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Nitrogen fertilizer production

Sources of producing nitrogen fertilizers

From 1830 until 1914 the main nitrogen fertilizer was Chilean nitrate, whose deposits are concentrated in South America. Ammonia from coke ovens of the metallurgical industry was also used as a nitrogen fertilizer.

By the beginning of the 20th century natural deposits of Chilean nitrate were almost exhausted, so the issue of industrial-scale production of nitrogen fertilisers arose. The use of atmospheric nitrogen for the production of nitrogen fertilizers was promising, which in a 15 kilometer layer of air over an area of 1 hectare amounts to about 78 thousand tons of molecular nitrogen.

At the end of the 19th century, a way was found in the laboratory to bind molecular nitrogen with oxygen by passing air through a voltaic arc discharge with a temperature of about 3000 °C:

N2 + O2 = 2NO.

The resulting nitrogen monoxide is oxidized by air oxygen to nitrogen dioxide, which in interaction with water forms nitric acid.

The first plant using this technology was built in Norway, where natural conditions allow obtaining relatively cheap electric power. Its production focused on the production of Ca(NO3)2. Hence the calcium nitrate was called “Norwegian nitrate”. The obvious disadvantage of this technology was high energy costs, and the calcium nitrate produced is very hygroscopic and inconvenient to use. Therefore, the method was not further disseminated.

A method of fixing atmospheric nitrogen was proposed, based on nitrogen fixing by calcium carbide at 700-800 °C:

N2 + CaC2 = CaCN2 + C.

The method of calcium cyanamide production is technologically simpler and cheaper, but it is also not widespread due to the discovery of a method for producing ammonia from molecular nitrogen and hydrogen.

The method of ammonia synthesis was discovered by the German chemist Gaber. Of all the ways to bind molecular nitrogen, his method turned out to be the cheapest and is currently the main one in the production of nitrogen fertilizers.

Ammonia production

Ammonia is produced by the interaction of nitrogen and hydrogen. For this purpose, a mixture of gases in a 1:3 ratio is compressed under high pressure and fed into a contact furnace (synthesis chamber), where ammonia is synthesized at 400-500 °C, pressure and in the presence of catalysts (iron with additions of aluminum and potassium oxides):

N2 + 3H2 = 2NH3

The source of nitrogen is air. One of the methods is used for nitrogen extraction from air:

Atmospheric air is passed through a generator filled with burning coke, the oxygen is completely burned out, and a mixture of nitrogen and carbon dioxide comes from the generator. The latter is absorbed by water at a pressure of 25 atm.
The air is liquefied and then separated into nitrogen and oxygen due to the difference in boiling point: oxygen boils at -183 °C, nitrogen – at -196 °C.

Up to 50% of the cost of ammonia production is spent on hydrogen production. Natural and associated petroleum gases or waste gases from coke ovens are used as hydrogen sources. Hydrogen can be produced by electrolysis of water. The latter method allows obtaining pure hydrogen, but requires high energy costs.

The ammonia produced is used directly as a fertilizer, to produce ammonium fertilizers, nitric acid, urea.

Obtaining nitric acid

Nitric acid is produced by catalytic oxidation of ammonia with air oxygen. This is the main method for producing nitric acid. The reaction takes place in several stages. First, ammonia is oxidized to nitric oxide:

4NH3 + 5O2 → 4NO + 6H2O.

Nitrogen oxide enters the oxidation towers, where it is oxidized to nitrogen dioxide by oxygen:

2NO + O2 = 2NO2.

NO2 enters absorption towers (absorbers) where it is absorbed by water to form nitric and nitrous acids:

2NO2 + H2O = HNO3 + HNO2;

3NO2 + H2O = 2HNO3 + NO.

Nitric acid HNO2 is unstable and decomposes quickly:

2HNO2 = NO + NO2 + H2O.

The resulting nitrogen oxides NO and NO2 are returned to the same oxidizing and absorbing units.

Ammonia and nitric acid obtained by industrial methods are the main sources of nitrogen fertilizers.

Classification of nitrogen fertilizers

Depending on the form of nitrogen, nitrogen fertilizers are classified into:

  • nitrate – sodium (NaNO3) and calcium [Ca(NO3)2] nitrate;
  • ammonium – sulfate [(NH4)2SO4] and ammonium chloride (NH4Cl),
  • carbonate [(NH4)2CO3] and ammonium bicarbonate (NH4HCO3);
  • ammonium-nitrate – ammonium nitrate (NH4NO3), ammonium sulfonitrate [(NH4)2SO4 ⋅ 2NH4NO3];
  • ammonia – anhydrous ammonia, ammonia water;
  • amide – urea [CO(NH2)2] and calcium cyanamide (CaCN2).

Nitrogen fertilizers may have mixed forms. A separate group includes slow-acting forms, such as urea-formaldehyde and encapsulated fertilizers.

Nitrate fertilizers

Nitrate fertilizers are nitrogen fertilizers that contain nitrogen in the form of the nitrate group NO3-. For example, sodium nitrate, or sodium nitrate, NaNO3 and calcium nitrate, or calcium nitrate, Ca(NO3)2. In Russia, the use of nitrate forms is less than 1%.

Sodium nitrate

Sodium nitrate, or sodium nitrate, or sodium nitrate, or Chilean nitrate – NaNO3. It contains 16% nitrogen and 26% sodium. It was the first mineral nitrogen fertilizer. The largest natural deposit was in Chile. Significant deposits were found in California, in southwest Africa.

Nowadays sodium nitrate is obtained as a byproduct in the production of nitric acid from ammonia. Nitric oxides NO and NO2 (the “tail gases”) that have not been adsorbed by the water in the absorption towers are passed through additional absorption towers sprayed with a solution of sodium carbonate or sodium hydroxide to produce a mixture of sodium nitrate and sodium nitrite:

Na2CO3 + 2NO2 = NaNO3 + NaNO2 + CO2.

Nitrite when acidified with dilute nitric acid turns into nitrate:

3NaNO2 + 2HNO3 = 3NaNO3 + 2NO + H2O.

Nitrogen monoxide returns to the oxidation tower. The acidified sodium nitrate solution is neutralized, evaporated and the NaNO3 precipitate is separated from the mother liquor.

Sodium nitrate is a fine crystalline salt of white, gray or brownish-yellow color, well soluble in water, hygroscopic, with high humidity is able to recrystallize into larger crystals. In dry condition and proper storage, it does not caking and retains its flowability.

Calcium nitrate

Calcium nitrate, or calcium nitrate, or calcium nitrate, or Norwegian nitrate – Ca(NO3)2. It contains 17% of nitrogen. First industrially synthesized in 1905 in Norway.

Currently, it is produced as a by-product of obtaining nitric acid from ammonia: in neutralization of tail gases (nitric oxides NO and NO2) by aqueous calcium hydroxide solutions Ca(OH)2 (milk of lime), as well as in the production of complex fertilizers by nitric acid decomposition of phosphate raw materials.

Calcium nitrate is highly hygroscopic (9.5 out of 10). Under normal storage conditions, it tends to get very damp, spongy and caked. It is transported and stored in moisture-proof bags. To reduce hygroscopic properties, hydrophobic additives (gypsum, paraffin oil) up to 0.5% of salt weight are added to commercial calcium nitrate.

To improve physical properties, 4-7% ammonium nitrate is added to calcium nitrate solution during production. Calcium nitrate is produced in granular form, which is obtained by adding 4-7% ammonium nitrate to evaporated concentrated nitrate solution and subsequent granulation.

Application of nitrate fertilizers

Nitrate fertilizers can be used on a variety of soils for all crops. Because of their low nitrogen content their use is more expensive economically, they are more often used in areas near industries.

Calcium and sodium nitrate are of equal value for most plants. The exception is sugar beet and other root crops: sodium nitrate is more effective due to the positive effect of sodium on these crops. The latter is due to the positive effect of sodium on the outflow of carbohydrates from the leaves to the roots, and thus increasing the yield of roots and the content of sugars in them.

When applied to the soil, nitrate fertilizers quickly dissolve in the soil solution, Na+ and Ca2+ cations enter into exchange reactions with the soil absorbing complex (SAC), pass into the exchange-absorbed state:

[SAC](Ca, K) + 3NaNO3 → [SAC]Na3 + Ca(NO3)2 + KNO3;

[SAC](H, NH4) + Ca(NO3)2 → [SAC]Ca + HNO3 + NH4NO3.

Systematic application of calcium nitrate contributes to the replenishment of SAC with calcium.

Nitrate ion NO3- forms soluble salts or nitric acid with cations displaced from the soil absorbing complex. It does not undergo physicochemical or chemical absorption. Nitrate can bind in the soil only through biological absorption during the warm period of the year. In the autumn-winter period, biological absorption is almost completely absent. For this reason, nitrate fertilizers are inexpedient to apply in autumn, especially in areas with flush water regime.

Sodium and calcium nitrates are used in spring for pre-sowing cultivation and as top dressing during the growing season. In summer, nitrates can be washed out in conditions of excessive moisture, irrigation and easily drained soils due to their high mobility. Therefore, in regions with humid climates and in irrigated areas, ammonium forms are applied for rice and other crops.

Sodium nitrate is also applied in rows with seeds, calcium nitrate is of little use because of its high hygroscopic properties. Sodium nitrate must not be applied on saline soils and salts.

Sodium and calcium nitrates are physiologically alkaline fertilizers, as plants absorb the anion NО3- more than Na+ or Ca2+ cations. Some of the cations remaining in the soil alkalize the soil solution. Prolonged use of nitrate fertilizers on acidic sod-podzolic and light low-buffered soils contributes to their neutralization. Therefore, on sod-podzolic soils nitrate fertilizers show greater efficiency than physiologically acidic ammonia fertilizers. On chernozems this advantage is lost.

Ammonium fertilizers

Ammonium fertilizers are a form of nitrogen fertilizer that contains nitrogen in the form of the ammonium group NH4+. They include ammonium sulfate, ammonium chloride, ammonium carbonate. Their production is relatively easier than that of nitrate fertilizers, as there is no stage of ammonia oxidation to nitric acid.

Ammonium sulfate

Ammonium sulfate, or ammonium sulfate, (NH4)2SO4 pure salt contains 21.2% of nitrogen, in the technical product – 20.5%. In the world production of nitrogen fertilizers its share is about 25%, in Russia – less than 6%. The large share of ammonium sulfate in world production is explained by its wide use in irrigated agriculture for rice and cotton and in areas of excessive moisture (tropics).

In Russia, production of ammonium sulphate began in Donbass at Shcherbinsky mine in 1899 by capturing and neutralizing ammonia from coking coal with sulphuric acid. The same technological scheme is used at present as well.

Ammonium sulfate can be produced by absorption of ammonia with sulfuric acid according to the reaction:

H2SO4 + 2NH3 = (NH4)2SO4.

The reaction proceeds with the release of heat, which is spent on evaporation of the solution, when the saturated solution is cooled ammonium sulfate precipitates as a crystalline precipitate, which is separated and dried. Sulphuric acid can be replaced with cheaper natural minerals: gypsum (CaSO4⋅2H2O), mirabilite (Glauber salt, Na2SO4⋅10H2O) or phosphogypsum, a waste product of phosphate fertilizers.

Finely ground gypsum is shaken in ammonia water, through which carbon dioxide is passed. The interaction of ammonia, carbon dioxide and gypsum produces ammonium sulphate:

2NH3 + CO2 + H2O = (NH4)2CO3;

(NH4)2CO3 + CaSO4 = (NH4)2SO4 + CaCO3.

Calcium carbonate insoluble in water is filtered off, and the solution containing (NH4)2SO4 is evaporated to crystallization, separated from the mother liquor and dried.

Due to the cheaper cost of ammonia obtained from the coke off-gases, coke-oven ammonium sulfate is obtained more cheaply.

