Nitrogen fixation is one of the most important processes
Nitrogen fixation is one of the most important processes ensuring the nitrogen cycle in nature, which influences the biological productivity of plants. The column of air above one hectare of surface contains 80 thousand tonnes of nitrogen. If wheat plants could assimilate nitrogen directly from the air, it would be enough to obtain a yield of 30 c/ha for more than half a million years. However, plants cannot assimilate molecular nitrogen on their own, which has forced humanity to seek ways of supplying plants with this important element through industrial synthesis of nitrogen compounds available to plants, but despite this, the natural process of nitrogen fixation by legumes in symbiosis with nodule bacteria remains an important and fairly cheap measure for raising soil fertility.
Historical background
Humanity has long known that crops grown after legumes give a higher yield than after other preceding crops. Mentions of this are found in the works of Greek and Roman philosophers in the 3rd–1st centuries BC. The first scientific explanation of this phenomenon was given by one of the founders of agricultural chemistry, the French scientist Boussingault. In his work published in 1838 he directly stated that legume grasses enrich the soil with nitrogen through nitrogen fixation, though mistakenly thinking that plants do this through the leaves, because of which he later renounced his discovery. Conducting experiments with legumes in calcined sand under a glass dome, where there were no nodule bacteria, he did not get nitrogen enrichment from the air. Only several decades later was it established that legume crops bind atmospheric nitrogen only in symbiosis with bacteria living on the roots in the form of thickenings, later named nodules. The discovery of the nitrogen fixation phenomenon was of extremely great practical importance. For example, the introduction in the 19th century in Europe of rotations with clover made it possible to raise cereal yields from 7 to 17 c/ha.
Obtaining high nitrogen fixation indicators
In modern agricultural production the share of legumes in a rotation can reach 20–25%. To guarantee high nitrogen fixation indicators, legume seed is treated before sowing with preparations containing a high titer (number) of nodule bacteria. The procedure is called inoculation. Legume crops differ substantially in their ability to fix nitrogen from the air. The most productive are: alfalfa up to 300 kg/ha, red clover 180 kg/ha, white sweet clover and lupin about 150 kg/ha, peas and soybean 50–70 kg/ha. Today a group of free-living nitrogen fixers in the soil from the genera Clostridium and Azotobacter is known. 
The activity of free-living nitrogen-fixing bacteria in the soil is limited by a lack of organic matter, so their productivity is insignificant, about 5 kg/ha per growing season. Nodule bacteria supply the legume plant with nitrogen fixed from the air and receive mineral salts and carbohydrate metabolism products from the plant. The productivity of the process of nitrogen assimilation from the air depends on many factors. The first and main condition is high virulence, which is a sign of a quality inoculant. Optimal soil moisture should be within 60–70% of full moisture capacity. The minimum soil moisture at which nodule bacteria can develop is 16–18% of full moisture capacity. At a lower soil moisture level, nodule bacteria go into an inactive state and partial dying off is observed. Excess moisture is also undesirable, since soil aeration is disrupted, which adversely affects the development of nodule bacteria. The nitrogen fixation process takes place at a temperature of +10°C and above. The optimal temperature at which nitrogen fixation reaches maximum values is considered to be 20–25°C. A temperature above 30°C adversely affects nitrogen fixation. The second important condition is soil reaction as an ecological factor influencing the activity and virulence of nodule bacteria. In acidic soils manganese and aluminum compounds have a toxic effect on root system development and reduce plant access to calcium, phosphorus, molybdenum and carbon dioxide, thereby adversely affecting nitrogen assimilation. A soil reaction close to neutral is considered optimal. Acidic soil reaction affects both the plant itself, by worsening root system development, and the nodule bacteria, through shortening the period of functioning of the bacteroid tissue and so reducing the volume of nitrogen fixation. In addition, it should be noted that molybdenum, the key microelement for legumes, is taken up better on soils with a neutral reaction. A certain ecological adaptation of species and strains of nodule bacteria to soil acidity is also observed. Clover nodule bacteria are more resistant to increased soil acidity than alfalfa nodule bacteria, whose crops are placed mainly on soils with a neutral and slightly alkaline reaction. The third feature is the mineral nutrition conditions of legume plants. They have a significant influence on nitrogen fixation, since nodule bacteria receive mineral nutrients and carbohydrates from the host plant.
