Researches on the study of predecessors and biologization techniques in field crop rotations were carried out in 2018-2020 in the dry steppe zone of the chestnut subzone of light chestnut soils of the Nizhneje Povolzhje region. It is necessary to grow biennial melilot for green manure as a fallow-growing crop in a grain-steam five-field crop rotation in order to increase the organic matter input into the soil. A significant increase in yield was achieved in chickpeas for winter wheat, its straw was plowed into the soil. The highest increase in the spring barley yield was provided during the chickpeas and sorghum cultivation, which straw entered the soil. When growing spring barley on safflower, its straw was plowed into the soil, the yield was also higher than the control variant. The highest grain yield was achieved in the control grain-fallow four-field crop rotation with complete fallow, where the cultivated crops straw was removed from the field and grain-fallow-grass green manure seven-field crop rotation with seed fallow, where straw and melilot were plowed into the soil. Grain-fallow seven-field and grain-fallow-grass-cultivated green manure seven-field crop rotations, where oats and phacelia straw and green manure mass entered the soil, were inferior to the control.

Soil fertility in biologized crop rotations depends on the saturation of them with legumes and sideral crops, perennial grasses, the involvement of grain crops in the organic matter cycle. The crop rotation was studied: 1) four-field grain and steam: clean steam winter wheat chickpeas spring barley (control); 2) five-field grain and steam: occupied steam (clover green manure) winter wheat chickpeas spring barley mustard + clover; 3) seven-field grain and grass: occupied steam (green manure oats) winter wheat mustard chickpeas safflower dyeing spring barley sainfoin (hatcher field); 4) semi-field grass and grassland: occupied steam (phacelia green manure) winter wheat spring wheat chickpeas grain sorghum spring barley alfalfa (hatchery field). The highest balance of organic matter was ensured in a five-field grain-steam crop rotation with clover for green manure +1.92 t/ha, in this crop rotation the highest balance was observed for nitrogen +23.8 kg/ha and phosphorus +1.3 kg/ha, grain harvest from 1 ha of arable land 0.51 t/ha. The greatest balance of potassium was ensured in the seven-field grain and grass-crop rotation with facet on green manure +8.8 kg/ha. The highest humus balance was observed in a seven-field grain-grass-crop rotation with oats per green manure +0.12 t/ha.

In the arid zone of light chestnut soils of the Lower Volga region, in order to increase the influx of organic matter and basic nutrients into the soil, winter rye and yellow two-year-old clover should be cultivated in occupied fallows as sideral crops. To increase the replenishment of potassium in the soil, it is also recommended to grow phacelia on siderate as a fallow crop. Plowing oats in the form of green fertilizer led to a decrease in the input of organic matter and mineral nutrition elements into the soil compared to the control. The highest content of nitrate nitrogen, mobile phosphorus and exchangeable potassium in the arable layer of the soil during the sowing of winter wheat, when its plants leave for winter and spring regrowth, as well as during harvesting, was provided by the predecessors of the occupied fallows by winter rye, two-year-old clover and phacelia for siderate. The highest yield of winter wheat was obtained by the predecessors the pure fallow and occupied fallows by winter rye, sweet clover and phacelia for siderate. According to the predecessors, the occupied fallows by spring ginger and oats for siderate showed a significant decrease in the yield of winter wheat.

Increasing the yield of spring barley on light chestnut soils of the Volgograd region is possible through the use of microbiological fertilizers that activate the processes of plant growth and development. The influence of the methods of basic tillage and the application of mineral and bacterial fertilizers “Azotovit” and “Phosphatovit” on the content of nitrogen in the soil was studied. The degree of decomposition of the linen cloths was the maximum for deep flat-cutting processing. According to the binary interaction, the best results were obtained on the variant with deep flat-cutting processing with double application of bacterial fertilizers. The introduction of bacterial fertilizers sharply increases the content of azotobacter, and especially when they are applied twice. The lowest toxicity of the soil in barley crops compared to the control was noted on the variant with deep flat-cutting treatment, and the maximum on the variant of dump treatment. The analysis of data on the structure of the crop also indicates the advantage of the variant of deep flat-cutting tillage with double application of microbiological fertilizers. The most cost-effective variants were with double application of bacterial fertilizers on the background of deep flat-cutting tillage.

An estimation of various agro-biological methods in crop rotation of dry-steppe zone of light chestnut soils of the Lower Volga region is given. The effectiveness of a grain-fallow four-field crop rotation has been established, where green manure crop mass of winter rye, straw and leaf-weed mass of crops are plowed in soil, which increases organic matter and nutrient content in soil, reduces humus loss, and stabilizes grain yields. It was noted that only in four-field grain-fallow green manure crop rotation there is a positive balance of the main nutrients in the soil per hectare, where additional organic matter enters arable layer of soil in the form of winter rye green manure crop. In this variant, the balance was: nitrogen +39.7; phosphorus +0.7 and potassium +49.9 kg/ha. In six-field grain crop rotation with ginger as a sidereal crop, a positive balance is achieved only for phosphorus +14.1 and potassium +35.5 kg/ha, for phosphorus there is a negative balance of -3.5 kg/ha. In the eight-field grain crop rotation with 50% of legume crops, a positive balance is achieved only for potassium, +29.8 kg/ha. The balance of humus can be regulated by the structure of areas, alternation of crops in field crop rotations, application of plant residues in the form of sidereal crops, straw, leaf-weed mass, decrease in the proportion of fallow and tilled crops in the structure of biotivated crop rotations. The most complete biologization is possible in green manure crops, where humus losses decrease 1.5 times or more. A positive balance of organic matter is ensured in four-field biologized grain-fallow crop rotation -+3.33 t/ha.