Ammonium sulphate is well soluble in water: 76.3 g (NH4)2SO4 per 100 cm3 of water at 20 °C. In the dry condition fertilizer has a small hygroscopic properties, little caking during storage, does not disperse in the air, retains loose and well dispersed by fertilizer aggregates.

Ammonium sulphate is a white crystalline substance with various paints depending on the method of production. It contains 0,2-0,3% of moisture, an impurity of Ca, Mg, SiO2, 0,025-0,05% (0,2-0,5%) of free sulfuric acid, which gives fertilizer slightly acidic reaction. Coke sulfate ammonium contains a small amount of organic impurities – resinous substances, phenol, up to 0.1% rhodanide ammonium (NH4SCN). These impurities may cause gray, bluish or reddish coloration.

Because of the toxicity to plants of ammonium rhodanide, its content should not exceed 0.1%, especially on soils with low humus and calcium content. Ammonium sulfate contains 24% sulfur, so it is a source of sulfur nutrition for plants.

After application to the soil, most of the NH4+ ammonium ions are included in the absorbing complex:

[SAC]Ca2 + (NH4)2SO4 = [SAC](Ca, (NH4)2) + CaSO4

The soil’s ability to absorb ammonium protects it from leaching; however, it may not be used in fertilization.

As a result of nitrification, some of the ammonia nitrogen is converted to the nitrate form, which leads to acidification of the soil solution. Acidification is also caused by the physiological acidity of the fertilizer. Systematic application of normal doses of ammonium sulfate leads to a change in the reaction of the soil environment. On acidic soils the negative effect appears after a few years. On chernozem soils it can be applied for a longer time. According to data from Myronivska experimental station in Ukraine, the use of (NH4)2SO4 in 14 years led to changes in soil reaction: pH from 6.0 to 4.9; exchange acidity increased by 1.5, hydrolytic – in 2.5 times. This did not affect the yields due to the high content of humus, high buffer and absorption capacity of chernozem. On chestnut soils and gray soils acidification of carbonate soils with physiologically acidic fertilizers is not dangerous.

On sod-podzolic soils in combination with liming ammonium sulfate is not inferior to other nitrogen fertilizers. However, prolonged use in high doses without liming on these soils deteriorates their properties, growth and productivity of plants. Such crops as oats, winter rye, flax, potatoes, rutabaga react weaker to acidifying action of ammonium sulfate than beets, corn, hemp, barley and spring wheat.

Because of the weak migration of ammonium ions, this fertilizer is effective on light soils and in areas of sufficient moisture. Ammonium sulfate is less effective than other nitrogen fertilizers when applied in rows and as a top dressing.

Ammonium-sodium sulfate

Ammonium-sodium sulfate – (NH4)2SO4⋅Na2SO4, contains up to 16% of nitrogen, 9% of Na2O, up to 2.5% of organic impurities, is a waste product of caprolactam production. It is a yellowish crystalline salt. It is a good fertilizer for sugar beet and cruciferous plants, which are responsive to sodium and sulfur. It can be used to fertilize hayfields and pastures.

Ammonium chloride

Ammonium chloride, NH4Cl, is a byproduct of the production of baking soda (sodium hydrogen carbonate):

NH3 + CO2 + H2O + NaCl = NaHCO3 + NH4Cl.

Ammonium chloride is a fine-crystalline white or yellowish powder, contains up to 26% nitrogen, dissolves in 100 cm3 of water at 20 °C 37.2 g, slightly hygroscopic, does not cake, well dispersed. It is characterized by high physiological acidity and contains up to 60% chlorine, which negatively affects chlorophobic crops such as potatoes, tobacco, grapes, onions, cabbage, hemp, flax, buckwheat, citrus fruits, vegetables, fruits and berries. Therefore, it is introduced in autumn to chlorine washed out of the root layer by atmospheric precipitation.

In soils, ammonium chloride enters into exchange reactions with the absorbing complex:

[SAC]Ca + 2NH4Cl = [SAC](NH4)2 + CaCl2.

In the soil is partially subjected to nitrification. Increase the effectiveness of ammonium chloride can also, as well as ammonium sulfate: liming, pre-neutralization fertilizer (1 kg NH4Cl 1.4 kg CaCO3), joint application with physiologically alkaline fertilizers, a combination with organic fertilizers.

NH4Cl is usually inferior to (NH4)2SO4 in its fertilizing effect. For crops at normal doses, the effectiveness of chloride and sulfate are equal. For crops sensitive to chlorine increased doses are not used, applied in advance as a basic fertilizer.

Ammonium carbonate and hydrogen carbonate

Ammonium carbonate (NH4)2CO3 and ammonium hydrogen carbonate NH4HCO3 are used as fertilizer in small quantities.

Ammonium carbonate is a white crystalline substance obtained by passing carbon dioxide through an aqueous ammonia solution, followed by evaporation of the resulting salt. Carbonate is unstable, and may decompose in the open air, releasing ammonia and forming ammonium hydrogen carbonate. The technical product contains 21-24% nitrogen; it is a mixture of ammonium carbonate, hydrocarbonate and carbamate.

Ammonium hydrogen carbonate, or bicarbonate, is produced by adsorbing gaseous ammonia and carbon dioxide with a solution of ammonium carbonate. It contains about 17% of nitrogen. It is relatively more stable than carbonate, but also has ammonia losses during storage, transportation and application. It should be incorporated into the soil immediately when it is applied on the surface.

Application of ammonium fertilizers

When applied to the soil, ammonium fertilizers are dissolved and the NH4+ ion enters into exchange reactions with the soil solid phase. Most of the NH4+ cations are included in the soil uptake complex, displacing an equivalent amount of cations from it:

[SAC](Ca, H) + NH4Cl = [SAC](Ca, NH4) + HCl;

[SAC]Ca2 + (NH4)2SO4 = [SAC](Ca, (NH4)2) + CaSO4.

When converted to the metabolized state, ammonium is fixed in the soil, thereby preventing its leaching. At the same time, in the exchanged-absorbed state, ammonium remains available to plants.

Partly under the influence of nitrification, ammonia nitrogen is converted into nitrate form. The rate of this process depends on temperature, humidity, aeration, biological activity and reaction of the soil, the degree of cultivation. Thus, in a microfield experiment conducted on poorly cultivated sod-podzolic soils in 15 days of nitrification was 12% ammonium sulfate, after 30 days – 24%, whereas in well-cultivated soils nitrification was 79 and 96% of the introduced amount, respectively.

Overwetting and increased acidity inhibit the process of nitrification. Lime application to acidic soils speeds up the process.

Ammonium chloride nitrifies slower than sulfate due to the inhibiting effect of chlorine on the activity of nitrifying bacteria.

Once ammonium nitrogen is converted into nitrate nitrogen becomes a nitrate fertilizer. In the process of nitrification, nitric, hydrochloric or sulfuric acid is formed in the soil:

NH4Cl + 2O2 = HNO3 + HCl + H2O,

or

(NH4)2SO4 + 4O2 = 2HNO3 + H2SO4 + 2H2O.

In soil, acids are neutralized by the hydrocarbonates of the soil solution and by the cations of the soil absorption complex:

2HNO3 + Ca(HCO3)2 = Ca(NO3)2 + 2H2O + 2CO2;

2HCl + Ca(HCO3)2 = CaCl2 + H2O + CO2;

[SAC]Ca2 + 2HNO3 = [SAC](Ca, H2) + Ca(NO3)2;

[SAC]Ca2 + 2H2SO4 = [SAC](Ca, H2) + CaSO4.

Neutralization of mineral acids is accompanied by consumption of soil solution hydrocarbonates and displacement of bases from the soil absorbing complex with hydrogen, which reduces the buffering capacity and increases soil acidity.

The change in reaction when ammonium fertilizers are applied is also related to their physiological acidity. From (NH4)2SO4 and NH4Cl plants absorb the cation faster than the anion, respectively, acidic residues accumulate. Their systematic application is accompanied by acidification of the soil environment. The degree of acidification is the greater the smaller the buffer capacity.

Table. Effect of fertilizers on acidity and the amount of absorbed bases of gray forest soil (according to the Research Institute of Bast Crops)

Fertilizer
Exchangeable acidity
Hydrolytic acidity
Amount of absorbed bases
Base saturation degree, %
mg⋅eq/100 g of soil
Control (without fertilizer)
0,4
11,3
14,3
55,8
Manure, 40 t/ha
0,4
9,8
17,7
64,4
NPK in doses equivalent to 40 tons of manure (nitrogen in the form of (NH4)2SO4)
0,5
14,2
9,3
39,4

On sod-podzolic and gray forest soils with low sum of absorbed bases and organic matter content acidification is manifested faster compared to chernozems and chestnut soils. Thus, long-term application of ammonium sulfate (as part of NPK) on gray forest soils led to an increase in hydrolytic acidity, a decrease in the amount of absorbed bases and the degree of saturation with bases.

To prevent the negative acidifying effect of ammonium fertilizers on such soils prior liming or neutralization of sulfate and ammonium chloride before making the calculation 130-140 kg of lime per 100 kg of fertilizer. Neutralisation of fertilizers is carried out just before application.

Features transformation of ammonium fertilizers in soils predetermine the technology of their effective use. These fertilizers are usually made before sowing as the main fertilizer, and both in spring and autumn, without fear of nitrogen washout.

The effectiveness of ammonium fertilizers depends on the acidity and buffer soils, and biological characteristics of crops.

On soils of the Non-Black Earth zone, ammonium fertilizers can increase the effectiveness of phosphate meal. Physiological acidity of these fertilizers contributes to the dissolution of calcium phosphate.

The effectiveness of ammonium fertilizers depends on the characteristics of the crops grown. Less sensitive crops such as rye, oats, potatoes, flax, buckwheat are less responsive to acidification. Sensitive crops (root crops, most vegetables and legumes, barley, wheat, sunflowers), respond negatively to acidification with repeated applications of ammonium fertilizers.

Crops sensitive to high chlorine content react negatively. For example, the starch content of potatoes decreases in excess chlorine. Therefore, ammonium sulphate is used for chlorophobic crops, or ammonium chloride is added in autumn.

Ammonium nitrogen due to low mobility is localized in the soil where it is applied. Therefore, ammonium fertilizers are of little use for inter-row fertilizing and local application. In the initial phases of growth, the root system of crops is poorly developed and may not reach the fertilizer localization zone.

Ammonium fertilizers are also not used for pre-sowing applications in rows or under pre-sowing cultivation due to the fact that the intensive entry of ammonium nitrogen in young plants can lead to “ammonia poisoning” due to its excessive accumulation.

Ammonium-nitrate fertilizers

Ammonium-nitrate fertilizers are a group of nitrogen fertilizers that include both ammonium and nitrate forms of nitrogen. This group includes ammonium nitrate, ammonium sulfo-nitrate, and lime ammonium nitrate.

Ammonium nitrate

Ammonium nitrate, or ammonium nitrate, ammonium nitrate, ammonium nitrate, NH4NO3, contains 35% nitrate and ammonium nitrogen in a 1:1 ratio. It is obtained by neutralizing nitric acid with ammonia:

HNO3 + NH3 = NH4NO3 + 144.9 kJ.

The resulting solution of ammonium nitrate is evaporated, recrystallized and dried. The heat of the neutralization reaction is used for evaporation. The result is a white crystalline substance containing up to 98-99% NH4NO3. Additives are used to improve the physico-chemical properties.

Ammonium nitrate is well soluble in water: at 20 °C in 100 cm3 of water dissolves 192 g of salt is very hygroscopic, the air dries up and cakes. Depending on the temperature it has five crystalline modifications. Transitions from one modification occur, at temperatures of +32.1 and -16 °C. If during the storage of ammonium nitrate there were sharp temperature changes, capturing these temperature points, there will be a recrystallization of one form into another with an increase in volume. The fertilizer will then thicken greatly, turning into lumps, clumps, and the bags in which it was stored may burst.