The most important nutrients
Among the most important nutrients that influence the process of symbiotic nitrogen fixation by legumes are nitrogen and molybdenum. Research has established that the higher the content of available nitrogen forms, the harder it is for bacteria to penetrate the root, and consequently fewer nodules form on the roots and their activity is low. The decrease in the share of atmospheric nitrogen assimilated by plants with increased mineral nitrogen supply is only relative. The absolute amount of nitrogen assimilated by bacteria from the air practically does not fall, and in some cases even rises, compared with trial variants with no nitrogen applied. The supply of phosphorus to legume plants is of great importance for nitrogen fixation; with low phosphorus content in the soil nodules may not form at all. With insufficient phosphorus nutrition, plant nitrogen assimilation worsens. Legumes are crops with increased potassium removal, so fertilization with potassium fertilizers positively influences the productivity of nitrogen fixation, which is evidently linked with the physiological influence of potassium in carbohydrate metabolism in plants. For legume plants molybdenum and boron are especially important microelements. Molybdenum shortage inhibits nodule formation and disrupts the synthesis of free amino acids and leghemoglobin. With boron shortage, vascular bundles do not form in the nodules and, as a result, development of the bacteroid tissue is disrupted. To overcome and prevent possible deficiency of these important mineral nutrients, production uses microfertilizers in pre-sowing seed dressing and by foliar application.
The most effective fertilizers for nitrogen fixation
Among the large number of microfertilizers on the domestic market, fertilizers of the Ecoline line are among the most effective in promoting nitrogen fixation processes. For pre-sowing treatment of legume seed and for supplying plants at the start of symbiotic nitrogen fixation, which begins with the appearance of 2–3 true (trifoliate) leaves, we recommend using Ecoline Universal Seed (Chelates) at 0.5 l/t, or in combination with Ecoline Phosphite (K) 0.5 l/t, or the new composition Gros Rootgrowth at 1.0 l/t. We recommend using molybdenum as the organo-mineral fertilizer Ecoline Molybdenum (Complex) at 0.5–1.0 l/ha at the moment of forming 2–3 true leaves of legume crops, that is, when the nitrogen fixation process begins. The microfertilizer Ecoline Molybdenum (Complex) contains this metal in the form of a phosphorus-molybdenum complex compound, which considerably raises the uptake of both elements thanks to the synergy of molybdenum with phosphorus. Supplying phosphorus in the initial period is important for a confident plant start. In case of weak soil supply, temporary cold spells or drought, it is advisable to add Ecoline Phosphite (K) at 1.0–1.5 l/ha to the tank mix with Ecoline Molybdenum (Complex). To supply plants with boron we recommend using Ecoline Boron (Premium), a liquid fertilizer in the form of an organic complex of boron with monoethanolamine with added amino acids. Active ingredients: boron, amino groups of monoethanolamine and amino acids of plant origin. The fertilizer does not lose its effectiveness even when plants are in a state of stress because of unfavorable weather.
Biological factors in the symbiosis of legume plants with nodule bacteria
Biological factors play a certain role in the symbiosis of legume plants with nodule bacteria. So-called phytophages can do great harm, hindering the penetration of bacteria into the legume root and causing lysis of nodule tissues.
Among beetles, several species of nodule weevils are known whose larvae damage root hairs and nodules. Weevils are most harmful in dry hot weather. Legume roots and nodules can also be damaged by nematodes. Recently on many crops, in particular soybean, an especially dangerous pest has spread, the red and other species of spider mite, which sucks sap from plants, leading to their infection with viral diseases and destruction of chlorophyll up to complete etiolation (bleaching) of plants and the stopping of photosynthesis, and then all the factors listed above have no practical importance.