Increasing the yield of barley cultivated in crop rotations and by sowing permanently is a major problem in the steppe zone of the Southern Urals. This requires to study factors affecting an increase in the crop yield. The aim of the study is to identify the effect of nitrate nitrogen, the biological activity of soil and the aftereffect of predecessors on barley productivity at different levels of mineral nutrition. In the experiment, methods such as field, ionometric and decomposition of flax fiber were used. The methods used make it possible to obtain new original results on the factors and experimental options. Over 18 research years, the highest barley yield was observed in millet and pea rotation using fertilizers. It amounted to 1.42 and 1.43 tons per 1 ha. These results were achieved due to the consumption of nitrate nitrogen up to 2.3 mg and accumulation in the amount of 7.7 mg per 100 g of soil with an activity of microorganisms of 8.3 and 8.1 %. The results are of scientific and practical importance in the field of agriculture and crop production.

The paper presents a study of the main factors affecting the yield level of soft spring wheat. This research is based on the study of the biological activity of the soil and the content of nitrates in the Urals southern chernozems. This paper is obtained as a result of field and laboratory studies. The purpose of the research work is to establish the influence of each factor individually on the yield of grain crops according to various predecessors and backgrounds of mineral nutrition. For the first time in the experiment, the methodology of the field experiment, decomposition of linen (applications) and the ionometric method are used. Over 18 years, rich experience has been accumulated, and soil fertilizer options have been developed to increase wheat productivity. Studies show that using balanced fertilizer it is possible to increase the yield of wheat after growing millet in this territory to 9.7 centners per 1 ha, the dependence on the influence of the activity of microorganisms is 71.51 %. In other research options, after the application of mineral fertilizers during the growing season, grain yield of 9.4 and 9.0 kg per 1 ha is noted, and the proportion of the effect of nitrate-nitrogen is 33.82 and 24.67 %. The results obtained are of scientific and practical importance in the field of microbiology, chemistry, agriculture, agronomy and agricultural production.

The purpose of the research is the yield increase of spring wheat under arid steppe of the Orenburg pre-Urals con-ditions. Stationary long term field experience was laid on the site of the Kuibyshev's experimental production farm located in Orenburg district. Crop rotations and hard spring wheat monocrop have been studied for 30 years. The two-factor experimental design, consisting of four repetitions was developed. The results of long-term stationary studies for 1990-2019 involving five rotations and hard spring wheat monocrop are presented. Weather conditions, the precursor type and nutrient status are the major factors affecting the hard spring wheat yield. The number of hyperarid years when hard spring wheat yield was less than 5 C per 1 ha in the first rotation amounted to 1 year, in the second – 2, in the third and the fourth – 3 years, in the fifth – 5 years. A significant hard spring wheat increase in yield on the basis of fertilized ground over 30 years was noted on weedfree fallow – 6 years, conservation tillage – 10, green manured – 8, winter – 12, corn – 13 and peas – 4 years, monocrop – 10 years. The highest yield of hard spring wheat for five rotations was on weedfree fallow, conservation tillage and green manured areas (average for two nutrient status), respectively, 11.4, 11.4 and 11.6 C per 1 ha, after winter crops – 10.3 C, corn for silage – 9.3 C, peas – 9.0 C and monocrop – 7.7 C per 1 ha. The yield increase from the fertilized ground was obtained within five rotations respectively for these precursors: 0.4, 0.6, 0.6, 1.0, 0.8, 0.3 and 0.9 C from 1 ha. The research was carried out in accordance with the plan for 2020-2021developed by the Federal State Budgetary Scientific Institution «Federal Scientific Center of Biological Systems and Agrotechnologies of the Russian Acade-my of Sciences» (№ 0761-2019-0003).

The detection of factors that affect the decrease in the productivity of durum wheat is the basis for the study of plant diseases in the soil and climatic conditions of the southern chernozems of the Cis-Ural region. On the basis of this premise, for the first time, studies on common root rot (Bipolaris sorokiniana) of durum wheat are conducted in field and laboratory conditions. There is the method of field experience, stationary accounting of agrometeorological conditions and disease determination is used. The results of six years research show that in the third variant of the experiment, the least spread and development of root rot is observed on a non-windy background of nutrition up to 28.6 and 10.2%. The dependence on the influence of air temperature is 94.4 and 70.0%. The highest crop yield of durum wheat was obtained in the average dry year of 2017 according to the green-manured fallow predecessor. It consists of 21.4 c on fertilized and 21.1 c per 1 ha on non-winded backgrounds of mineral nutrition. The maximum effect of root rot development on the decrease in durum wheat productivity is noted in the fourth version of the experiment and it consists of 80.04 and 89.66% on two nutrition backgrounds. The study results have a scientific and practical importance in the field of phytopathology, farming and intensive agriculture.