To prevent caking of ammonium nitrate it is added hydrophobic and hardening additives: ground limestone, chalk, phosphate flour, phosphogypsum, kaolinite, magnesium nitrate, fatty acids and their amines, and others. The total content of additives ranges from 3.0% to 5.0%. The additives may impart a yellow tint. Fuchsin, which imparts a red color, may be introduced as an additive.

The physical properties of ammonium nitrate depend on the size and shape of the resulting crystals and pellets. The chemical industry produces ammonium nitrate in the form of 1-4 mm granules and flakes (flake nitrate). Granulated ammonium nitrate is characterized by good physical properties.

Moisture content should be no more than 0.3-0.4%, the reaction is neutral or weakly acidic, and the content of insoluble impurities – no more than 0.1%.

To prevent moisture and reduce caking ammonium nitrate is packed in dense, tightly sealed containers – polyethylene or laminated paper bags. For storage, the bags must not be stacked in high piles or stacks, as the bottom layers of the pile strongly compact the bags and they become caked.

To improve physical properties, nitrate can be mixed with precipitate and phosphate flour (for podzolic soils) during storage. Immediately before application to podzolic soils ammonium nitrate may be mixed with 30-40% calcium carbonate, which greatly reduces hygroscopicity and increases the convenience of machine sowing.

Ammonium nitrate is flammable and may explode under certain conditions. At temperatures above 200-270 °C, it decomposes with the release of heat and strong oxidants that accelerate combustion. Rapid heating to 400-500 °C leads to an explosion. Mixtures with combustible materials (sawdust, diesel fuel, paper dust, dry peat, oil) contribute to the manifestation of flammable and explosive properties.

For the first time pure ammonium nitrate was used in our country. Due to the high nitrogen content, the cost of transportation and application is much lower than that of other nitrogen fertilizers except urea and liquid ammonia. Due to the combination of mobile nitrate nitrogen and less mobile ammonium nitrogen, it is possible to vary the methods, doses and timing of its application depending on the soil and climatic conditions and biological characteristics of crops.

When applied to the soil, ammonium nitrate is dissolved by soil moisture. Nitrogen NH4NO3 is absorbed by microorganisms, and at their dying out and mineralization becomes available to plants. In soil, ammonium reacts with the soil absorbing complex:

[SAC]Ca2 + 2NH4NO3 = [SAC](Ca, (NH4)2) + Ca(NO3)2.

When calcium deficiency on acidic podzolic soils, the application of ammonium nitrate leads to acidification of the soil solution. The experiments of D.N. Pryanishnikov established that from the solution of ammonium nitrate the cation NH4+ is absorbed faster than NО3. Therefore, ammonium nitrate refers to physiologically acidic fertilizers. However, its physiological acidity is lower than that of ammonium fertilizers.

On soils saturated with bases (chernozem, gray soil), the systematic application of high doses of ammonium nitrate does not lead to acidification of the soil solution. Local acidification is temporary, but can have a negative effect on the initial phases of plant growth and increase the mobility of toxic compounds of aluminum, manganese and iron.

On acidic sod-podzolic soils, the application of ammonium nitrate can lead to even greater acidification, which is temporary in nature: the absorption of nitrate nitrogen restores the reaction of the environment to its original value.

Ammonium can undergo nitrification, which also temporarily acidifies the soil. Part of the nitrate nitrogen is lost during denitrification in the form of gaseous compounds (N2, N2O, NO). In the first year after application 40-50% of nitrogen is used; 10-20% of nitrate nitrogen and 20-40% of ammonia nitrogen are transformed into an organic form (immobilized), and only 10-15%, i.e. 2-3% of the introduced nitrogen is assimilated by plants in the second year. The process of immobilization is accelerated by stocking crop residues with low nitrogen content and high carbon content, such as straw, straw manure. Fertilizer nitrogen mobilizes soil nitrogen, which leads to higher utilization factor.

Ammonium nitrate in high doses on low-buffered light soils increases the nitrate content in plants. The use of such plants in animal feed can lead to metabolic disorders and poisoning. Microflora in the rumen of ruminants reduce nitrates to nitrites, which, once in the blood, bind hemoglobin and block its ability to carry oxygen. Increased concentration of methemoglobin in the blood of animals leads to asphyxiation and death in severe poisoning.

The effectiveness of ammonium nitrate when applied to acidic soils is influenced by timely liming. The negative effect of potential acidity can be eliminated by neutralizing the fertilizer with lime or dolomite at the rate of 1 ton of CaCO3 per 1 ton of fertilizer.

Ammonium nitrate is used as a pre-sowing (basic) and in-line (at sowing) fertilizer, for fertilizing during the growing season.

Under irrigation conditions, sufficient or excessive moisture, especially on light soils with granulometric composition, the introduction of ammonium nitrate in autumn under autumn plowing is inexpedient because of the possible washout of nitrate nitrogen. In these circumstances, it can be used directly at the time of the highest nitrogen consumption by plants. In small doses of 10-15 kg/ha nitrate is made in conjunction with phosphorus and potash fertilizers in the rows when sowing sugar beets and vegetable crops, in the wells when planting potatoes. High efficiency is noted when feeding winter cereals and row crops.

Ammonium nitrate is also used for early spring fertilizing of winter crops and perennial grasses. It can be used for top dressing of row crops and vegetable crops during inter-row cultivation with embedding to a depth of 10-15 cm by plant-feeder cultivators.

Ammonium sulfo-nitrate

Ammonium sulfo-nitrate, or ammonium sulfate-nitrate, leyna-selitra, montane-selitra, (NH4)2SO4⋅2NH4NO3 with an admixture of (NH4)2SO4. It contains up to 25-27% of nitrogen, including in the ammonium form – 18-19%, in the nitrate form – 7-8%. It is a grayish fine-crystalline or granular substance.

It is produced by mechanical mixing of 65% ammonium sulphate and 35% ammonium nitrate or by adding dry ammonium sulphate to nitrate alloy, followed by drying and grinding the mixture. The product obtained by the latter method is also called leyna-selitra. Another method of production is the neutralization of sulfuric and nitric acids with ammonia – montan-selitra.

Ammonium sulfate-nitrate is well soluble in water, less hygroscopic than ammonium nitrate. When stored in dry conditions, it does not caking and retains its flowability.

Its efficiency is similar to that of ammonium sulfate. It has a significant potential acidity, so its use on acidic soils requires prior liming or neutralizing fertilizer before applying.

Lime-ammonium nitrate

Lime-ammonium nitrate, NH4NO3⋅СаСО3. It is obtained by fusion of ammonium nitrate with limestone. Available in the form of granules with different ratios of NH4NO3:CaCO3 – from 80:20 to 53:47. Optimal physical and mechanical properties of the product has 60:40 with a nitrogen content of 20.5%.

This fertilizer is less hygroscopic, non-explosive and can be transported in bulk (bulk) mode compared to ammonium nitrate. It is widely used in Western Europe. It is not produced in Russia due to the high cost of transportation (the lower the content of the active substance, the more expensive transportation).

Liquid ammonia fertilizers

Liquid ammonia fertilizers are liquid (anhydrous) and aqueous solutions (ammonia water) of ammonia as well as ammoniates. In terms of their effect on plants, they show the same effectiveness as solid nitrogen fertilizers. Their production is cheaper than that of solid nitrogen fertilizers. Thus, the unit cost of nitrogen of liquid ammonia is about 35-40% cheaper than that of ammonium nitrate (the cheapest of solid nitrogen fertilizers). It is used on the largest scale in the United States.

The use of liquid ammonia fertilizers allows full mechanization of loading and unloading operations and their application. It requires 2-3 times less labour than solid nitrogen fertilisers. Liquid fertilizers are more evenly distributed in the soil, do not have caking and segregation (stratification).

The use of liquid fertilizers has several disadvantages: storage requires special large-capacity tanks, requires the organization of the distribution points, the use of special equipment for the application, the fleet of road and rail tank for transportation.

Liquid nitrogen fertilizer is made by special machines with immediate embedding to a depth of at least 10-12 cm on heavy soils and 14-18 cm – on light soils to avoid loss of ammonia. Losses are possible on the highly carbonate soils with alkaline reaction. Superficial application of liquid ammonia fertilizers is unacceptable. Shallow embedding in the dry topsoil is also associated with large losses of ammonia.

In all cases, anhydrous ammonia is embedded at a depth of at least 14-15 cm, aqueous solution – at least 10-12 cm. In the case of coarse lumpy soil, the depth of embedding is increased by 1.2-1.5 times. They are applied as a basic fertilizer under autumn plowing in autumn, in spring – under pre-sowing cultivation and top dressing of row crops in doses (nitrogen), as well as for solid nitrogen fertilizers. On light soils with low absorption capacity, applying high doses in the fall associated with a possible loss of ammonia, as it can not be completely adsorbed by the soil absorbing complex.

Since liquid ammonia fertilizers are made locally, the coulters of fertilizer machines for solid crops are set at 20-25 cm in meadows and pastures – 30-35 cm when feeding row crops is determined by the width of the row spacing. The technology of using liquid ammonia fertilizers requires higher qualification of specialists.

When fertilizing to avoid possible damage to young plants, excessive ammonia, fertilizers are applied in the middle of the row spacing or at a distance of 15-10 cm from the rows. For uniform distribution in the soil, carry out subsequent inter-row tillage. With nitrification, the formed nitrates become mobile and are transported with soil moisture to the root zone. The intensity of nitrification is determined by soil properties: in chernozem and cultivated sod-podzolic soils, it proceeds faster than in acidic podzolic soils. Synthetic aqueous ammonia undergoes nitrification faster than coke ammonia, as impurities contained in the latter inhibit the activity of nitrifying bacteria.

When liquid ammonia fertilizers are used properly, their effectiveness is not inferior to ammonium nitrate.

Liquid ammonia

Liquid ammonia, NH3 is the most concentrated ballast-free nitrogen fertilizer and contains 82.3% of nitrogen. It is produced by liquefying gaseous ammonia under pressure. It is a colorless liquid with a density of 0.61 kg/m3 at 20 °С. Freezing point is -77.7 °С, boiling point -33 °С. At normal temperature it quickly turns into gas. When stored in open vessels, ammonia quickly evaporates with strong cooling. Elasticity of liquid ammonia vapors:

Ammonia vapor pressure, Pa
192⋅103
293⋅103
424⋅103
616⋅103
859⋅103
116⋅104
178⋅104
Temperature, °C
-20
-10
0
10
20
30
40

To prevent volatilization of liquid ammonia, it is stored and transported in special steel tanks designed for pressure of 2.5-3.0 MPa. At 20-40° its vapor pressure is 9-18 atm. Elasticity of vapor, density and nitrogen content in 1 m3 depend on temperature. When ammonia is stored in closed vessels under pressure, it is separated into two phases: liquid and gaseous. Due to the high elasticity of vapors, storage and transportation tanks are not filled completely. Liquid ammonia corrodes copper, zinc and their alloys, does not react with iron, cast iron and steel.

Liquid ammonia is a highly toxic substance; mixture with air at volume concentration of ammonia 15-27% is explosive. An explosion may be caused by a spark and any open source of fire. Skin contact causes burns, frostbite when evaporated.

Liquid ammonia turns into gas in soil, adsorbed by soil colloids and absorbed by soil moisture. Well soluble in water: under normal conditions (at 20 °C and atmospheric pressure) in 1 volume of water dissolves 702 volumes of ammonia.

