Nitrogen is a component of plant proteins, chlorophyll, DNA (genetic code), enzymes, and many other components necessary for plant growth. Plants take up nitrogen as nitrate (NO3–) and ammonium ions (NH4+). The dominant form is nitrate. Ammonium is preferred during the early growth stage of plants, but the need for nitrogen increases throughout the growing season; plants take up most of it in the form of nitrate.

The amount of nitrogen fertilizer required by a plant depends on the supplying capacity of the soil. The dynamic range of available nitrogen sources includes:

1) organic nitrogen sources such as manure, wastewater, and compost;

2) nitrogen fixed by Rhizobium bacteria from plant residues;

3) nitrogen fixed by microbes;

4) nitrogen fertilizers;

5) soil-bound nitrogen.

All of these nitrogen sources are mineralized into nitrate over time.

SOURCES OF NITROGEN FERTILIZER

Agronomists are very interested in a stable source of nitrogen fertilizer that will be used by their crops. Their main concerns are volatilization, immobilization, and availability of nitrogen fertilizer. All nitrogen sources work effectively when applied properly.

There are many sources of nitrogen fertilizer that have been used for many years. Nitrogen is most commonly used as a mixture of ammonium nitrate and urea (28-0-0 and 32-0-0), urea (46-0-0), ammonium nitrate (34-0-0), anhydrous ammonia (82-0-0), and ammonium sulfate (21-0-0-24).

Choosing a nitrogen fertilizer source is a major decision for the grower. Each has its own advantages and disadvantages. There may be situations where one source is preferable to another.

The primary source of nitrogen fertilizer is anhydrous ammonia (NH3). Ammonia is produced by reacting atmospheric nitrogen (78% of the atmosphere is nitrogen, N2) with natural gas at high temperature and pressure. As energy prices rise, so do the prices of nitrogen fertilizers. Ammonia becomes liquid at temperatures below –33°C. Ammonia is therefore stored under pressure to maintain its liquid state. Ammonia passed over a platinum catalyst becomes nitric acid (HNO3). Nitric acid mixed with ammonia yields ammonium nitrate NH4NO3 (34–0–0). Urea (NH2)2CO (46–0–0) is the result of excess ammonia reacting with carbon dioxide (CO2). Ammonia reacting with sulfuric acid yields ammonium sulfate; ammonia reacting with phosphoric acid yields ammonium phosphate. For many years, anhydrous ammonia (NH3) was the main source of nitrogen fertilizer in conventional tillage. Anhydrous ammonia must be injected deep enough into the soil to avoid loss of gaseous ammonia. Application can be done at seeding or at any other time. This method of application is quite harsh on the microflora and microfauna of the soil. Today, no-till farmers use other sources of nitrogen fertilizers that are gentler.

NITROGEN VOLATILIZATION FROM UREA

Urea (NH2)2CO (46–0–0) is a dry nitrogen fertilizer commonly used by farmers. Urea, when applied to the soil or plant residues, reacts with water and is quickly converted into ammonium by the enzyme urease. This is the so-called hydrolysis of urea. The ammonium cation (NH4+) is converted into ammonia (NH3). Since ammonia is a gas, it evaporates into the atmosphere. If ammonium is captured by soil particles, it is retained in the soil and does not evaporate. Rainfall or irrigation of 850 mm is normally sufficient to move urea into the soil. Since the reaction of converting urea to ammonium is an enzymatic reaction, the rate of conversion increases with increasing temperature. It is best to apply urea during cool periods with high rainfall.

Dry urea can be applied in strips. A strip of 38 cm is used for small grains or across the width of the row. Urea can also be applied as a starter fertilizer 5–8 cm away from the seed. The nitrogen rate is 70–100 kg/ha.

Today, farmers use different methods of applying nitrogen. For example, by mixing solutions of urea and ammonium nitrate, they obtain a liquid fertilizer containing approximately half urea and half ammonium nitrate. Such solutions are good sources of nitrogen for crops. To avoid NH3 volatilization, such fertilizers can be injected into the soil. The depth of application is not critical, but it should be large enough; a rotary cultivator copes well with this task.

The solution must be injected to reduce urease activity. Repeated application of this mixture also works effectively. However, to avoid nitrogen loss through volatilization, it is necessary to adhere to the application rules.

Other nitrogen fertilizers do not have the problem of NH3 loss. NH3 volatilization occurs because urease breaks down urea to NH3 and CO2. NH3 dissolves in water to form ammonium (NH4+). CO2 lowers the pH of the solution. Ammonium, which is part of ammonium nitrate, sulfate, and phosphate, does not evaporate. The pH of solutions of these ammonium salts is low because nitric, sulfuric, and phosphoric acids are strong, and NH4OH is a weak base. The strong acidic nature of the salts prevents NH3 loss. CO2 is a weaker acid than NH3, a weak base; thus, acidity increases with increasing CO2.

APPLICATION TIMING

The optimal time to apply nitrogen fertilizer depends on:

1) the crop being grown;

2) the nitrogen uptake characteristics;

3) the soil texture;

4) the root zone;

5) the climate;

6) the amount of nitrogen needed.

Nitrogen management is more important for shallow-rooted crops grown on sandy soils than for deep-rooted crops grown on loamy soils. Maximum nitrogen uptake occurs during the period of rapid growth.

