Main elements of nutrition content in the soil for maize crops, depending on the predecessors and methods of soil treatment

. The article presents data on the study of the main nutrition elements in the soil under maize crops, depending on the predecessors and methods of soil treatment in the conditions of ordinary Chernozem. The object of research is a Krasnodarsky 194MV hybrid of maize. Variants: 1. Fallow arable land (control); 2. Lupine as green manure; 3. Peas as green manure; 4. Binary sowing of lupine and peas as green manure; 5. Soy as green manure. The experiment was repeated three times, field placement was randomized, and area of each was 120 m². Agrotechnics of maize cultivation in the experiment corresponded to the recommendations for this zone. Green manure was plowed in during its reproduction phase. In the variants with green mass plowing to a depth of 23 – 25 cm, the largest amount of nitrate nitrogen was in the soil layer 0 – 20 cm. In the specified soil horizon, before sowing maize, the content of nitrate nitrogen ranged, depending on the variant, from 24.7 mg/kg to 42.8 mg/kg. In the soil layer 20 – 40 cm, the amount of nitrogen was lower, just 19.4 – 29.5 mg/kg. Similar dependence was observed for maize during its flowering phase. Tillage methods had almost no effect on the accumulation of nitrate nitrogen in the upper soil layer (0 – 20 cm). As in all the variants, the differences in this indicator were within the experimental error. However, the content of nitrate nitrogen during the growing season was different. The greatest amount of it was observed during the spring period, both in versions with plowing to a depth of 25 – 28 cm and in versions with blade loosening to a depth of 10 – 12 cm. The smallest amount of available phosphorus, regardless of the method of soil preparation, in the layer 0 – 20 cm was on fallow arable land – 122 mg/kg during the sowing period and 104 mg/kg during the flowering period. In variants with cultivation of legumes as green manure, the content of available phosphorus in the specified horizon was significantly chigher, being 147 – 171 mg/kg. The highest content of exchangeable potassium was provided by lupine – 209 – 213 mg/kg in the 0 – 20 cm soil layer, and lupine sown together with peas – 196 – 207 mg/kg. The minimum amount of exchangeable potassium was 143 – 146 mg/kg in fallow arable land, depending on the method of soil treatment.


Introduction
Nowadays, it is important not only to obtain high yields of agricultural products, but also to preserve the fertility of soils. Restoring fertility of soils and creating zerodeficit balance of nutritional elements is the most important task of agriculture, solution of which is based on application of organic fertilizers. [1][2][3][4][5].
The most common way to provide soil with organics is to use manure [6][7][8][9]. However, in recent years there has been a shortage of cattle manure (CM). In this regard, the use of green manure as organic fertilizer, which in its properties is equivalent to manure, is highly important. When plowing in the green mass of green manure in the amount of 35-40 t/ha, the soil gets up to 150-200 kg/ha of nitrogen. It is important that the nitrogen mineralization rate of the crushed mass of plants in the first year after the introduction is twice as high as that of manure [10][11][12].
Domestic and foreign experience shows that in the current state of agricultural production, green manure should be considered an important link of energy and resource-saving technologies for the production of crops [15][16][17][18].
Maize is one of the leading grain and forage crops not only in Russia, but also worldwide [19][20][21].
Intensification of maize cultivation for silage production requires not only placing it in specialized crop rotations, but also making correct scientific justification for the selection of green manure, ensuring the increased productivity of high-quality silage mass with the lowest costs, while preserving the fertility of the soil [16,[22][23][24].
The goal of the study is to study the nutrient regime of the soil for growing maize, depending on the type of green manure, on the black-earth soils of the Central Black Earth region (CBER).

Materials and methods
The object of the study is a maize hybrid Krasnodarsky 194MV.
The experiment was repeated three times, the field placement was randomized, each field's area was 120 m². Agrotechnics of maize cultivation in the experiment corresponded to the recommendations for the area. Green manure was grinded and mixed into the soil at the reproduction phase. In the control variant, 20 t/ha of cattle manure was applied.
Nitrate nitrogen content in the soil was determined according to GOST 36107-84, mobile forms of nitrogen and potassium were determined by the Chirikov method in the CRIAS (Central Research Institute of Agricultural Services) (GOST 26204-91) modification.