The rate and degree of ammonia adsorption by soil is determined by the absorption capacity and humidity, method and depth of application. On heavy soils with a high organic matter content and normal moisture content, the absorption is greater than on light, humus-poor soils. On light or dry soils, ammonia is retained in gaseous form for a long time, which leads to volatilization losses.

After liquid ammonia application the soil reaction shifts to an alkaline pH of 9 in the first days. In the zone of fertilizer application the soil is temporarily sterilized, which suspends the nitrification process of ammonia nitrogen. In 1-2 weeks the microbiological activity is restored. Under optimal conditions, complete ammonia nitrification occurs within a month.

In terms of payback on additional yield liquid ammonia is comparable to solid nitrogen fertilizers, on light soils, under irrigation or excessive moisture surpasses them.

Ammonia water

Ammonia aqueous solution, or ammonia water, NH3 + H2O. It is a clear liquid, sometimes with a yellowish tint. In aqueous ammonia solution there is always an equilibrium between ammonia absorbed by water and gaseous above the solution surface, which causes its loss when stored in open vessels.

Two grades of ammonia solution are produced: the first with 20.5% nitrogen, or 25% ammonia, and the second with 16.4% nitrogen, or 20% ammonia. Coke-oven aqueous solution contains impurities of hydrogen sulfide, phenols, rhodanides and cyanides.

Ammonia water has a small elasticity of ammonia vapor (25% solution – 0.15 kgf/cm2 at 40 ° C), does not corrode ferrous metals, freezes at temperatures: 25% – at -56 ° C, 20% – at -33 ° C). The density at 15 ° C of the first grade – 0.910 kg/m3, the second – 0.927 kg/m3.

It is stored and transported in leak-tight carbon steel tanks designed for pressure up to 0.4 kgf/cm2. Ammonia water corrodes non-ferrous metals (copper, zinc, tin) and their alloys (bronze, brass), therefore all technological units must be made of ferrous metals. It is inert towards aluminum and rubber.

When applied to soil, ammonia is adsorbed by soil colloids and therefore weakly migrates. Over time, ammonia nitrogen is nitrified, with increased mobility. The use of ammonia water is technically easier and safer than liquid ammonia. The intensity of ammonia absorption by soil is influenced by the granulometric composition, humus content, moisture, depth of embedding. On heavy, well-cultivated soils with high organic matter content, ammonia absorption is higher than on light, dry soils poor in humus, losses from volatilization on which are much greater.

The disadvantage of ammonia water is low nitrogen content, which leads to higher costs for transportation, storage and application. Therefore, its use is advisable in farms located near the sites of fertilizer production.

Amide fertilizers

Urea

Urea, or urea, CO(NH2)2, contains 46.7% nitrogen, one of the most concentrated solid nitrogen fertilizers. The nitrogen in urea is in the amide form of carbamic acid. It is obtained from ammonia and carbon dioxide at a pressure of 30.3⋅105 to 202⋅105 Pa and a temperature of 150-220 ° C. At the first stage of the process ammonium carbamate is formed:

2NH3 + CO2 → NH4COONH3,

followed by urea during its dehydration:

NH4COONH2 → CO(NH2)2 + H2O.

Urea is a white or yellowish crystalline substance, well soluble in water: at 20 ° C in 100 cm3 of water dissolves 51.8 g of urea. It is characterized by a relatively small hygroscopicity; at 20 °С it is close to ammonium sulfate in hygroscopicity, at higher temperatures it absorbs moisture more strongly. It can freeze during storage.

It is available in granular form with 1 – 3 mm granules. During granulation it can be covered with hydrophobic additives. Granulated urea has good physical and mechanical properties, practically does not caking, retains flowability and dispersibility.

In the process of granulation under the influence of elevated temperatures an impurity is formed – biuret:

2CO(NH2)2 → (CONH2)2HN + NH3.

When its content exceeds 3%, it becomes toxic to plants.

Biuret decomposes in the soil within 10-15 days. Therefore, applying urea with high content of biuret 1 month before sowing does not have a negative effect on plants. At present, granulated urea is produced with biuret content not exceeding 1%, which does not have a depressing effect on plants regardless of the application period.

In the soil, urea dissolves with soil moisture and under the influence of the enzyme urease of plant residues and microflora undergoes ammonification, turning into ammonium carbonate:

CO(NH2)2 + 2H2O = (NH4)2CO3.

Under favorable conditions in cultivated soils, the conversion occurs within 1-3 days.On poorly fertile sandy and overwatered soils the process takes up to 3 weeks. Urea dissolved in the soil solution before ammonification can be washed out.

The resulting ammonium carbonate is unstable, decomposes in the air to form ammonium hydrocarbonate and ammonia gas:

(NH4)2CO3 → NH4HCO3 + NH3.

Therefore, at surface application of urea without embedding and with insufficient moisture content, ammonia losses may occur. Losses are enhanced on soils with neutral and alkaline reactions. Ammonium carbonate undergoes hydrolysis with the formation of ammonium hydrogen carbonate, NH3 and water, which leads to an alkaline environment:

(NH4)2CO3 + H2O = NH4HCO3 + NH3 + H2O.

Over time, the ammonium is nitrified and the soil reaction shifts to the acidic side. As plants absorb nitrogen, no alkaline and acidic residues of fertilizer remain in the soil, and the reaction of the medium is restored.

Urea is used as the main fertilizer on all types of soils for any crop. In rainfed conditions its effectiveness is equal to ammonium nitrate, in irrigated conditions – to ammonium sulfate. Under conditions of leaching water regime of the soil urea is more effective than ammonium nitrate due to the fact that the amide nitrogen quickly turning into ammonia nitrogen is absorbed by the soil without washout from the root layer.

Urea is used for fertilizing winter crops in early spring with immediate incorporation by harrowing. According to experiments, embedding urea even 1.5 cm dramatically reduces ammonia losses. Urea is used for top dressing of row crops and vegetable crops by plant feeder cultivators. However, in hayfields and pastures surface introduction of urea shows the effectiveness by 15-20% lower than ammonium nitrate due to the significant losses of ammonia from ammonification of urea.

Urea is the best form for foliar fertilizing of plants, especially wheat, especially to increase grain protein content, due to the fact that even in higher concentrations (1% solution) it does not lead to leaf burns and is well absorbed by plants. Urea is absorbed by leaf cells as a whole molecule and assimilated by plants both in the form of ammonia after ammonification, and by direct involvement in the cycle of nitrogen transformations. For foliar dressing it is desirable to use the crystalline form, because its content of biuret is lower than 0.2 – 0.3%.

The use of urea as a pre-sowing fertilizer (in rows) may lead to retardation of seed germination due to the inhibiting effect of excess free ammonia.

Because of its high nitrogen content, it is essential to apply urea evenly to the soil. For uniform spreading, it is thoroughly mixed with other fertilizers immediately before application.

In the global range of nitrogen fertilizers, the proportion of urea use is increasing steadily. Urea production technologies are continuously improved to produce higher quality urea at lower production costs.

Urea is used in production of complex and slow acting nitrogen fertilisers. Due to higher cost effectiveness of urea and other high-nitrogen fertilizers low-nitrogen fertilizers are losing their importance in the nitrogen fertilizer consumption balance.

Calcium cyanamide

Calcium cyanamide, CaCN2 contains 20-21% nitrogen. It is a light powder of black or dark gray color, dusty when dispersed, can lead to inflammation when in contact with eyes and respiratory tract. It is a physiologically alkaline fertilizer as it contains up to 20-28% CaO. Factory technical cyanamide contains impurities of carbon – 9-12%, silicic acid, iron and aluminum oxides.

Its systematic use on acidic soils leads to improvement of their physicochemical properties through neutralizing acidity and enrichment with calcium. It is applied 7-10 days before sowing or in autumn under the winter tillage. It is not recommended for top dressing as calcium cyanamide undergoes hydrolysis in soil and interacts with absorbing complex to form cyanamide H2CN2, which is toxic for plants.

It is almost never used as a fertilizer; it is more commonly used for preharvest removal of cotton and sunflower leaves during harvesting for seed.

Mixed nitrogen fertilizers

Ammonia complexes

Ammonium complexes (ammoniacs) are nitrogen fertilizers that are aqueous solutions of ammonia and ammonium nitrate, ammonium and calcium nitrate, urea or ammonium nitrate and urea. They contain from 30 to 50% nitrogen. Ammonium nitrates are produced in special mixers by introducing a hot solution of ammonium nitrate (urea or calcium nitrate) into a 10-15% ammonia solution.

Ammonia complexes are liquids of light yellow color; depending on the composition, ammonia vapour elasticity at 32 °C is from 0.2 to 3.6 atm. According to vapour elasticity, ammonia compounds are divided into two groups:

  • with moderate vapor elasticity – 0.2-0.7 atm, with 35-40% of nitrogen;
  • with higher vapour elasticity – 0.7-3.6 atm, with 40-50% of nitrogen.

Ammonia complexes differ in crystallization start temperature: from 14 to 70 °С. Ammonia is produced with low crystallization temperature in winter and with higher one in summer.

Ammonia complexes can corrode copper and ferrous metals alloys, that is why tanks and equipment are made of alloyed steel, aluminum and its alloys, or steel tanks with protective anti-corrosion coating (epoxy resins) are used, as well as tanks of polymeric materials. Nitrogen is transported and stored in special, sealed tanks designed for low pressure. 20-40% of nitrogen in ammonia complexes are in the form of ammonia and 60-80% in the form of ammonium salt or urea.

The application of ammonia complexes requires the same application conditions as ammonia liquid fertilizers, that is, compliance with the depth of embedding, depending on the granulometric composition. In soil, the diffusion of ammonia is usually not more than 8-10 cm, so the distance between the coulters when applying the ammonia complexes should be no more than 20-25 cm. When making ammonia complexes as fertilizer for row crops the distance between the coulters is set equal to the width of the row spacing.

By the effect on crops, ammonia complexes are equal to the solid nitrogen fertilizers. In Russia, the most widely used carbon-ammonium complexes – ammonia solutions of ammonium carbonate and hydrocarbonate and urea, containing 4-7% ammonia and 18-35% of total nitrogen.

Urea-ammonium nitrate

Urea-ammonium nitrate (UAN) is a nitrogen fertilizer, which is an aqueous solution of urea and ammonium nitrate.

UAN with a nitrogen content of 28-32% has several advantages over solid and liquid nitrogen fertilizers: they do not contain free ammonia, so they are more technologically advanced and easy to use; they can be stored in open tanks without nitrogen losses. Urea and ammonium nitrate in solutions produce a mutual dissolution effect, which allows obtaining more concentrated fertilizers without the risk of crystallization. UAN solutions are transparent or yellowish liquids with a density of 1.26-1.33 g/cm3 and a neutral or slightly alkaline reaction.

UAN solutions are prepared in industrial conditions from unevaporated melts of urea and ammonium nitrate. By eliminating the stages of evaporation, pelletizing, conditioning and packaging, the cost of their production is reduced.

Changing the ratio of urea and ammonium nitrate allows to regulate the crystallization temperature (salting-out), which allows to use them in different regions, terms and seasons.

UAN solution grades are selected taking into account storage and use temperatures to prevent crystallization. 

Table. Composition and properties of solutions of different brands of UAN[1] Yagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.

Composition and properties of solutions
UAN-28
UAN-30
UAN-32
Composition by weight, %:
NH4NO3
40,1
42,2
43,3
CO(NH2)2
30,0
32,7
36,4
H2O
29,9
25,1
20,3
Density at 15.6 °C, t/m3
1,28
1,30
1,33
Temperature of crystal precipitation, °C
-18
-10
-2

UAN solutions can be used as a basis for complex fertilizers with macro- and microelements, such as salts of cobalt, boron, copper, molybdenum, herbicides, retardants.

