The emergence of the modern problem of nitrogen in the mineral nutrition of plants in the agriculture of different countries of the world can be explained by three main reasons. These reasons are primarily related to the presence of three strong covalent bonds between atoms in the nitrogen molecule, the breaking of which requires a large amount of energy, the low content of mineral forms of nitrogen in the soil, and insufficient use of the capabilities of leguminous crops in replenishing soil reserves with biological nitrogen.

Let's consider each of these reasons in more detail.

First, agricultural plants, except for legumes, are not able to directly consume nitrogen from the air, which makes up 78.2% of its volume and 75.6% of its mass. In terms of physical weight, the amount of molecular nitrogen in the atmosphere is very large. So, over each hectare of the Earth's surface, there are about 70,000 tons of molecular nitrogen in the atmosphere, which, unfortunately, is not available for plants.

On our planet, the reserves of molecular gaseous nitrogen are practically inexhaustible - 3.8·1015 tons. However, despite this large amount of molecular nitrogen, the amount of chemically bound nitrogen normally produced in the atmosphere during lightning discharges is small, up to 2–5 kg/ha annually. This nitrogen, in the form of ammonia and nitrates, enters the soil with precipitation, but its quantity is insignificant, and it is in no way able to solve this important problem, even partially. That is, due to thunderstorm discharges that occur in the atmosphere in spring and summer during rainy weather during lightning, it is impossible to fully satisfy the plants' need for mineral nitrogen. Therefore, the non-biological processes of nitrogen fixation due to lightning discharges and under the influence of ultraviolet rays are quantitatively insignificant, since together they give no more than 0.5% of chemically bound nitrogen.

Only leguminous plants, with the help of nodule bacteria, are able to assimilate this element of mineral nutrition directly from the atmospheric air, converting it into ammonium and nitrate forms in significant quantities. For this purpose, the seeds of leguminous crops are inoculated with special bacterial preparations, which contain nodule bacteria in their composition, which penetrate through the cell membranes into the cytoplasm, multiply intensively and fix atmospheric nitrogen, providing it to leguminous crops during their growing season. Leguminous crops, in turn, provide these bacteria with ready-made organic substances. Thus, a symbiosis occurs between them.

Under such conditions, microbiological binding of atmospheric molecular nitrogen ensures its transition from inert to mobile and is carried out by a large group of microorganisms distributed in various soils. In this case, the relationship between nodule bacteria and leguminous plants is mutually beneficial.

Due to nodule bacteria, 80 to 200 kg/ha of biological nitrogen can enter the soil. All other plants, except legumes, are not able to solve the problem of nitrogen nutrition in this way, they need to be provided with ready-to-consume nitrogen in mineral form. Free-living nitrogen-fixing soil bacteria are also unable to solve this problem, since they convert atmospheric molecular nitrogen into mineral form in small quantities. Therefore, it is necessary to look for other, more reliable sources of entry into the soil of this rather important element of mineral nutrition of plants.

Secondly, the main source of nitrogen in the soil for plant nutrition is the content of mineral forms of nitrogen (ammonium and nitrate), as well as labile nitrogen-containing organic matter capable of rapid mineralization under favorable meteorological conditions.

However, unfortunately, almost all nitrogen in the arable soils of Ukraine is contained in non-hydrolyzed organic compounds, that is, in a form that is difficult to dissolve and is not available to plants.

The source of organic compounds entering the soil is organic fertilizers and various plant residues. Let's make simple mathematical calculations and see how much mineral nitrogen available to plants can be obtained by applying organic fertilizers.

To obtain 100–150 kg of mineral forms of nitrogen available for plant nutrition (taking into account that humus contains about 5% nitrogen), 2–3 t/ha of humus must be mineralized. Simple calculations show that one ton of manure on chernozems produces about 0.07 tons of humus, that is, to restore 2–3 tons of humus, it is necessary to apply annually at least 30–40 t/ha of manure, which is not available in such quantity in our country. This is due to a sharp reduction in the number of cattle. The consequence of this is the disappointing result: there is very little available nitrogen for plants in the form of nitrates and ammonium compounds in the soils of Ukraine. In addition, the main mass of soil nitrogen is included in the composition of various organic compounds, plant residues and humus substances and is in a state unavailable for plant nutrition. The main mass of soil nitrogen is concentrated in them.

The content of total nitrogen in the topsoil of different types of soils is low: it varies from 0.05 to 0.3% and directly depends on the presence of organic substances in them. Its reserves in the arable layer of the soil are from 2.4 to 8.7 t/ha. Studies conducted on the basis of a large array of data have shown that there is a direct relationship (correlation coefficient r>0.9) between the content of total nitrogen and humus in the soil. Soil deposits of the non-chernozem zone contain the following amounts of gross nitrogen: sandy loam 0.05–0.07%, loam 0.10–0.20, clay 0.10–0.23, peat 0.5–1%.

Humus plays an extremely important role as a source of nitrogen in the soil, but its reserves are gradually decreasing in most soils of Ukraine. The least amount of humus is in the soils of Polissia (2.0%), much more - in the forest-steppe (3.6%) and steppe (3.5%). This indicator varies quite a lot in individual administrative regions: Polish - from 1.6 to 2.8%, forest-steppe - from 2.8 to 4.9%, steppe - from 2.3 to 4.5%.

It should be noted that humus in the soil is in a state of constant renewal, and therefore it is necessary to ensure that its synthesis exceeds decomposition, and the balance is always only positive. Only in this case, there will always be a sufficient amount of mineral forms of nitrogen in the soil, which in most cases is not enough to meet the needs of plants.

