Nitrogen fertilizers

Sources of nitrogen fertilizers

From 1830 until 1914, the main nitrogen fertilizer was Chilean nitrate, whose deposits are concentrated in South America. Ammonia from the waste gases of coke ovens in 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 question of producing nitrogen fertilizers on an industrial scale became urgent. The use of atmospheric nitrogen for the production of nitrogen fertilizers was promising; in a 15-kilometer layer of air over an area of 1 ha, it amounts to about 78 thousand t of molecular nitrogen.

At the end of the 19th century, under laboratory conditions, a method was found for binding molecular nitrogen with oxygen by passing air through an electric-arc discharge at a temperature of about 3000 °C:

The resulting nitric oxide is oxidized by atmospheric oxygen to nitrogen dioxide, which, upon interaction with water, forms nitric acid.

The first plant using this technology was built in Norway, where natural conditions allow relatively cheap electricity to be obtained. Its production focused on the manufacture of Ca(NO₃)₂. Hence calcium nitrate received the name “Norwegian nitrate.” The obvious disadvantage of this technology was high energy consumption, and the resulting calcium nitrate has very high hygroscopicity and is inconvenient to use. Therefore, the method did not become widespread.

A method was proposed for binding atmospheric nitrogen based on binding nitrogen with calcium carbide at a temperature of 700–800 °C:

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

The method for ammonia synthesis was discovered by the German chemist Haber. Of all the methods for binding molecular nitrogen, his method proved to be the cheapest and is currently the main one in the production of nitrogen fertilizers.

Production of ammonia

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

The source of nitrogen is air. To isolate nitrogen from air, one of the following methods is used:

1. Atmospheric air is passed through a generator filled with burning coke; oxygen burns completely, and a mixture of nitrogen and carbon dioxide leaves the generator. The latter is absorbed by water at a pressure of 25 atm.
2. Air is liquefied and then separated into nitrogen and oxygen due to the difference in boiling points: oxygen boils at −183 °C, nitrogen at −196 °C.

Up to 50% of the costs in ammonia production are for hydrogen production. Natural and associated petroleum gases or coke-oven waste gases are used as sources of hydrogen. Hydrogen can also be obtained by electrolysis of water. The latter method allows pure hydrogen to be obtained, but requires high energy inputs.

The resulting ammonia is used directly as a fertilizer, for the production of ammonium fertilizers, nitric acid, and urea.

Production of nitric acid

Nitric acid is produced by catalytic oxidation of ammonia with atmospheric oxygen. This method is the main one for nitric acid production. The reaction proceeds in several stages. First, ammonia is oxidized to nitric oxide:

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

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

Nitrous acid HNO₂ is unstable and rapidly decomposes:

The resulting nitrogen oxides NO and NO₂ are returned to the same oxidation and absorption units.

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

Sodium nitrate

Sodium nitrate, or sodium nitrate, or nitric-acid sodium, or Chilean nitrate, is NaNO₃. It contains 16% nitrogen and 26% sodium. It was the first mineral nitrogen fertilizer. The largest natural deposit was located in Chile. Significant deposits have been found in California and southwestern Africa.

Currently, sodium nitrate is produced as a by-product in the production of nitric acid from ammonia. Nitrogen oxides NO and NO₂ not absorbed by water in the absorption towers (“tail gases”) are passed through additional absorption towers irrigated with a solution of sodium carbonate or hydroxide; this produces a mixture of sodium nitrate and nitrite:

Upon acidification with dilute nitric acid, nitrite is converted to nitrate:

Nitric oxide is again returned 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, highly soluble in water, hygroscopic, and at elevated humidity it can recrystallize into larger crystals. In a dry state and with proper storage, it does not cake and retains flowability.

Calcium nitrate

Calcium nitrate, or calcium nitrate, or nitric-acid calcium, or Norwegian nitrate, is Ca(NO₃)₂. It contains 17% nitrogen. It was first synthesized industrially in 1905 in Norway.

Currently, it is produced as a by-product in the production of nitric acid from ammonia: by neutralizing “tail gases” (nitrogen oxides NO and NO₂) with an aqueous solution of calcium hydroxide Ca(OH)₂ (milk of lime), and also in the production of compound fertilizers by the nitric-acid decomposition of phosphate raw materials.