UAN solutions are transported by carbon steel railroad tanks or by tank-cars using corrosion inhibitors. UAN-32 contains 1.3 times more nitrogen than granulated urea at equal volume, and 1.5 times more than ammonium nitrate, which reduces transportation and storage costs. For on-farm transportation and application of UAN to soil, the same technique can be used as for other liquid fertilizers, ammonia water or herbicides.

UAN solutions are used as the main fertilizer and top dressing. For the main application, you can use the application directly into the soil or superficially, followed by embedding. They can be used for root fertilization of row crops and for foliar fertilization of cereals. UAN solutions can be applied together with irrigation water by sprinkler systems.

Slow-acting nitrogen fertilizers

The production of slow-acting fertilizers is developing in different directions, for example:

  • obtaining compounds with limited solubility in water (ureaforms);
  • coating fertilizer particles with substances that slow down dissolution (wax, paraffin, oils, resins, polymers);
  • introduction of nitrification inhibitors into the fertilizer.

Advantages of slow-acting fertilizers:

  • reduced nutrient losses from application to plant uptake;
  • higher coefficient of fertilizer use;
  • reducing the negative impact on the environment;
  • higher product quality due to lower nitrate content;
  • reducing labor costs by replacing fractional application with a single application;
  • preservation of fertilizer quality during storage and transportation.

The largest producers of slow-acting fertilizers are the USA and Japan.

Aldehydes are used to produce slow acting fertilizers: formaldehyde, acetaldehyde, croton and isobutyric aldehyde etc. This produces, respectively: urea-formaldehyde fertilizer, or ureaform, with a nitrogen content of 38-40%, 28-32% of which is insoluble in water, crotonylidene-diurea with a nitrogen content of about 32%, isobutylene-diurea, with 31% of low-soluble nitrogen, urea-form-acetaldehyde.

The use of low-soluble forms of nitrogen fertilizers is promising in conditions of excessive moisture and irrigation, as well as when applied to vegetable crops, grasses, grasses on sports grounds and lawns, for which nitrogen is applied in large doses and in several steps.

In normal doses in the first year after application, these fertilizers are less effective than urea. However, at high doses, they do not create an excessively high concentration, the nitrogen is almost not washed out, less subjected to denitrification, as it decomposes over a long period is used by plants. Slow-acting nitrogen fertilizers can be made in high doses once every 2-3 years without fear of nitrogen loss.

The disadvantages of slow-acting fertilizers are high cost compared with traditional nitrogen fertilizers, the rate of nitrogen release does not always correspond to the rate of absorption by most crops during the growing season, which causes less efficiency compared to urea in the first year after application.

Encapsulated nitrogen fertilizers

The development of encapsulated nitrogen fertilizers is promising. Encapsulated fertilizers are the usual water-soluble forms, but their granules are covered with films that slow down dissolution. Encapsulated fertilizers have good physical and mechanical properties: less hygroscopic, granules are more durable, do not caking. When released into the soil from granules, nitrogen is gradually released and assimilated by plants as the capsules break down. Depending on the composition and thickness of the capsules it is possible to adjust the dissolution rate of fertilizer in accordance with the biological needs of crops and the frequency of feeding.

Paraffin, polyethylene emulsion, sulfur compounds, acrylic resin, polyacrylic acid are used for encapsulation.

Experiments show that the use of encapsulated nitrogen fertilizers is promising for rice, meadows and long-term pastures, vegetable crops, especially in areas with excessive moisture and under irrigation. In cereal crops the advantages of encapsulated fertilizers over conventional fertilizers are practically absent. The main disadvantage is the high cost, so these fertilizers are used in agriculture to a limited extent.

Nitrification inhibiting fertilizers

Among nitrification inhibitors, cianguanidine (dicyandiamide), the American drug N-serve, or nitripyrine (2-chloro-6-trichloromethyl)pyridine or the Japanese drug AM (2-amino-4-chloro-6-methylpyrimidine) are used. In Russia, the inhibitors picochlor and jacos, which are nitripyrin derivatives, are produced. The application of these inhibitors in mixtures with solid or liquid ammonia fertilizers in the doses of N-serve 0.5-1%, AM 1-3% of the nitrogen content inhibits the nitrification processes up to 1.5-2 months, that is, for the period of intense nitrogen consumption by plants.

The rate of decomposition of inhibitors in the soil and, accordingly, the duration of their action is affected by the granulometric composition of the soil, moisture, environmental reaction, temperature, and humus content.

Inhibitors, by slowing down nitrification, reduce nitrogen losses in gaseous form, flushing with surface runoff and leaching. This leads to increased yields, primarily of cotton, rice, vegetable crops, corn for grain and silage, row crops and fodder crops grown under irrigation or excessive moisture.

The use of inhibitors positively affects product quality because it prevents accumulation of nitrates and reduces the incidence of some diseases. Due to the increased coefficient of nitrogen use, doses of nitrogen fertilizers are reduced, and fractional application is replaced by applying the whole dose in one go.

Table. Effect of nitrification inhibitor N-Serve on the efficiency of nitrogen fertilizers and nitrate accumulation in green mass of winter rape (All-Russian Institute of Fertilizers and Agrochemistry)

Experience option
Мочевина
Сульфат аммонния
Yield, t/ha
Increase, t/ha
Content N-NO3 in rape, %
Yield, t/ha
Increase, t/ha
Content N-NO3 in rape, %
from nitrogen
from inhibitor
from nitrogen
from inhibitor
Without nitrogen
26,2
-
-
0,017
26,2
-
-
0,017
N45
37,0
10,8
-
0,026
38,4
12,2
-
0,026
N45 + inhibitor
38,4
12,2
1,4
0,027
40,0
13,8
1,6
0,028
N90
45,9
19,7
-
0,103
47,3
21,1
-
0,105
N90 + inhibitor
48,0
21,8
2,1
0,073
50,0
23,8
2,7
0,082
N135
53,0
26,8
-
0,226
53,8
27,6
-
0,243
N135 + inhibitor
58,2
32,0
5,2
0,156
59,2
33,0
5,4
0,165

Urea-formaldehyde fertilizer (UFF)

Urea-formaldehyde fertilizers (UFF), or ureaform, ureaform, are products of the chemical condensation of urea CO(NH2)2 and formaldehyde (CH2O). Condensation takes place in concentrated solutions at equivalent ratios of urea and formaldehyde in an acidified medium to pH 3, at a temperature of 30-60°. This produces monomethylurea CONHCH2NH2OH, which reacts again with urea and converts to methylenediurea NH2CONHCH2NHCONH2 with evolution of water. The resulting condensate is filtered off, dried, crushed and, if necessary, pelleted. The reaction product is usually a white, crumbly powder that does not cake and retains its flowability even at high humidity.

The nitrogen content of UFF is 38-40%, the water-soluble part accounts for 8-10%, the insoluble part remains available to plants.

One of the main indicators of UFF is the index of assimilation – the amount of nitrogen insoluble in water, which dissolves after boiling for 1 hour. It is expressed as a percentage of water-soluble nitrogen. The digestibility index depends on the reaction, temperature, molar ratio of urea to formaldehyde, and duration of condensation. It varies from 15 to 55%.

In some foreign countries, the index of assimilation is conventionally taken equal to the amount of nitrogen, which is nitrified for 6 months of location of fertilizer in the soil. The degree of nitrification of MFP – an indicator of their effectiveness, depending on the assimilability index and soil properties. UFF with a high digestibility index corresponds to a greater and more rapid accumulation of nitrate nitrogen in the soil.

Acidic soil reaction reduces the rate of conversion of UFF, so liming increases the rate of nitrification. High doses of UFFs alkalize the soil, with gradual acidification occurring as they mineralize.

Under certain conditions of the reaction of condensation, for example, at a temperature of 30-40 °, get UFFs with a high content of available nitrogen for plants, approaching the soluble nitrogen fertilizers. In this case, they lose their purpose as a slow-acting fertilizer.

Urea-formaldehyde fertilizers production is promising because all nitrogen fertilizers are well soluble, but applying them in large doses creates a high concentration and osmotic pressure of the soil solution, which adversely affects plants in the initial stages of growth, especially crops sensitive to high concentrations of salts, such as corn and flax. In addition, in areas of sufficient moisture, especially on light soils, and with irrigation, nitrogen losses from leaching are possible.

On sod-podzolic soils with different degrees of cultivation and different links of field crop rotations the advantages of UFF over soluble nitrogen fertilizers in terms of yield and product quality were not revealed. On heavy sod-podzolic soils the effectiveness of MFP on the yield of green mass of maize was lower.

Fertilizer nitrogen use coefficients

Effective use of nitrogen fertilizers is possible only when taking into account their properties and peculiarities of nitrogen transformation in soils. All nitrogen fertilizers, with the exception of slow-acting forms, are well soluble in water. Nitrate fertilizers migrate in the soil with soil moisture and, except for biological, no type of absorption. Biological absorption occurs only in warm seasons. Therefore, nitrates in the conditions of flushing water regime of soil can be leached, especially on light soils. At higher doses on soils of light granulometric composition in fallow fields under conditions of excessive moisture or irrigation losses of nitrate nitrogen can reach 10-25% of the applied.

Ammonium and ammonium forms when entering the soil are absorbed by the soil absorbing complex. In this form, they lose their mobility, but remain available to plants, are not washed out, except for light soils with low absorption capacity. Under favorable conditions, as a result of nitrification, they are transformed into nitrates, acquiring their properties. Similarly, urea behaves in the soil after it is converted into ammonium forms as a result of the activity of urobacteria.

All nitrogen fertilizers initially or during nitrification accumulate in the soil as nitrates. Nitrates are subject to denitrification processes, which are characteristic of almost all soils, and the main losses of nitrogen are associated with them. According to experiments, nitrogen losses from denitrification for ammonium and amide forms are about 20%, for nitrate – up to 30% of the applied amount. In pure steam and with increasing doses nitrogen losses increase up to 50%.

From an ecological point of view, denitrification has a positive value, as it “frees” the soil from the excess of unused nitrates, preventing their penetration into groundwater and water bodies.

In the soil, part of the nitrogen of fertilizers as a result of microorganisms’ activity is transformed into organic forms that are inaccessible to plants. As a result of immobilization approximately 10-12% of nitrate nitrogen and 30-40% of ammonium, ammonia and amide fertilizers are fixed in organic form. The intensity of immobilization increases with the introduction of organic fertilizers with low nitrogen content and high carbon (stubble, straw, straw manure).

Previously, it was assumed that plants in the first year of nitrogen fertilizer application use 60-70% of the nitrogen received. These data were obtained in field experiments using the difference method by comparing nitrogen export in control variants (without fertilizer) and in variants with fertilizer. Later research using labeled nitrogen atoms showed that under field conditions plants absorb 30-50% of nitrogen from fertilizers, but at the same time the use of soil nitrogen by plants increased by 20-30% in the fertilized variants. As a result, total nitrogen removal in fertilized variants increases by 20-30%, and as a result nitrogen use coefficients calculated by the difference method are by 20-30% overstated from the actual ones.

Nevertheless, for practical purposes, such as the calculation of nitrogen balance and doses of nitrogen fertilizers, the nitrogen use coefficient obtained by the difference method is used, because it characterizes the total nitrogen consumption by plants. Balance calculations made in multi-year experiments, including several crop rotation rotations, confirm these conclusions. The coefficients of the use of fertilizer nitrogen by the balance method are 60-70%.

Most of the fertilizer nitrogen applied to the soil is spent during the growing season for plant consumption, immobilization, denitrification, leaching and erosion. Therefore, the effect of nitrogen fertilizers is not taken into account.

Nitrogen fertilizer efficiency

The efficiency of nitrogen fertilizers is associated with an increase in the use of nitrogen by plants and with a decrease in irretrievable losses. The general solution is to optimize the conditions and regimes of nitrogen nutrition, improve the level of agrotechnics and reclamation measures.

