soils > drip irrigation > sprinkling. The soil macrostructure is influenced mostly when exposed to agricultural use. The most favorable indicators of the structural state are kept only under virgin conditions: the content of agronomical valuable aggregates (0.25–10 mm) at “dry” and “wet” sieving is 73 and 68 % respectively, which means a good evaluation. The soil use in rainfed conditions reduces the amount of agronomical valuable aggregates at “wet” sieving by 32.4 % and in irrigated conditions with sprinkling – by 51.5 %. For drip irrigation, the content of clod fraction increases more than two times. Data analysis for soil layer 0–30 cm resulted in a direct linear relationship y = 19.02 x – 34,41 with the approximation validity coefficient 75 %, where x – humus content,%; y – the content of agronomical valuable aggregates at “wet” sieving. According to this dependency the reduction of humus content by 1 % leads to reduction of the quantity of water stable aggregates by 19.02 %. The usage of this dependence has shown that to improve the content of water stable aggregates to optimal values for ordinary chernozem (70–90 %) humus content should be increased to 5.5–6.5 %.
Keywords: soil structure, soil texture, soil microstructure, sprinkling irrigation, drip irrigation, water stability, humus content.
