The work reflects the analysis of changes in the productivity of crops of crop rotation during the long-term use of fertilizer systems in a stationary experiment for the period from VI to VIII rotation (1999-2001 - 2016-2018). The influence of various doses of nitrogen, phosphorus and nitrogen-phosphorus fertilizers on the change in the content of nitrate nitrogen and mobile phosphorus in the conditions of the southern chernozem for the specified period of research was determined. The responsiveness of crop rotation crops to the application of nitrogen, phosphorus, and nitrogen-phosphorus fertilizers in minimum, medium and high doses has been established.

The paper analyzes the results of the effectiveness of the use of various systems of mineral fertilizers in the six-field crop rotation, obtained under conditions of long-term stationary experience for the period 1969-2018. The nature and degree of influence of the changing agro-climatic conditions of the Volga steppe zone on the productivity of crop rotation on both fertilized and non-fertilized variants of the experiment have been established. In the period from I-IV to V-VIII rotation, there was an increase in the sum of active temperatures for May-July, an increase in the average daily air temperature in July by an average of 1.1 °C, a decrease in hydrothermal coefficient values. The increase in aridity of the climate had a negative impact on the effectiveness of various fertilizer systems in the experiment. There is a tendency to decrease the productivity of crop rotation in the period from I-IV to V-VIII rotation. At the same time, there is an increase in the share of increases in the resulting crop from the use of mineral fertilizers. The most effective in the experiment were medium and high doses of nitrogen-phosphorus fertilizers.

In the conditions of a long stationary experiment conducted in the Saratov region on the Southern Chernozem, it was found that for eight rotations of a six-field grain-pair crop rotation (48 years) in the variant without fertilization, carbon losses from the 0-40 cm layer were the highest. Fertilizers N32P16K7 and N38P13K8 per 1 ha of crop rotation area reduced its losses by 29-32%. When applying N54P17K7, an increase in the migration of organic carbon into the underlying layer of 40-100 cm was noted. The total nitrogen content in the control decreased by 8.7% compared to the initial data. The introduction of N15P13K8 and N38P13K8 per 1 ha of crop rotation area contributed to the preservation of nitrogen compounds in the humus layer of the soil. The content of total phosphorus in both non-fertilized and fertilized soil has changed slightly over 48 years of research. Systematic application of fertilizers led to an increase in the fraction of calcium phosphates.

The results of the effectiveness of the use of mineral fertilizers on oat crops in a six-field crop rotation (8 rotations, 24 years of cultivation) under conditions of a long stationary experiment on the chernozem of the southern steppe Volga region are presented. The nature and direction of natural changes in agro-climatic parameters during the period of oat cultivation and the level of their influence on its productivity when applying various doses of mineral fertilizers are determined. During the studied period, there was a tendency to increase the aridity of the climate, which was expressed in a decrease in the values of the hydrothermal coefficient and an increase in the sum of active temperatures. In the experiment, in the period from 1-4 to 5-8 rotation of crop rotation, there was a tendency to increase the average monthly air temperature in July. It was found that these climatic changes contributed to a decrease in the productivity of oats in the experiment. At the same time, the share of increases in relation to the control in the obtained yield values on fertilized variants increased. The most effective were medium and high doses of nitrogen-phosphorus fertilizers.

The paper reflects the results of the effectiveness of the use of various doses of mineral fertilizers in a long stationary experiment on spring soft wheat for nine rotations of a six-field crop rotation (27 years of cultivation) in the conditions of the steppe zone of the Volga region. The systematic use of medium and high doses of nitrogen-phosphorus fertilizers contributed to obtaining the highest yield increases to control in the experiment throughout the entire research period. The average level of correlation between hydrothermal conditions in the period May-July and crop productivity has been established. Separate calculations by month showed an average level of correlation with June. Based on the results of regression analysis, the degree of influence of hydrothermal conditions as a factorial feature on the productivity of spring soft wheat in the experiment was determined. The analysis showed that the effect of the obtained values of the hydrothermal coefficient in May and July on the production process of vegetating plants was insignificant, and in June it reached 30% according to experience.