Optimization of plant nitrogen nutrition conditions includes:

  1. Application of optimal doses and forms of nitrogen fertilizers, taking into account biological characteristics of crops and properties of fertilizers, soil and climatic conditions, the results of plant nutrition diagnostics.

The basis for determining the optimal doses of nitrogen fertilizers based on the results of field experiments carried out in different soil and climatic zones. These data are compared with the results of soil and plant diagnostics, which are the basis for recommendations for the use of fertilizers, methods and ways of adjusting doses depending on specific conditions.

Methods to optimize nitrogen fertilizer doses by the content of mineral nitrogen in the soil are widespread. Effective doses for different crops depend on soil and climatic conditions.

Table. Winter wheat grain yield and efficiency of nitrogen fertilizers depending on the content of mineral nitrogen in the soil (according to the All-Russian Institute of Fertilizers and Agrochemistry)

Group of soil nitrogen content
Nmin content in soil layer 0-40 cm, kg/ha
Average yield, t/ha (control)
Yield increase (t/ha) with nitrogen application at a dose, kg/ha
0+45
30
30+45
45
45+45
60
60+45
90
90+45
120
120+45
НСР0,95, t/ha
I
0-60
2,38
0,86
0,93
1,33
0,93
1,04
1,15
1,47
1,13
1,37
1,06
1,18
0,18
II
60-80
2,94
0,72
0,76
1,32
1,02
1,39
1,10
1,28
1,13
1,26
1,26
1,26
0,11
III
80-100
4,06
0,40
0,27
0,57
0,44
0,58
0,42
0,47
0,52
0,52
0,38
0,36
0,12
IV
100-130
4,32
-0,02
-0,06
0,05
0,11
0,06
-0,02
-0,02
-0,07
-0,08
-2,3
-2,7
0,22
V
> 130
5,00
-0,18
-0,12
-0,36
-0,08
-0,19
-0,35
-0,45
-0,45
-0,47
-0,41
-0,49
0,25

Depending on the provision of soils with mineral nitrogen they are divided into 5 groups: from very low – group I, to high – group V. Provision of soils with mineral nitrogen affects the effectiveness of nitrogen fertilizers: very high in soils with a nitrogen content of less than 80 kg/ha, low with a content of over 130 kg / ha. The largest increases in grain yield were observed when fertilizers were applied in two applications: the first application in spring, the second – in the phase of the emergence of the tube.

For areas of Western Siberia in Russia, depending on the content in the soil layer 0-40 cm developed a scale of grain crops need for nitrogen fertilizers.

Table. Nitrogen fertilizer demand of grain crops depending on N-NO3 content in the soil layer 0-40 cm in autumn or spring (by Kochergin)

N-NO3
Soil nitrogen supply to plants
Nitrogen fertilizer requirements
Approximate doses of nitrogen fertilizers, kg/ha a.s.
mg/kg of soil
kg/ha
At low to medium plant phosphorus levels (up to 100 mg P2O5 per 1 kg of soil, according to Franzesson)
0-5
0-25
Very low
Very strong
60
5-10
25-50
Низкая
Strong
45
10-15
50-75
Medium
Medium
30
> 15
> 75
High
No need
0
If the plant has a high phosphorus supply (150-200 mg P2O5 per 1 kg of soil, according to Franzesson)
0-10
0-50
Very low
Very strong
80
10-15
50-75
Низкая
Strong
60
15-20
75-100
Medium
Medium
45
> 20
> 100
High
No need
0

The same methods with changes and modifications are used in other regions of the country.

For optimization of nitrogen fertilizer doses, balance-calculation methods are also used, which are based on nitrogen removal with the planned yield.

  1. Maximum shift in the timing of fertilizer application to the period of intensive consumption of nitrogen by plants, fractional application of the total dose of fertilizer in several steps.

Table. Nitrogen use of ammonium sulfate depending on the timing of its application (according to the All-Russian Institute of Fertilizers and Agrochemistry)

Experience option
Quantity of nitrogen, % of applied nitrogen
under barley
under millet
used
fixed
losses
used
fixed
losses
РК + (15NH4)2SO4 before sowing
58,2
22,4
19,4
54,8
28,9
16,3
РК + (15NH4)2SO4 piecemeal
68,2
16,9
14,9
63,2
25,4
10,4

Closer timing of spring nitrogen dressing of winter wheat and perennial grasses to the beginning of intensive nitrogen consumption significantly increases the effectiveness of dressing. The period of active nitrogen consumption after overwintering occurs 15-20 days after snow melting, i.e. after soil warming. Before that time nitrogen fertilizers are not absorbed in significant quantities and can be washed out and subjected to denitrification. For example, in experiments, VIUA on average for 3 years yield increase of winter wheat grain in 3 times lower when making nitrogen fertilizers on the snow layer 5-7 cm than when making 10-15 days after snowfall.

Experiments were carried out on light soils with a granulometric composition in Yegoryevsk district, Moscow region, and showed a sharp increase in the effectiveness of nitrogen fertilizers in meadows when making the application at the beginning of active grass growth. The application of ammonium nitrate immediately after snowfall on a dry hayfield with temporary excessive moisture increased hay yield by 2 times. Fertilization in the same doses in 20-30 days after the snow melt and drainage of excess moisture increased hay yield 4 times. At the same time collection of protein from 1 hectare increased by 2 times, the coefficient of utilization of nitrogen fertilizer increased by 4 times.

Table. Effect of ammonium nitrate on hay yield (dryland haying of temporary excess moisture) depending on the timing of its application (All-Russian Institute of Fertilizers and Agrochemistry, average for 3 years)

Experience option
Hay yield, t/ha
Increase from nitrogen fertilizer, t/ha
Payment of 1 kg of nitrogen by hay, kg
Protein yield, kg/ha
Nitrogen utilization factor, %
P60K60
1,34
-
-
116,1
-
N90P60K90 after the snow melts
2,49
1,15
12,7
231,1
18,3
N90P60K90 20-30 days after the snow melts
5,89
4,56
50,5
591,5
77,7
  1. Application of slow-acting and encapsulated forms of nitrogen fertilizers with controlled rate of nitrogen release.
  2. Application of nitrification inhibitors, resulting in a 10-15% increase in fertilizer nitrogen utilization factor.

Unproductive nitrogen losses can be reduced by introducing intermediate and stubble crops.

The introduction of 1 kg of nitrogen mineral fertilizers yields 8-15 kg of grain, 50-70 kg of potatoes, 20-30 kg of meadow grass hay, 30-40 kg of sugar beet root crops, about 3 kg of flax fiber.

In the zonal aspect the efficiency of nitrogen fertilizers depends on moisture conditions and the level of natural soil fertility. There are high efficiency and sustained effect of nitrogen fertilizers in the Non-Black Earth zone on humus-poor sod-podzolic, gray forest soils, podzolic and leached chernozems, and the more leached, the higher the efficiency. High efficiency is shown on light soils characterized by constant nitrogen deficiency.

Table. Effects of nitrogen fertilizers on winter wheat by natural-agricultural zones (CINAO)

Natural-agricultural zone
Number of experiments
Mineral fertilizer dose, kg/ha a.s.
Yield, t/ha
Grain yield increase (t/ha) from doses of nitrogen fertilizers, kg/ha
Precipitation per year, mm (liters per 1 m2)
of phosphate
of potassium
without fertilizer
on a background of РК
30
60
90
120
Southern taiga-forest
114
61
53
1,98
2,30
0,37
0,62
0,84
1,06
500-800
Forest-steppe
306
58
43
2,62
2,92
0,25
0,36
0,43
0,48
400-600
Steppe
259
60
37
2,78
3,14
0,18
0,25
0,31
0,36
350-500
Dry-steppe
35
70
41
3,01
3,48
0,14
0,20
0,25
0,29
250-350
Mountainous areas
47
64
59
1,83
2,09
0,32
0,54
0,74
0,93
300-600
On average in Russia
-
60
43
2,55
2,88
0,24
0,36
0,46
0,54
-

On drained peat-bog soils, their effect is reduced, as potassium and phosphorus are in the first minimum. However, in the first years of peatland development in the central and northwestern areas of the Non-Black Soil Zone, nitrogen efficiency increases.

Moving from north to south and from west to east within the European part of Russia, the continentality of the climate increases, and the amount of precipitation decreases, which affects the efficiency of nitrogen fertilizers. The provision of soils with nitrogen also changes.

The efficiency increases in the series sod-podzolic soils → gray forest → black earth. Decrease is observed in steppe areas due to decrease of moisture supply and more favorable conditions of nitrogen nutrition.

Nitrogen fertilizers are also effective in the eastern regions of the country: in the forest-steppe of the Trans-Urals and Eastern Siberia it is higher, in the forest-steppe of Western Siberia with a more continental climate – lower. In the Trans-Urals 1 kg of mineral fertilizer nitrogen gives an increase in grain yield of spring wheat – 10 kg/ha, in Eastern Siberia – 11 kg/ha, in Western Siberia – 5 kg/ha.

In the steppe regions of the European part of Russia on heavy, ordinary and southern black soils due to high nitrogen content in the soil and moisture deficit, the effectiveness of nitrogen fertilizers decreases and becomes unstable. To an even greater extent it is noted on chestnut and light chestnut soils of arid regions of the south-east.

Measures for the accumulation, conservation of soil moisture, as well as under irrigation, the effectiveness of low doses of nitrogen fertilizers in these areas is quite high, and it turns out more than phosphorus and potassium fertilizers.

Table. Increase in potato yield from mineral fertilizers on different soils (All-Russian Institute of Fertilizers and Agrochemistry)

Soils
Yield without fertilizer, t/ha
Increase from the application of mineral fertilizers, 100 kg/ha
complete fertilization
from nitrogen
from phosphorus
from potassium
Podzolic sandy loam
11,7
6,0
3,5
1,3
1,6
Podzolic loamy
15,4
6,9
3,9
1,8
2,8
Grey forests
15,9
7,3
4,3
1,0
0,9
Leached black soils
20,3
5,6
3,1
2,0
1,3

Importance of nitrogen fertilizers

Nitrogen fertilizers increase the yield and quality of agricultural products: the protein and gluten content of cereal grains increases, which improves the baking quality of flour.

Assessment of the quality of wheat grain is carried out taking into account a number of indicators. For example, soft wheat is considered to be strong if it has a raw protein content of at least 14%, gluten content of at least 28%, gluten quality not lower than the first group, flour strength of 200-300 conventional units, volume yield of bread over 500 cm3/100 g of flour.

Late fertilization of crops with nitrogen fertilizers has little effect on grain yield, but it increases the protein and gluten content and improves the technological quality.

Table. Effect of mineral fertilizers on the yield and grain quality of winter wheat variety Mironovskaya 808 (Central Experimental Station of the All-Russian Institute of Fertilizers and Agrochemistry)

Experience option
Yield, t/ha
Weight of 1000 grains, g
Raw protein, %
Gluten, %
Flour strength, conditional units
Flour swellability, ml
Bread volume, cm3/100 g of flour
Without fertilizers
2,85
42,3
10,6
23,1
176
34
576
P90K90
2,91
43,1
11,1
23,7
191
34
530
N90P90K90
3,69
41,3
12,0
25,4
205
36
595
N180P90K90
3,65
39,9
12,8
28,8
213
44
658
N180P120K90
3,82
39,6
12,8
30,0
200
46
626
N90P120K90 + N90 in the spring
3,81
39,5
13,1
32,4
206
50
641
N90P120K90 + N90 in the spring + N60 at the flowering time
3,83
40,0
14,3
34,4
260
56
723

Balanced use of nitrogen fertilizers increases the content of vitamins, ascorbic acid, carotene, thiamine, riboflavin and myosin. Nitrate forms contribute to the accumulation of ascorbic acid in plants more than ammonium forms.