Results and Discussion
The results of the study showed that green manure affected the nitrate nitrogen content in the soil for growing maize (table 1). In variants with plowing green mass to a depth of 23-25 cm, the largest amount of nitrate nitrogen was in the soil layer 0-20 cm. In this horizon, the nitrate nitrogen content ranged between 24.7 mg/kg and 42.8 mg/kg, depending on the predecessor. In the soil layer 20-40 cm content of nitrogen was less -19.4-29.5 mg/kg. We have observed a similar pattern during the flowering phase of maize. In that phase, content of nitrogen was significantly less. In the layer 0-20 cm it was 17.9-29.1 mg/kg and in the layer 20-40 cm, respectively, 10.6-18.5 mg/kg.
Among the variants studied, the largest content of nitrate nitrogen in the spring (sowing phase) was in variants with green manure. In fallow arable land (control), despite the application of 20 tons/ha of manure, content of the nitrate nitrogen was 1.5-1.7 times lower. This can be explained by the fact that the rate of mineralization of manure in the first year is twice lower than that of green mass of plants. This pattern continued in the summer (flowering phase).
The use of lupine as a green manure crop contributed to the greatest accumulation of nitrate nitrogen in the soil. The amount of nitrogen before sowing in this variant was 42.8 mg/kg in the 0-20 cm layer of soil and 27.4 mg/kg in the 20-40 cm layer. The same accumulation of nitrogen was provided by joint sowing of lupine and peas: 41.2 mg/kg in the 0-20 cm layer and 25.1 mg/kg in the 20-40 cm layer. The difference between these options was within the experimental error. A slightly lower nitrate nitrogen content was after plowing peas and soy into the soil, both before planting and during flowering. Decrease of nitrogen content in these cases can be explained by a decrease in mass of plants plowed into the soil.
The methods of soil treatment had little or no effect on the accumulation of nitrate nitrogen in the upper layer (0-20 cm). In all variants, the differences in this indicator were within the experimental error.
In the soil layer 20-40 cm in the variants with plowing, the nitrate nitrogen content was significantly higher compared to similar variants when blade loosening the soil to a depth of 10-12 cm. So, when plowing in lupine to the depth of 25-28 cm, the amount of nitrogen in the 0-20 cm layer in spring was 27.4 mg/kg, and when blade loosening to a depth of 10-12 cm, its amount was 17.3 mg/kg or 1.6 times less.
When blade loosening, the green manure is concentrated at a depth of up to 15 cm, where the mineralization of rotted plants happens. Formation plowing to the depth of 25-28 cm contributed to a deeper layering of green manure, which further affected the increase in nitrate nitrogen in this horizon. As we know, maize has a well-developed root system, which penetrates to great depths. Therefore, the presence of a large amount of nitrate nitrogen in the root zone will ensure better plant growth and higher productivity.
The nitrate nitrogen content varied during the growing season. Its largest amount was in the spring period, both in variants with plowing to a depth of 25-28 cm, and in variants with blade loosening to a depth of 10-12 cm. So, when sowing, the nitrate nitrogen content in the soil layer 0-20 cm on average for 4 years amounted to 21.3-42.8 mg/kg, depending on the variant. During the flowering period, the nitrate nitrogen content in similar variants decreased to 17.4-29.1 mg/kg. As we know, in the initial growth period, maize plants consume a rather small amount of nutrients, including nitrogen. Maximum nitrogen consumption happens during the The content of available phosphorus in the soil also largely depended on predecessors and ways of soil preparation (table 2). The lowest amount of available phosphorus, regardless of the method of soil preparation, for fallow arable land in the 0-20 cm layer, was: 122 mg/kg during the planting of maize and 104 mg/kg during flowering. In variants with the cultivation of legumes as green manure, the content of available phosphorus in this horizon was significantly higher -147-171 mg/kg. At the same time, the largest amount of available phosphorus on average for 4 years was observed in the variant with the single-component sowing of lupine and in the variant with joint sowing of lupine and peas. In these variants the mobile phosphorus contents was 179-181 mg/kg and 169-171 mg/kg respectively. There was a bit less phosphorus with single-component sowing of peas and soy: 157-160 mg/kg and 147-151 mg/kg respectively. The content of active phosphorus in the soil layer 0-20 cm depended mainly on the amount of accumulated mass of green manure plants and the content of phosphorus in it.
In the soil layer 20-40 cm, the content of available phosphorus was influenced by the way the soil was processed. Plowing to a depth of 25-28 cm contributed to the greater accumulation of this element relatively to surface treatment. In the first case, the phosphorus content was 1.2 to 1.4 times higher.
During the flowering phase, the content of available phosphorus in all variants was significantly lower than during maize planting, which is also explained by the maximum consumption of it in summer.
The content of exchangeable potassium, as well as nitrogen and phosphorus is mostly dependent on the predecessor, timing of vegetation and depth of sampling (table 3).
The highest content of exchangeable potassium was provided by lupine green manure -209-213 mg/kg in the 0-20 cm layer of soil and lupine planted together with peas -196-207 mg/kg. The minimum amount of exchangeable potassium was in fallow arable land, 143-146 mg/kg, depending on the way the soil was processed.
Plowing the soil to a depth of 25-28 cm, similarly to other elements, contributed to a greater accumulation of exchangeable potassium in the soil layer 20-40 cm, relative to surface treatment to a depth of 10-12 cm. Higher potassium consumption by maize plants happened in the flowering phase, as in other cases, which explains the lower content of it relative to the spring period.

Conclusion
1. Green manure crops provided greater accumulation of nutrients in the soil compared to fallow arable land, where their number was 1.4-1.7 times less. The highest amount of nutrients is observed in the soil after applying lupine green manure. 2. In the upper horizons of the soil (0-20 cm) the methods of soil treatment had no effect on the content of nitrate nitrogen, available phosphorus and exchangeable potassium. surface-treated variants, their content was 10-16% lower. 4. During the flowering phase, the contents of movable forms of nitrogen, phosphorus and potassium were reduced by 20-25% compared to the spring period, which is due to their maximum consumption by maize plants.