In the soil, under the influence of ammonifying and nitrifying bacteria, nitrogen-containing organic compounds of plant residues are gradually transformed into NH4+ cations and NO3– anions and then absorbed by plant roots, if favorable conditions for mineralization are created in it.

First, ammonifying bacteria, actinomycetes, and microscopic fungi conduct mineralization of organic matter in the soil with subsequent release from it of ammonium nitrogen available for plants. In the future, bacteria of the genus Nitrosomonas, Nitrocystis and Nitrosospira participate in the oxidation of ammonia salts to nitric acid (the first phase), and bacteria of the genus Nitrobacter participate in the oxidation of nitric acid (the second phase).

The basis of nitrification is the dehydrogenation of ammonia, which is carried out by the enzyme dehydrase, and the connection of nitrogen with oxygen with the help of appropriate oxidases. This process is possible under certain conditions, namely: optimal (22–25°С) soil temperature, close to the neutral reaction of the soil solution, the presence of oxygen and moisture, special nitrifying microorganisms. In fact, the nitrification process lasts from late spring to mid-autumn. Thus, the transformation of nitrogen is most closely related to the soil microflora, and the nitrogen regime of the soil depends on its activity, that is, the amount of mineral forms of nitrogen in it.

The process of mineralization of organic matter takes place most actively during the creation of a favorable hydrothermal regime in the soil. There is no process in the soil in which the microflora does not take an active part. During the agricultural use of soils, engaged in farming, a person plows them, introduces various types of toxic substances in the form of pesticides (herbicides, fungicides, insecticides, defoliants, etc.), heavy metals, etc., which, due to their cumulative effect, reduce the amount of useful substances in the soil microflora, as a result of which the nitrogen regime of the soil deteriorates.

Along with mineral nitrogen, easily hydrolyzed nitrogen can also be available to plants, which serves as the first reserve for enriching soils with mineral forms of nitrogen. It is mainly represented by the nitrogen of amides and amino acids, which can quickly mineralize and provide plants with mineral forms of nitrogen throughout the growing season. But the weighted average content of easily hydrolyzed nitrogen in the soils of Ukraine is low and amounts to only 105.4 mg/kg of soil. In general, a very low content of easily hydrolyzed nitrogen prevails in the soils of Ukraine on most of the arable land areas (93.1% of the areas), including 90.5% in Polissia, 94.3% in the forest-steppe, and 93.3% in the steppe available cultivated areas.

Hardly hydrolyzed and non-hydrolyzed nitrogen, respectively, is poorly available and almost inaccessible to plants in the soil. Heavily hydrolyzed nitrogen is a possible further reserve for enriching soils with mineral forms of nitrogen; this nitrogen is represented only by amines, a part of non-exchangeable ammonia and humic nitrogen, and non-hydrolyzed by melanins, bitumens, non-exchangeable ammonium and almost does not participate in the nitrogen exchange between the soil and the plant.

Thirdly, in the conditions of modern agriculture, in addition to replenishing the soil with nitrogen, it is constantly consumed as a result of its use by plants and removal with the harvest, as well as as a result of leaching of the nitrate form of nitrogen from the soil by precipitation and irrigation water. In plants, and especially in their generative organs, there is a large amount of nitrogen, which is removed from the fields with the main products every year. A significant amount of nitrogen is lost due to the strong development of erosion processes and especially denitrification and for other reasons both from the soil itself and from nitrogen fertilizers introduced into it. Significant losses of nitrogen are caused by denitrification, which takes place under the influence of denitrifying bacteria and leads to the loss of this nutrient in the form of molecular nitrogen or its oxides NO and N2O. Denitrification is carried out by a large group of bacteria, which have the general name of denitrifiers (Bact. denitrificans, Bact. stutzeri, Bact. fiuorescens, etc.).

As evidenced by the results of research, the largest share of nitrogen losses from all the above-mentioned species nevertheless occurs due to the processes of denitrification and the entry of various gaseous forms of nitrogenous compounds into the atmosphere. As a result, the already small reserves of mineral nitrogen in the soil become depleted.

Man's intervention in the centuries-old process of soil formation, primarily the plowing of virgin lands, has made significant changes to the natural closed cycle of nutrients that has been constantly occurring in the soil for centuries.

The use of soils for agricultural production led to the intensification of the mineralization of soil organic matter, disruption of the complex process of reproduction of the main indicators of their potential fertility. Instead, various types of soil degradation associated with the deterioration of physical, agrochemical, and physicochemical indicators began to actively develop. Of particular concern is dehumification, which has covered all types of soils in Ukraine without exception and causes a systematic gradual decrease in the content of humus in them, at the same time impoverishing their nitrogen fund. Under such conditions of cultivation of agricultural crops in our fields, intensive decomposition of humus occurs, which significantly exceeds its synthesis and leads to a deficiency of nutrients in the soil.

It is quite obvious that in the current conditions of management on Ukrainian soils of modern agricultural holdings, corporations and farms with different forms of ownership, all the efforts of their managers are directed to the maximum use of available reserves of nutrients, especially soil nitrogen, due to the introduction of intensive cultivation technologies into production, especially those agricultural crops that ensure high profitability are mainly sunflower, rapeseed and corn, and this is happening against the background of the practical removal of legumes and perennial grasses from the crop rotation.

Under such management conditions, the nitrogen fund of the soil may soon be completely exhausted. Therefore, it is necessary to constantly return the nutrients removed from it by the crops to the soil by applying fertilizers and to restore the lost reserves of humus.