Nitrogen fertilizers affect the quality of sugar beet. Thus, the introduction of N60 increases the sugar content of root crops by 0.2-0.4%, the dose of N120, by contrast, reduces by 0.1-0.2%. Doses of nitrogen fertilizer above the optimum increases the content of “harmful” nitrogen in root crops.

Excess nitrogen in the soil at high doses of fertilizers leads to an accumulation of nitrates and nitrites in plants.

Ways to increase the efficiency of nitrogen fertilizers

The effectiveness of nitrogen fertilizers depends on: 

  • zonal characteristics;
  • complex of agronomic and reclamation measures used in crop rotation and for specific crops;
  • technology of nitrogen fertilizers application (timing and methods of application, doses, forms);
  • methods of plant nutrition diagnostics.

A set of agrochemical methods has been developed in Russia to increase the efficiency of nitrogen fertilizers:

  1. Compliance with the agronomic technology of application of nitrogen fertilizers, taking into account the doses, forms, timing and methods of application.
  2. Optimal ratio of nitrogen and other macro- and microelements, taking into account soil fertility and biological characteristics of the crop.
  3. Optimization of nitrogen nutrition of crops during the growing season.
  4. Take into account the direct effect of fertilizers as a source of nutrition, and indirect effects associated with the mobilization of nitrogen due to the activation of mineralization processes of soil organic matter. This is important, as the amount of nitrogen formed from mineralization is the most difficult to take into account by existing methods. Nitrogen from mineralization of organic matter can create an excess of nitrogen in the soil, which leads to lodging of crops, deterioration of product quality, a negative impact on groundwater.
  5. Application of nitrification inhibitors. Despite the fact that this technique is temporary, nitrification inhibition can help to reduce nitrogen losses at the stage of developing comprehensive measures to improve efficiency.
    Improvement of slow-acting forms of nitrogen fertilizers: technology for the production of urea-formaldehyde forms (MFU) and encapsulated nitrogen fertilizers, as well as the search for new forms of mineral fertilizers with a gradual transfer of fertilizer nutrients into the soil.
  6. The use of lime in the rotation in the systematic application of nitrogen fertilizers on acidic soils, especially sod-podzolic soils.
  7. Application of agrotechnical methods aimed at regulating nitrogen mobilization and immobilization processes and humification processes.

Soil nitrogen mineralization when applying nitrogen fertilizers is influenced by:

  • the degree of cultivation of sod-podzolic soils: easily hydrolyzed humus compounds prevail in cultivated soils;
  • activity of soil microorganisms;
  • Increased absorptive capacity of the root system of plants;
  • form of nitrogen fertilizer, e.g. ammonium forms promote better nitrogen assimilation than nitrate forms;
  • liming;
  • application of organic fertilizers, which increase the amount of microflora.

Nitrate and ammonium forms of nitrogen can be immobilized by interaction with soil organic matter, fixation by soil microorganisms, and fixation by clay minerals of ammonium forms. Immobilization absorbs 20-60% of the applied nitrogen, its value depends on:

  • forms of nitrogen fertilizers: amide and ammonium forms are fixed 1.5-2 times more than nitrate forms;
  • fertilizer dosage: with the increase of dosage the absolute amount of immobilized nitrogen increases while the relative amount – the share of applied nitrogen – decreases;
  • the amount of fixed nitrogen: in soils with high humus content the nitrogen content is higher than in low humus soils;
  • content of energy material, which simultaneously with mineral fertilizers increases immobilization of fertilizer nitrogen;
  • C : N ratio in the soil: the more carbon and less nitrogen, the more is immobilized.

Zonal features

In areas with high efficiency of nitrogen fertilizers, per 1 ton of applied nitrogen you get an additional 10-15 tons of grain, 30-40 tons – root crops of sugar beet, 5-6 tons – raw cotton, about 2 tons – flax fiber, 20-30 tons of meadow grass hay. 

The effect of nitrogen fertilizers can manifest itself differently within large agricultural regions of Russia. For example, in the Nonchernozem zone from the introduction of 1 kg of nitrogen in optimum doses allows for additional 8-15 kg of grain, 50-70 kg – potatoes, 3.5 kg – flax fiber, 70-100 kg of silage corn. The high effect of nitrogen fertilizers in this zone is shown on sandy loam and sandy soils, where nitrogen deficiency is almost always observed. Under conditions of washed water regime due to nitrogen losses in the autumn-winter-spring period, application in spring is much more effective than in autumn.

On the podzolized and leached chernozem forest-steppe zone of Ukraine return on nitrogen fertilizer is more than in the right-bank forest-steppe, but less than the left-bank.

In different regions of the steppe zone, for example, in Moldova, large yield increases from the application of nitrogen fertilizers are obtained on typical black soils, smaller increases – on ordinary and carbonate soils. On 1 kg of nitrogen fertilizer you get an additional 6 kg of grain of winter wheat, up to 7 kg of corn, 2.5-3 kg of sunflower seeds, 40-60 kg of root crops of sugar beets.

In the steppe regions of Ukraine on ordinary chernozems nitrogen fertilizers are effective on crops of winter wheat, sugar beet and corn. Their effect weakens from west to east. In the steppe of the European part of Russia a positive effect is noted on ordinary and carbonate chernozems of Kuban, in the foothills of the North Caucasus, on North-Azov chernozems. On carbonate chernozems of Rostov region and ordinary chernozems of the Volga region the effect of nitrogen fertilizers is reduced.

The effect of nitrogen fertilizers on chestnut soils with low humus content is shown in regions with good moisture, such as Ukraine, Transcaucasia, the mountainous regions of the Northern Caucasus. In conditions of severe arid climate of Stavropol, Rostov region, the Volga region, Northern Kazakhstan the effect of nitrogen fertilizers on chestnut soils is often weak. The same is typical for ordinary and southern chernozems and chestnut soils of the Asian flat part of Russia.

Influence of agromeliorative measures on nitrogen fertilizer efficiency

Efficiency of nitrogen fertilizers is connected with timely and quality agronomic, reclamation and soil-protection measures, with high culture of agriculture: absence of weed vegetation, provision of optimal soil regimes, sowing of highly productive varieties and hybrids, comprehensive system of plant protection. All measures for reproduction of fertility and cultivation contribute to increase of efficiency of nitrogen fertilizers and greater payback. The balance of soil organic matter should be positive or deficit-free.

Organic fertilizers reduce the negative effects of high doses of mineral nitrogen and contribute to its more efficient use. Mineral nitrogen should be in the soil in an optimal ratio with other nutrients, corresponding to the requirements of the cultivated crop.

Liming of acidic soils significantly increases the effectiveness of nitrogen fertilizers by better use of nitrogen, increasing its mobilization, improved phosphorus nutrition. In the arid steppe and dry-steppe areas the positive effect of nitrogen fertilizer is possible under the conditions of optimal doses and regimes of irrigation.
Under conditions of risk of erosion processes efficiency of nitrogen fertilizers depends on soil-protecting complex and anti-erosion soil treatment system, such as contour-meliorative plowing across the slope, combined plowing, slotting slopes, creating shafts, furrowing of plowed autumn soil, which reduce water runoff and soil washout.

Doses of nitrogen fertilizers

In the future, the relevance of nitrogen in agriculture and its share in the composition of mineral fertilizers will increase, which is associated with the lability and inability to be fixed in the soil. Increasing the content of other biogenic elements in the soil, its fertility and cultivation leads to the fact that nitrogen becomes a limiting factor in the size and quality of the crop. This trend is observed in a number of countries where high doses of phosphorus-potassium and other mineral fertilizers have been used for decades and a sufficient supply of these elements in the soil has been created.

The need of agriculture in nitrogen fertilizers, depending on the natural and economic zones of the country is projected according to the Geographic Network of field experiments, as well as scientific institutions. On the basis of these studies, the norms of nitrogen fertilizer inputs for crop yield increase have been developed. Modern computer technology allows you to summarize the data of field experiments, to carry out their analysis and to develop programs which allow to adjust the optimal nitrogen demand depending on the availability of fertilizers, the structure of cultivated areas, the planned yields, weather forecasts, etc.

The optimal solution for determination of optimal doses of nitrogen fertilizers for a specific crop is based on the data of field experiments held under local soil and climatic conditions, as well as on the results of agrochemical analysis of soils for the content of organic matter, easily hydrolyzable forms of organic nitrogen, nitrification ability of soil and the content of mineral nitrogen forms. 

Optimization of nitrogen fertilizer doses depending on the content of mineral (nitrate and ammonia) nitrogen in the soil (NMIN method) is widespread in the practice of world agriculture.

Effective doses of nitrogen for specific crops depend on zonal characteristics, so methods of nitrogen nutrition diagnostics have their zonal modifications. There are different approaches to determine nitrogen fertilizer doses by mineral nitrogen content in soil when applying NMIN method.

  1. It is allowed for plants to consume equal amounts of mineral nitrogen from soil and fertilizer. Knowing the plant’s nitrogen requirement for the planned yield and the mineral nitrogen content in the soil, the difference is compensated by applying nitrogen fertilizer.

This method does not take into account:

  • effects of organic and mineral fertilizers,
  • mobilization of additional “extra” nitrogen from activation of mineralization of soil organic matter,
  • the effect of the preceding crop rotation on the nitrogen regime of the soil,
    nitrification ability of the soil,
  • periodicity of nitrogen nutrition of plants,
  • depth of soil sampling for agrochemical analysis by crop and in the zonal aspect,
  • coefficient of soil nitrogen and fertilizer use depending on crop, soil properties, climatic conditions.

Therefore, this method requires refinement.

Table. Doses of nitrogen fertilizer required to obtain the planned yields of winter wheat depending on the provision of soils with assimilable nitrogen before sowing (0-60 cm layer) (Nikitishen, 1986)

Planned yield, t/ha
Average nitrogen removal with the crop, kg/ha
Quantity of nitrate and ammonium nitrogen, kg/ha
72-96
96-120
120-144
144-168
168-192
192-216
Typical black earth
4.0
96
45
20
-
-
-
-
4.5
112
75
50
25
-
-
-
5.0
128
100
75
55
30
-
-
5.5
144
125
100
80
55
> 30
-
6.0
160
155
130
105
80
60
30
6.5
176
180
155
130
105
85
50
Gray forest soil
4.0
111
50
25
-
-
-
-
4.5
129
75
50
30
20
-
-
5.0
147
100
75
55
40
25
20
5.5
164
125
100
80
65
50
40
6.0
182
150
125
105
90
75
65
6.5
200
175
150
130
115
100
90
  1. The second variant of the NMIN method consists in determining the indices of soil nitrate nitrogen availability and establishing the crop nitrogen requirement and dose. This method is well grounded scientifically and received wide practical application in diagnostics of crop nitrogen nutrition for Siberian regions. For example, a scale of nitrogen fertilizer requirements for grain crops was developed for the regions of Western Siberia.

    To calculate the doses of nitrogen fertilizers for the planned yield the formula is also used:

where R – nitrogen removal with the planned yield of the main and by-products, kg/ha; Nin – nitrate nitrogen in the soil layer 0-50 cm before sowing, kg/ha; Ncurrent – nitrogen of current nitrification during the growing season of the crop, kg/ha; n – coefficient of nitrate nitrogen use in the soil; K – coefficient of mineral fertilizer nitrogen use by plants. Conventionally accepted n = 0.8 and K = 0.6; coefficients are different for each zone.