Calcium nitrate is strongly hygroscopic (9.5 points out of 10). Under ordinary storage conditions, it becomes very damp, deliquesces, and cakes. It is transported and stored in moisture-proof bags. To reduce hygroscopic properties, hydrophobic additives (gypsum, paraffin-containing fuel oil) are introduced into commercial calcium nitrate up to 0.5% by weight of the salt.

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

Application of nitrate fertilizers

Nitrate fertilizers can be used on various soils under all agricultural crops. Due to their low nitrogen content, their use is economically more expensive; they are more often used in areas located near production facilities.

Calcium and sodium nitrates are equivalent 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 explained by the positive effect of sodium on the outflow of carbohydrates from leaves to roots and, consequently, an increase in root yield and sugar content in them.

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

Systematic application of calcium nitrate helps enrich the SAC with calcium.

The nitrate ion NO₃^- forms soluble salts or nitric acid with cations displaced from the soil absorbing complex. It is not subjected to physicochemical or chemical absorption. Nitrate can be bound 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, it is inexpedient to apply nitrate fertilizers in autumn, especially in areas with a leaching water regime.

Sodium and calcium nitrates are applied in spring before presowing cultivation and as side-dressings during the growing season. In summer, nitrates, due to their high mobility under conditions of excessive moisture, irrigation, and on easily drained soils, can be leached. Therefore, in regions with a humid climate and in irrigated areas for rice and other crops, ammonium forms are applied.

Sodium nitrate is also applied in rows with seeds; calcium nitrate is poorly suited for this because of its high hygroscopicity. Sodium nitrate must not be used on saline soils and solonetzes.

Sodium and calcium nitrates are physiologically alkaline fertilizers, since plants absorb the NO₃^- anion more than Na⁺ or Ca²⁺ cations. Some of the cations remain in the soil and alkalinize the soil solution. Long-term use of nitrate fertilizers on acidic sod-podzolic and light, low-buffer soils promotes their neutralization. Therefore, on sod-podzolic soils, nitrates show greater efficiency than physiologically acidic ammonium fertilizers. On chernozems, this advantage is lost.

Ammonium sulfate

Ammonium sulfate, or sulfuric-acid ammonium, (NH₄)₂SO₄, in pure form contains 21.2% nitrogen; in the technical product, 20.5%. In 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, ammonium sulfate production began in the Donbass at the Shcherbinka mine in 1899 by capturing and neutralizing ammonia formed during coal coking with sulfuric acid. The same technological scheme is used at present.

Ammonium sulfate can be obtained by absorbing ammonia with sulfuric acid according to the reaction:

The reaction proceeds with the release of heat, which is used to evaporate the solution. Upon cooling of the saturated solution, ammonium sulfate precipitates as a crystalline solid, which is separated and dried. Sulfuric acid can be replaced by cheaper natural minerals: gypsum (CaSO₄⋅2H₂O), mirabilite (Glauber’s salt, Na₂SO₄⋅10^H₂O), or waste from the production of phosphorus fertilizers — phosphogypsum.

Finely ground gypsum is agitated in ammonia water through which carbon dioxide is passed. As a result of the interaction of ammonia, carbon dioxide, and gypsum, ammonium sulfate is formed:

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 coke-oven waste gases, coke-chemical ammonium sulfate is cheaper.

Ammonium sulfate is highly soluble in water: 76.3 g (NH₄)₂SO₄ per 100 cm³ of water at 20 °C. In a dry state, the fertilizer has low hygroscopicity, cakes little during storage, does not deliquesce in air, retains flowability, and is spread well by fertilizer spreaders.

Ammonium sulfate is a white crystalline substance with various colors depending on the production method. It contains 0.2–0.3% moisture, impurities of Ca, Mg, SiO₂, 0.025–0.05% (0.2–0.5%) free sulfuric acid, which gives the fertilizer a slightly acidic reaction. Coke-chemical ammonium sulfate contains a small amount of organic impurities — resinous substances, phenol, up to 0.1% ammonium thiocyanate (NH₄SCN). These impurities may cause a gray, bluish, or reddish color.

