Stepan P. Poznyak, Lviv Nation University named by Ivan Franko, Doroshenka str. 41, Universytetska str., 1, Lviv, Ukraine
Ukraine occupies a leading place in the world among the countries on the areas of which chornozems are spread. The total area of world chornozems equals about 300 mln ha, among them Ukraine occupies 26.5 mln ha. Chornozems embrace the main area of agricultural lands (67.7%), which is the basic fund for obtaining agricultural produce. Chornozems of Ukraine are being studied for a long time. Characteristic feature of chornozems is being in the zone of moderate continental climate, where evaporation prevails over atmospheric moistening. Chornozem is a type of humus, calcium, montmorilonite soils of complicated and lasting history of the process of soil forming - from early hydromorphic to present automorphic stages of the development of aqua - accumulative valleys. Positive balance of biogenic substances and space energy was leading in the history of chornozem forming, which led to the creation of the system of soil horizons, enriched by the following basic fertility elements: humus, living overground and underground biomass, rich deposits of nitrogen, phosphorus, potassium, microelements, optimum aqua-air regime, active intrasoil biology and biochemistry. Huge supplies of potentially active chemical energy in humus amounting 3-4 e9 kilcal/ha is the basics of great biologic productivity of chornozems, their stable capacity to resist different impacts of ecologic situation and provide a high effectiveness of plants photosynthesis. These natural features of chornozems stipulate their exceptional meaning in agriculture of the world and Ukraine in particular. Complete cultivation of chornozems and introduction of industrial technology in agriculture during the XIX-XX centuries radically changed physics, biogeochemistry, aqua-air and other regimes of chornozems. Tillage of chornozems for a long time practically without fertilizers caused considerable changes of the structure, composition and properties. Tillage of chornozems of Ukraine amounts 68 percent in foreststeppe zone, plastic clay and clayey chornozems in particular - 94 percent in particular; in steppe zone - 65 percent, plastic clay and clayey chornozems in particular - up to 96 percent. Tillage, constant cultivation causes a considerable decrease (by 2-6 times) of living organisms in chornozem and they gradually seem to be "sterilizing", which leads to the loss of structuralness and selfcondensation of soil mass. Usage of heavy agricultural equipment accelerates this process which leads to the condensation of root contained horizons to the values 1.4-1.6 g/sm3. Drought and overmoistening are more sharply distinguished on the condensed horizons causing the depression of plants. Harvest on overcondensed chornozems decreases by 15-30 percent, but when the condensation amounts to 1.5-1.6 g/sm3, the yield decreases by 50-75 percent. Researches of chornozems of Ukraine of many years showed that average year losses of humus in typical chornozems are equal to 0.7-1.9 t/ha, in ordinary chornozems - 0.3-0.7 t/ha, in the southern chornozems - 0.3-0.6 t/ha. Decrease of humus in chornozems intensifies the development of eroded processes. The analysis of researches showed that 24.9 percent of chornozems of Ukraine are eroded. Among them slightly eroded chornozems amount 17.8 percent, average eroded chornozems amount 5.27 percent and heavily eroded amount 1.9 percent. Eroded arable lands amount 30.4 percent, the area of eroded feeding lands amounts 45.2 percent and washed off lands amount 1.9 percent. Linear form of erosion is the most distinct one. Many-scaled irrigation of chornozems caused new problems. The main part of irrigated lands is situated in the southern steppe subzone - 1.6 mln ha, where chornozems predominate. On irrigated plots of chornozems new processes untypical to these soils take place, namely: the raising of ground waters level, waterflooding, secondary salinity, salinization, alkalization, carrying-out of nutrients, deterioration of physical and physical-mechanical properties. Nowadays the cases of contamination by toxic substances are taking place. Technogenic pollution has a local spreading in the regions with a high background content (Donbas, Pre-Dnieper area). For the last 40 years considerable changes took place in chornozems due to intensification of agrotechnogenic impact. Due to extensive system of agricultural production on the first stage the process of evolution of soil fertility is characterized by a gradual degradation. In typical chornozems humus content decreased by 21-38 percent, absorbed calcium - by 26-37 percent, water-proof aggregates (more than 0.25 mm) - by 33 percent, mineral nitrogen, soluble phosphates and metabolic potassium - correspondingly by 34-40 percent, 39-40 percent and 22-24 percent. Reducing to the minimum the quantity of alienation of chornozems for other but not agricultural usage, introducing a range of measures for preventing degradation, raising the culture of agriculture, especially in the process of chemisation and reclamation, renovation of chornozems are the important factors of chornozems productivity increase.