Table. Nitrogen fertilizer demand of grain crops depending on N-NO3 content in the soil layer 0-40 cm in autumn or spring (by Kochergin)

N-NO3
Soil nitrogen supply to plants
Nitrogen fertilizer requirements
Approximate doses of nitrogen fertilizers, kg/ha a.s.
mg/kg of soil
kg/ha
At low to medium plant phosphorus levels (up to 100 mg P2O5 per 1 kg of soil, according to Franzesson)
0-5
0-25
Very low
Very strong
60
5-10
25-50
Низкая
Strong
45
10-15
50-75
Medium
Medium
30
> 15
> 75
High
No need
0
If the plant has a high phosphorus supply (150-200 mg P2O5 per 1 kg of soil, according to Franzesson)
0-10
0-50
Very low
Very strong
80
10-15
50-75
Низкая
Strong
60
15-20
75-100
Medium
Medium
45
> 20
> 100
High
No need
0

Table. Content of mobile nitrogen in the soil in the layer 0-40 cm before sowing winter crops and the need for nitrogen fertilizers on chernozem[2]Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al; ed. by V.G. Mineev. - M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, … Continue reading

N-NO3, mg/kg of soil
Nitrogen fertilizer requirements
Approximate fertilizer doses, kg/ha
Initial nitrate content
Nitrate content after 7 days of composting (nitrification capacity)
N
P
< 5
< 15*
strong
90
40
5-10
15-25
medium
60
40
10-15
25-30
weak
20-30
60
> 15
> 30
not available
30**
60-90

*Nitrification capacity without deduction of initial nitrate content

**Feeding only

  1. Determination of the need for nitrogen fertilizers and indicative doses for specific crops can be carried out by the content of mineral nitrogen in the soil and the value of nitrification capacity. The method was developed for the districts of the Volga region and the Orenburg region and is recommended for wide application in farming practice.

    In various modifications these diagnostic methods are used in other regions of Russia.

    Calculations of nitrogen fertilizer doses can be carried out by the balance method. In our country, A.V. Sokolov, Z.I. Zhurbitsky, I.S. Shatilov, and N.K. Boldyrev contributed to the development and improvement of these methods.

    Calculation of the amount of effective nitrogen (Nef), which enters the plants from the soil itself during the growing season, by the content of NMIN at the beginning of the growing season is determined by the formula:

and calculation of the nitrogen dose for the planned yield or its increase by the formula:

where DN – dose of nitrogen for the planned yield, kg/ha; R – nitrogen removal with the planned yield, kg/ha; NMIN – mineral (nitrate and ammonium) nitrogen content in soil, mg/kg; Nef (kg/ha) – amount of effective nitrogen that plants receive from soil (a certain soil layer), taking into account the current nitrification capacity determined by the mineral nitrogen use factor of soil, %; FNMIN – coefficient of mineral nitrogen use by soil, % (for nitrate nitrogen in 0-30 cm layer of chernozem soil equals 200%); Ffertilizer – coefficient of mineral fertilizer nitrogen use; d – volume weight of 1 cm3; h – soil layer depth (cm); dh/10 – soil layer weight, million kg, for converting mineral nitrogen from mg/kg to kg/ha; 100 – converting percent FNMIN and Ffertilizer, %.

Example. Calculation of nitrogen fertilizer doses by the balance method: the content of nitrate nitrogen in the layer 0-30 cm of ordinary chernozem is 10 mg/kg, h = 30 cm, d = 1.2 g/cm3; FNMIN = 200%, Ffertilizer N = 60%, B – nitrogen withdrawal at a grain yield of 4.0 t/ha is 120 kg/ha.

then according to the formula:

Y = Nef / NY,

where NY is the nitrogen content in the grain, kg/100 kg.

The yield of wheat (Y) due to soil nitrogen will be equal to:

72 (kg/ha) / 3 (kg/100 kg) = 2.4 t/ha.

And the dose of nitrogen according to the balance method will be:

Of the 80 kg of nitrogen applied to wheat, the plants use 60%, i.e. 48 kg, which will provide a yield increase equal to:

48 (kg/ha) / (3 kg/100 kg) = 1.6 t/ha.

As a result, the planned yield is provided by soil nitrogen (2.4 t/ha) and fertilizer (1.6 t/ha).

Experimental data allow some researchers to disregard FNMIN and Ffertilizer for nitrogen. In this case, the formula for calculating nitrogen doses takes the form:

The disadvantage of the balance method is the need for optimal values of the indicators included in the above equation.

Modifications of NMIN or balance method allow to set nitrogen fertilizer dosage accurately enough to obtain the planned yield of winter wheat. The method of plant diagnostics allows controlling the level of nitrogen nutrition in plants and timely correcting spring and late nitrogen fertilizing of crops. The combination of soil and plant diagnostics allows to regulate the level of nitrogen nutrition of winter wheat taking into account the soil and climatic and agrotechnical factors.

In Czechoslovakia, a system was developed to monitor the nutritional conditions of cereal crops based on plant analysis data. For this purpose, plant samples are taken in the phase of tubing (5 leaves) and the content of nitrogen, phosphorus, potassium and other elements is determined. According to the ratio of these elements determine the need of crops in fertilizers and the optimal doses for fertilizing.

Table. Optimization of fertilizer nitrogen doses during fertilization of winter wheat in the trumpeting phase based on chemical analysis of plants (by Bayer)

Criterion and plant analysis data
Degree of plant nitrogen demand
Optimal nitrogen doses for top dressing at the beginning of trumpeting (at yield > 4.0 t/ha), kg/ha
Р, %
N : Р
(100xK) / N
> 0,30
< 7,5
-
very high
80-100
7,5-8,5
> 100
100 and below
medium
high
60-80
80-100
8,6-10,0
> 100
weak
40-60
10,1-12,5
100 and below
> 100
medium
very weak
60-80
30-40
> 12,5
100 and below
> 100
weak
plants are provided for
40-60
feeding is unnecessary
< 0,30
10.0 and below
100 and below
> 100
very weak
very weak
30-40
30-40
10,1-12,5
100 and below
> 100
weak
plants are provided for
40-60
feeding is unnecessary
> 12,5
100 and below
very weak
30-40

Diagnosis of plant nitrogen nutrition

Nitrogen nutrition of plants due to the mobility of nitrogen in the soil and the need for periodic plant nutrition requires optimization during the growing season through fractional fertilizer application. This is possible with the use of complex soil and plant diagnostics of nitrogen nutrition of plants. In our country the issues of plant diagnostics and optimization of mineral nutrition of crops were addressed by K.P. Magnitsky, V.V. Tserling, N.K. Boldyrev, Y.I. Ermokhin.

V.V. Tserling proposed to determine nitrogen security of winter cereals by the content of nitrate and total nitrogen in plants in bushing and trumpeting phases.

Table. Nitrogen supply in winter cereals by its content in plants by development phases

Level of security
Tillering, 3 leaves
Tubing, 4-5 leaves
N-NO3, mg/kg raw material
total nitrogen, % of dry matter
N-NO3, mg/kg raw material
total nitrogen, % of dry matter
Very weak
0-100
2,5
0-50
2,0
Weak
101-200
2,5-3,0
51-100
2,0-2,8
Medium
220-710
5,0-5,5
101-220
2,9-3,7
High
> 710
> 5,5
> 220
3,8-4,4

Based on data on the content of nitrate and total nitrogen in plants according to the state of crops after the wintering and at the time of sampling, taking into account the planned harvest, determine doses of nitrogen fertilizers to fertilize winter crops in the appropriate phase.

Table. Application of nitrogen fertilizer doses for winter crops fertilization in the phase of tubing (4-5 leaves) according to the results of plant analysis, d.v. kg/ha[3]Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al; ed. by V.G. Mineev. - M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, … Continue reading

Level of security
N-NO3, mg/kg raw material
total nitrogen, % of dry matter
Planned yield, t/ha
2.1-3.0
3.1-4.0
> 4.0
Very weak
0-50
2,0
40-60
60-80
80-100
Weak
51-100
2,0-2,8
20-40
40-60
60-80
Medium
101-220
2,9-3,7
20
20-40
40-60
High
> 220
3,8-4,4
0
20
40

The entire aboveground part or individual (indicator) organs are used for plant diagnostics. Portable field laboratories are used for plant diagnostics. For analysis, the lower part of the stem (1-2 mm) of selected plants is cut off, placed on a glass, sap is squeezed out with a glass pestle, and 1-2 drops of 1% diphenylamine solution in concentrated sulfuric acid are applied to it. The intensity of the color is compared with the comparison scale. For an objective assessment, at least 10-15 determinations with subsequent averaging of the results are carried out.

Forms, timing and methods of nitrogen fertilizer application

Forms

The effectiveness of nitrogen fertilizers depends not only on the optimal nutritional regime of the crop during the growing season, but also on the form, timing and method of application. At present, in Russia there is a tendency to increase the share of urea application.

Urea has a number of advantages: it is well absorbed by the soil, travels little in the soil profile, its effect surpasses ammonium nitrate. Its effectiveness increases when applied as the main fertilizer in irrigation conditions and sufficient moisture, especially on light soils, not inferior to ammonium sulfate. Surface application of urea, for example, when fertilizing winter crops in the spring, in meadows and pastures, its effectiveness is lower than that of ammonium nitrate, which is associated with nitrogen loss.

In practice, it is often necessary to combine the application of several forms of fertilizers for the same crop. For example, to obtain a high quality yield of raw cotton 30-50% of the total dose of nitrogen is applied as ammonium and amide forms before sowing, and the rest is in the form of ammonium nitrate as fertilizer. Such a combination is also important in the cultivation of winter and row crops. Or, under conditions of irrigation or excessive moisture can be made slow nitrogen fertilizers as the main, and during the growing season to optimize nitrogen nutrition by feeding ammonium nitrate during the growing season.

For foliar fertilizing of winter wheat to increase protein content in grain urea is preferred because the amide form is well absorbed by plant leaves and does not cause burns even in high concentrations (20-30%).

Liquid nitrogen fertilizers show high efficiency when applied mainly to all crops and when fertilizing row crops. The most effective is liquid ammonia. 

Timing of fertilizer application

Under field conditions the nitrogen utilization rate of mineral fertilizers is on average 40-50%. The main part is fixed by the soil in the form of hard-to-hydrolyze organic compounds, which are not available to plants, or is lost as a result of denitrification and leaching. The amount of losses depends on the timing and methods of application, biological characteristics of crops, soil and climatic conditions. Therefore, for nitrogen fertilizers it is important to apply in periods of intensive nitrogen consumption.

Thus, the period of active nitrogen consumption by winter crops in spring begins about 5-15 days after snow melting. By this time, fields are free from excess moisture and the soil is sufficiently warmed. The same applies to meadows, which are fertilized 1 to 3 weeks after snow melt and the outflow of excess water. Application of nitrogen fertilizers immediately after snow melt on dry grasslands with excessive moisture reduces the efficiency of feeding due to gaseous nitrogen losses.

Table. Influence of temperature and humidity on the size of gaseous losses of nitrogen[4]Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al; ed. by V.G. Mineev. - M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, … Continue reading

Soil moisture, % of the smallest moisture capacity
Nitrogen losses at temperature
28º
NH4NO3
(NH2)2CO
NH4NO3
(NH2)2CO
60
8,5
27,0
15,8
31,9
90
20,3
37,0
49,7
61,1

In the forest-steppe, especially in the southern regions, and in the steppe zone, the soil dries up quickly in spring, so a late fertilization of winter crops with nitrogen reduces the effectiveness of this technique. In these regions there is no water flow along the soil profile. Therefore, on flat fields, winter crops are fertilized immediately after snowmelt. In the most continental areas of the steppe with little snowy winters, for example, in the Volga region, the North Caucasus and Ukraine, the same effect of nitrogen fertilization of winter crops in early spring and late fall, when a steady cold snap occurs, and even under the winter is often observed on flat land.

Sources

Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.

Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific and Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.

Fundamentals of Agronomy: Tutorial/Y.V. Evtefeev, G.M. Kazantsev. – M.: FORUM, 2013. – 368 p.: ill.