Because ammonium thiocyanate is toxic to plants, its content must 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 plant nutrition.

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

The ability of the soil to absorb ammonium protects it from leaching; however, during side-dressing it may not be used.

As a result of nitrification, part of the ammonium nitrogen passes into 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 usual doses of ammonium sulfate leads to a change in the reaction of the soil environment. On acidic soils, the negative effect appears after several years. On chernozem soils, it can be used for a longer time. According to data from the Mironovka Experimental Station in Ukraine, application of (NH₄)₂SO₄ for 14 years led to a change in soil reaction: pH from 6.0 to 4.9; exchangeable acidity increased 1.5 times, hydrolytic acidity 2.5 times. This did not affect yield due to the high humus content, high buffering capacity, and absorption capacity of chernozem. On chestnut soils and gray soils (sierozems), acidification of carbonate soils upon application of physiologically acidic fertilizers does not pose a danger.

On sod-podzolic soils, when combined with liming, ammonium sulfate is not inferior to other nitrogen fertilizers. However, long-term use in high doses without liming on these soils worsens their properties, plant growth, and productivity. Crops such as oats, winter rye, flax, potato, rutabaga react less to the acidifying effect of ammonium sulfate than beets, maize, hemp, barley, and spring wheat.

Due to the weak migration of ammonium ions, this fertilizer is effective on light soils and in areas with sufficient moisture. When applied in rows and as side-dressing, ammonium sulfate is less effective than other nitrogen fertilizers.

Ammonium-sodium sulfate

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

Ammonium chloride

Ammonium chloride, or hydrochloric ammonium, NH₄Cl, is a by-product in the production of baking soda (sodium bicarbonate):

Ammonium chloride is a fine-crystalline white or yellowish powder, contains up to 26% nitrogen; 37.2 g dissolves in 100 cm³ of water at 20 °C; it is slightly hygroscopic, does not cake, and spreads well. It is distinguished by high physiological acidity and contains up to 60% chlorine, which has a negative effect on chlorine-sensitive crops. Therefore, it is applied in autumn so that chlorine is leached from the root-inhabited layer by atmospheric precipitation.

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

In the soil, it is partially subjected to nitrification. The efficiency of ammonium chloride can be increased by liming, preliminary neutralization of the fertilizer (1.4 kg CaCO₃ per 1 kg NH₄Cl), joint application with physiologically alkaline fertilizers, and combination with organic fertilizers.

In terms of fertilizing effect, NH₄Cl is usually inferior to (NH₄)₂SO₄. For cereal crops at usual doses, the effectiveness of chloride and sulfate is equivalent. For chlorine-sensitive crops, high doses are not used; it is applied in advance as a basic fertilizer.

Ammonium carbonate and bicarbonate

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

Ammonium carbonate, or carbonic-acid ammonium, is a crystalline white substance obtained by passing carbon dioxide through an aqueous ammonia solution followed by evaporation of the resulting salt. Carbonate is unstable; in open air it can decompose with the release of ammonia and the formation of ammonium bicarbonate. The technical product contains 21–24% nitrogen and is a mixture of ammonium carbonate, bicarbonate, and carbamate.

Ammonium bicarbonate, or bicarbonate, is obtained by adsorption of gaseous ammonia and carbon dioxide by an ammonium carbonate solution. It contains about 17% nitrogen. It is relatively more stable than carbonate, but also has ammonia losses during storage, transportation, and application. When applied superficially, it should be immediately incorporated into the soil.

Application of ammonium fertilizers

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

Passing into an exchange-absorbed state, ammonium is fixed in the soil, thereby preventing its leaching. At the same time, in the exchange-absorbed state, ammonium remains available to plants.

Partially, under the action of nitrification, ammonium nitrogen passes into the nitrate form. The rate of this process depends on temperature, moisture, aeration, biological activity, soil reaction, and degree of cultivation. Thus, in a microfield experiment conducted on a weakly cultivated sod-podzolic soil, after 15 days of the experiment, 12% of ammonium sulfate underwent nitrification; after 30 days, 24%; whereas on a well-cultivated soil, nitrification was 79 and 96% of the applied amount, respectively.

