Structural and aggregate composition features of southern variant natural forest edaphotopes of Ukrainian steppe zone

The natural forests of the southern variant of the steppe zone of Ukraine (according to the classification of O. L. Belgard) are located on the territory of the former porous part of the Dnieper. The properties of edaphotopes of these forests were studied in detail by the staff of the Comprehensive expedition of the Oles Honchar Dnipro National University for the study of forests in the steppe zone of Ukraine (A. P. Travleev, N. A. Bilova, A. A. Dubina, K. M. Bozhko). However, there are no data on the structural and aggregate composition of the genetic horizons of edaphotopes of natural forests of the southern variant of the steppe zone of Ukraine, established by classical methods of determination. This work is devoted to the coverage of these features. As a result of research it is established that the best conditions for the formation of structural aggregates are characteristic of the upper horizons of all studied soils and especially for the forest chernozem in the conditions of northern exposure. The highest content of agronomically valuable structural aggregates is characterized by forest chernozem in the conditions of northern exposure of the ravine, the lowest – meadow-forest soil of thalweg, and forest chernozem in the conditions of southern exposure occupies an intermediate value. The eluvial horizons of ravine edaphotopes differ most contrastingly from the illuvial horizons in the conditions of northern and southern exposures. Eluvial horizons are characterized by a reduced fraction content of > 10 mm and increased values ​​of the structural coefficient compared to illuvial horizons. In the thalweg, these differences are not so clear. The upper horizons have an increased content of water-resistant fraction size > 5 mm compared to the horizons below. With depth in all studied edaphotopes there is an increase in the content of the fraction size < 0.25 mm with depth. Among the upper horizons, the highest content of water-resistant fraction size > 5 mm and the lowest content of fraction < 0.25 mm differs meadow-forest soil in thalweg conditions. Very close to the meadow-forest soil in terms of the values ​​of the respective fractions is forest chernozem in the conditions of northern exposure. Forest chernozem in the conditions of southern exposure differs by the lowest content of water-resistant fraction size > 5 mm and the highest content of fraction size < 0.25 mm among all studied soils. Eluvial horizons of forest chernozems of northern and southern exposures differ from illuvial horizons by reduced content of water-resistant fractions of > 5 and 5–3 mm. The eluvial horizons of the meadow-forest soil of the thalweg differ from the illuvial horizon by a sharp decrease in the content of the water-resistant fraction of 1–0.5 mm. A comparative analysis of the structural and aggregate composition of the studied edaphotopes revealed that the best conditions for the formation of agronomically valuable fractions are characteristic of the forest chernozem of the northern exposure. As a result of comparing the characteristics of eluvial and illuvial horizons, it was found that the change of eluvial process to illuvial is accompanied by a sharp decrease in forest chernozems content of agronomically valuable fractions during dry sieving, as well as an increase in water resistance. In meadow-forest soils, this transition is characterized by a sharp decrease in the content of agronomically valuable fractions in the illuvial horizon during wet sieving compared to the eluvial horizon.