The alternation of crops for a long time will remain a fundamental element of the zonal technologies of their cultivation. The use of mathematical modeling methods will allow to reach a new level in solving the issues of effective use of arable land through the use of models that optimize it. Especially, the results of studies based on long-term observations will be valuable. We used data from long-term stationary experiments to study the alternation of crops in crop rotations that took more

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... n five rotations. The stationary experiments are located in the forest-steppe zone of Western Siberia in the fields of the Omsk Agrarian Scientific Center. The purpose of the research was to optimize the use of arable land based on mathematical modeling methods to preserve soil fertility. In the work, a model that takes into account the conditions of the agroecosystem is used (humus balance, labor costs, energy costs, etc.). It allows one to optimize the cultivated area, while maintaining soil fertility, ensuring the production of products at a consistently high level, with increasing economic and energy efficiency. The article shows that in grain-crop crop rotations, the yield is generated due to the natural fertility of the soil, as evidenced by the negative humus balance, which reaches 0.18-0.78 t / ha. During the alternation of the fruit-changing alternation, the crop was formed against the background of the reproduction of the organic matter of the soil by increasing the plant residues from the use of fertilizers. The humus balance was positive (0.15-0.28 t/ha). When using employed steam along with clean steam, rape as siderate, and also straw as an organic fertilizer for grain crops, the amount of 2.01 t/ha of marketable grain was obtained (with a 16.8% share of pure steam, as well as occupied steam (11%) and spring grain (72.2%), of the arable land area). Increasing the area in the crop rotation of Western Siberia using straw, green manure, and occupied vapors as organic fertilizer, the soil fertility reproduction with a certain decrease in the yield of marketable grain up to 2 t/ha can be achieved.