Waterlogging and increased acidity inhibit nitrification processes. Liming acidic soils accelerates this process.

Ammonium chloride nitrifies more slowly than sulfate, which is associated with the inhibitory effect of chlorine on the activity of nitrifying bacteria.

After ammonium nitrogen is converted to nitrate, it acquires the properties of nitrate fertilizers. During nitrification, nitric, hydrochloric, or sulfuric acid is formed in the soil:

or

In the soil, acids are neutralized by bicarbonates of the soil solution and cations of the soil absorbing complex:

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

The change in reaction upon application of ammonium fertilizers is also associated with their physiological acidity. From (NH₄)₂SO₄ and NH₄Cl, plants absorb the cation faster than the anion; accordingly, acid residues accumulate. Systematic application is accompanied by acidification of the soil environment. The degree of acidification is greater, the lower the buffering capacity.

On sod-podzolic and gray forest soils with a small total exchangeable bases and organic matter content, acidification appears faster compared to chernozems and chestnut soils. Thus, long-term application of ammonium sulfate (as part of NPK) on gray forest soil led to an increase in hydrolytic acidity and a decrease in the total exchangeable bases and base saturation.

To prevent the negative acidifying effect of ammonium fertilizers on such soils, liming is carried out beforehand or sulfate and ammonium chloride are neutralized before application at the rate of 130–140 kg of lime per 100 kg of fertilizer. Neutralization of fertilizers is carried out immediately before application.

The specific transformations of ammonium fertilizers in soils determine the technology of their effective use. These fertilizers are applied, as a rule, before sowing as basic fertilizers, both in spring and autumn, without fear of nitrogen leaching.

The effectiveness of ammonium fertilizers depends on the acidity and buffering capacity of soils and the biological characteristics of crops.

On soils of the Non-Chernozem zone, ammonium fertilizers can increase the effectiveness of phosphorite meal. The physiological acidity of these fertilizers promotes the dissolution of calcium phosphates.

The effectiveness of ammonium fertilizers depends on the characteristics of the crops grown. Crops less sensitive to acidic reaction, for example, rye, oats, potato, flax, buckwheat, react less to acidification. Sensitive crops (root crops, most vegetables and legumes, barley, wheat, sunflower) react negatively to acidification with repeated application of ammonium fertilizers.

Crops sensitive to increased chlorine content react negatively. Thus, the starch content in potato decreases with excess chlorine. Therefore, for chlorine-sensitive crops, ammonium sulfate is used or ammonium chloride is applied in autumn.

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

Ammonium fertilizers are not used for pre-sowing application in rows or before presowing cultivation because the intensive entry of ammonium nitrogen into 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 simultaneously contain ammonium and nitrate forms of nitrogen. This group includes ammonium nitrate, ammonium sulfonitrate, and lime-ammonium nitrate.

Ammonium nitrate

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

The resulting ammonium nitrate solution is evaporated, recrystallized, and dried. The heat released by the neutralization reaction is used for evaporation. The result is a white crystalline substance containing up to 98–99% NH₄NO₃. Additives are introduced to improve physicochemical properties.

Ammonium nitrate is highly soluble in water: at 20 °C, 192 g of salt dissolves in 100 cm³ of water; it is very hygroscopic, becomes damp in air, and cakes. Depending on temperature, it has five crystalline modifications. Transitions from one modification occur at temperatures of +32.1 and −1 °C. If during storage of ammonium nitrate there were sharp temperature changes covering these temperature points, recrystallization of one form into another will occur with an increase in volume. The fertilizer becomes strongly compacted, turning into lumps and blocks, and the bags in which it was stored may burst.

To protect ammonium nitrate from caking, hydrophobic additives and additives that increase granule strength are added: ground limestone, chalk, phosphorite meal, phosphogypsum, kaolinite, magnesium nitrate, fatty acids and their amines, and others. The total content of additives is from 3.0 to 5.0%. Additives may give a yellow tint. Fuchsin may be introduced as an additive, which gives a red color.

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

The moisture content should be no more than 0.3–0.4%; the reaction is neutral or slightly acidic; the content of insoluble impurities is no more than 0.1%.