Wheat, for example, has its most rapid growth and therefore maximum nitrogen uptake during the booting phase. Most or all of the nitrogen fertilizer should be applied early enough to allow time for microorganisms to mineralize the nitrogen fertilizer to nitrate and make the nitrogen available to the plant. Cooler soil temperatures slow mineralization processes, so nitrogen fertilizers should be applied early.

The fastest nitrogen uptake in corn occurs in the phase from the 8th leaf to panicle ejection. Most of the nitrogen should be applied 2 weeks before the phase of maximum uptake, provided that this nitrogen is in a form available to the plant – in the form of nitrate.

NITROGEN LOSSES DURING WASHING AND DENITRIFICATION

Losses of nitrogen from the soil can occur when:

1) washing out (leaching);

2) denitrification;

3) volatilization of NH3.

The issue of NH3 loss is discussed above. Leaching is the process of washing soluble nitrate with water. Soils with a high water-holding capacity can accumulate a significant amount of it together with nitrate.

For soils with a light texture, nitrogen fertilizer can be applied immediately before sowing or as top dressing during the period of the most active growth. The introduction of nitrogen should be carried out in time to avoid leaching into the deeper layers of the soil. In areas with a small amount of rain during the growing season, accordingly, this problem is not relevant. But in areas with a large amount of precipitation, the time of fertilizing is critical.

For soils with a light texture, with poor aeration due to increased moisture, it is very important to apply most of the nitrogen fertilizer after the soil dries. If nitrogen is applied in advance before sowing, the main potential of nitrogen is lost due to denitrification.

For sandy soils, part of the nitrogen can be applied with herbicide or starter fertilizer. The remaining part must be applied before the phase of maximum nitrogen consumption (as discussed above).

The amount of leached nitrogen depends on the properties of the soil and its ability to hold water. Improving the structure of the earth undoubtedly reduces the amount of leached nitrates. When macropores are developed, water can enter the ground by gravity, moving vertically down through the pores and spreading horizontally. Washing takes place through capillaries. Capillary movement of water carries soluble nitrate.

The standard estimate of leached nitrate can be calculated according to the following formula:

d = 100a/Pv,

where d is the washing depth, Pv is the capacity (water-absorbing capacity), and is the amount of washing water. For example, if the water-absorbing capacity of loam is 46% and 2.5 cm of water moves along the root zone, then nitrate will move to a depth of 5.5 cm. On sandy soils with half the water-absorbing capacity, nitrate is washed to a depth of 2 times greater - 11 cm.

Denitrification is a microbial process by which anaerobic soil bacteria (bacteria capable of living without air oxygen) of the earth utilize (process) nitrate oxygen (NO3) to support their vital processes.

The process of denitrification is the transformation of a ready supply of nitrate into various forms of nitrogen that can be lost in the atmosphere. The process of denitrification can be represented by the following diagram: 2NO3 —> 2NO2 —> 2NO —> N2O —> N2.

To reduce potential losses of nitrogen due to denitrification, it is necessary to synchronize the time of nitrogen application with the phase of maximum nitrogen consumption. If nitrogen fertilizers have to be applied before the phase of maximum nitrogen consumption, it is possible to use a nitrification retarder to reduce nitrogen from fertilizer to nitrate.

CONSUMPTION AND EXTRACTION OF NITROGEN BY CULTURES

In the process of vegetation, nutrients are spent on the formation of leaves, grains, stems, etc. p. The amount of nitrogen fertilizers needed by the plant depends on the yield and the amount of nitrogen removed with the crop.

Nitrogen removal is used to show how much nitrogen is removed from the soil by the crop. Nitrogen requirement is the amount of nitrogen consumed by the plant for the growth of leaves, stems, roots and grain. Crop residues contain final nitrogen. This organic nitrogen will be "released" after some time. However, for the No-till technology, it is necessary to add a certain amount of nitrogen fertilizers for faster processing of nitrogen into an accessible form. Only after 3–4 years, nitrogen from crop residues after rotting becomes available. Subsequently, this fact will be taken into account when reducing the dose of nitrogen fertilizers. Leguminous crops mostly use their "own" nitrogen, which can be available for subsequent, non-leguminous crops as well.

Thus, the value of the loan can be subtracted from the total amount of necessary nitrogen fertilizers.

CONCLUSIONS

This article examines the functions of nitrogen as a fertilizer and as a nutrient for plants. Nitrogen is necessary for growing a healthy and high-quality crop. The amount of nitrogen that needs to be applied depends on many factors, such as the final nitrate in the soil, past/current legume crops, nitrogen consumption by the crop, yield potential, immobilization of crop residues with nitrogen fertilizers, degree of nitrate mineralization, soil organic matter, grain price, protein level, protein level and other indicators of crop quality.

The efficiency of nitrogen fertilizer application can increase with the correct selection of the nitrogen source. There are many effective ways of applying nitrogen fertilizers. Based on time, equipment, and other factors, the best application method is what we can generally do. Nitrogen injection into the ground is usually the most effective method. However, all other application methods work well. It is also advisable to take into account the indicators for nitrogen losses during leaching, denitrification and weathering, given in the article.