To prevent moistening and reduce caking, ammonium nitrate is packed in dense, hermetic containers — polyethylene or laminated paper bags. During storage, bags must not be stacked in high piles or stacks, since in the lower layers of the pile the bags become strongly compacted and cake.

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

Ammonium nitrate is fire-hazardous and under certain conditions can explode. At temperatures above 200–270 °C, it decomposes with the release of heat and strong oxidizers that accelerate combustion. Sharp 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 fire- and explosive properties.

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

When applied to the soil, ammonium nitrate dissolves in soil moisture. The nitrogen of NH₄NO₃ is absorbed by microorganisms, and when they die and mineralize, it becomes available to plants. In the soil, ammonium enters an exchange reaction with the soil absorbing complex:

With a lack of calcium on acidic podzolic soils, application of ammonium nitrate leads to acidification of the soil solution. Experiments by D. N. Pryanishnikov established that from an ammonium nitrate solution the NH₄⁺ cation is absorbed faster than NO₃^-. Therefore, ammonium nitrate is classified as a physiologically acidic fertilizer. However, its physiological acidity is lower than that of ammonium fertilizers.

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

On acidic sod-podzolic soils, application of ammonium nitrate can lead to even greater acidification, which is temporary: absorption of nitrate nitrogen restores the reaction of the medium to the initial 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 and 20–40% of ammonium nitrogen are transformed into organic form (immobilized); only 10–15%, that is, 2–3% of the applied amount, is assimilated by plants in the second year. The immobilization process is accelerated by plowing in plant residues with low nitrogen and high carbon content, for example, straw, straw manure. Fertilizer nitrogen mobilizes soil nitrogen, which leads to an increase in the utilization coefficient.

Ammonium nitrate in high doses on low-buffer light soils increases the nitrate content in plants. The use of such plants for animal feed can lead to metabolic disorders and poisoning. The rumen microflora of ruminants reduces nitrates to nitrites, which, entering the blood, bind hemoglobin and block its ability to carry oxygen. An increased concentration of methemoglobin in the blood of animals leads to suffocation; in severe poisoning, to death.

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

Ammonium nitrate is used as a pre-sowing (basic) and row (at sowing) fertilizer and for side-dressing during the growing season.

Under irrigation, sufficient or excessive moisture, especially on soils light in texture, application of ammonium nitrate in autumn under fall plowing is inexpedient due to possible leaching of nitrate nitrogen. Under these conditions, it can be used directly at the moment of greatest nitrogen consumption by plants. In small doses of 10–15 kg/ha, nitrate is applied together with phosphorus and potassium fertilizers in rows when sowing sugar beet and vegetables, and in holes when planting potato. High efficiency is noted in side-dressing winter cereals and row crops.

Ammonium nitrate is also used for early spring side-dressing of winter crops and perennial grasses. It can be used for side-dressing row and vegetable crops during inter-row cultivation with incorporation to a depth of 10–15 cm by cultivator-fertilizer units.

Ammonium sulfonitrate

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

It is obtained by mechanical mixing of 65% ammonium sulfate and 35% ammonium nitrate or by introducing dry ammonium sulfate into molten nitrate followed by drying and grinding the mixture. The product obtained by the latter method is also called Leuna nitrate. Another production method is neutralization of sulfuric and nitric acids with ammonia — Montan nitrate.

Ammonium sulfate-nitrate is highly soluble in water and less hygroscopic than ammonium nitrate. When stored in a dry room, it does not cake and retains flowability.

In effectiveness, it is close to ammonium sulfate. It has significant potential acidity; therefore, its use on acidic soils requires preliminary liming or neutralization of the fertilizer before application.

Lime-ammonium nitrate

Lime-ammonium nitrate, NH₄NO₃⋅CaCO₃. It is obtained by fusion of ammonium nitrate with limestone. It is produced in the form of granules with various ratios of NH₄NO₃:CaCO₃ — from 80:20 to 53:47. The product with a 60:40 ratio and a nitrogen content of 20.5% has optimal physical and mechanical properties.

Compared with ammonium nitrate, this fertilizer has lower hygroscopicity, is non-explosive, and can be transported in bulk. It is widely used in Western European countries. It is not produced in Russia because of the high cost of transportation (the lower the active substance content, 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 their effect on plants, they show the same effectiveness as solid nitrogen fertilizers. Their production is cheaper than that of solid fertilizers. Thus, the cost per unit of nitrogen of liquid ammonia is approximately 35–40% cheaper than ammonium nitrate (the cheapest of the solid nitrogen fertilizers). They are used on the largest scale in the USA.

The use of liquid ammonia fertilizers makes it possible to fully mechanize loading and unloading operations and their application. Application requires 2–3 times less labor than solid nitrogen fertilizers. Liquid fertilizers are distributed more evenly in the soil and do not have caking or segregation (stratification).

The use of liquid fertilizers has a number of disadvantages: storage requires special large-capacity tanks, the organization of distribution points is required, the use of special application equipment, and a fleet of road and railway tank cars for transportation.

Liquid nitrogen fertilizers are applied with special machines with immediate incorporation to a depth of at least 10–12 cm on heavy soils and 14–18 cm on light soils to exclude ammonia losses. Losses are possible on strongly carbonate soils with an alkaline reaction. Surface application of liquid ammonia fertilizers is unacceptable. Shallow incorporation into the dry topsoil is also associated with large ammonia losses.

In all cases, anhydrous ammonia is incorporated to a depth of at least 14–15 cm; aqueous solution, at least 10–12 cm. In the case of coarse-cloddy soil, the incorporation depth is increased by 1.2–1.5 times. They are applied as a basic fertilizer under fall plowing in autumn, in spring before presowing cultivation, and for side-dressing row crops in doses (by nitrogen) as for solid nitrogen fertilizers. On light soils with low absorption capacity, application of high doses in autumn is associated with possible ammonia loss, since it may not be completely adsorbed by the soil absorbing complex.

Since liquid ammonia fertilizers are applied locally, the openers of side-dressing machines are spaced for continuous-sowing crops at 20–25 cm, on meadows and pastures at 30–35 cm; for side-dressing row crops, the width is determined by row spacing. The technology of using liquid ammonia fertilizers requires higher qualifications of specialists.

During side-dressing, to exclude possible damage to young plants by excess ammonia, fertilizers are applied in the middle of the inter-row or at a distance of 15–10 cm from the rows. For uniform distribution in the soil, subsequent inter-row soil cultivation is carried out. As nitrification proceeds, the resulting nitrates acquire mobility and are transferred with soil moisture to the root zone. The intensity of nitrification is determined by soil properties: on chernozem and cultivated sod-podzolic soils it proceeds faster than in acidic podzolic soils. Synthetic aqueous ammonia undergoes nitrification faster than coke-chemical ammonia, since the impurities contained in the latter inhibit the activity of nitrifying bacteria.

With proper use of liquid ammonia fertilizers, their effectiveness is not inferior to ammonium nitrate.

Liquid ammonia

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

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

Liquid ammonia is a highly toxic substance; a mixture with air at a volume concentration of ammonia of 15–27% is explosive. An explosion can occur from a spark or any open source of fire. Contact with the skin causes burns; evaporation causes frostbite.

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

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

After application of liquid ammonia, in the first days the soil reaction shifts toward alkalinization to pH 9. In the zone of fertilizer application, temporary soil sterilization occurs, which suspends the process of nitrification of ammonium nitrogen. After 1–2 weeks, microbiological activity is restored. Under optimal conditions, complete nitrification of ammonia occurs within a month.

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

Ammonia water

Aqueous ammonia solution, or ammonia water, NH₃ + H₂O. It is a transparent liquid, sometimes with a yellowish tint. In an aqueous ammonia solution, equilibrium always exists between ammonia absorbed by water and gaseous ammonia above the surface of the solution, which causes its loss during storage in open vessels.

Ammonia solution is produced in two grades: the first with a content of 20.5% nitrogen, or 25% ammonia; the second with 16.4% nitrogen, or 20% ammonia. Coke-chemical aqueous solution contains impurities of hydrogen sulfide, phenols, thiocyanate and cyanide compounds.

Ammonia water is distinguished by a low vapor pressure of ammonia (25% solution — 0.15 kgf/cm² at 40 °C), does not corrode ferrous metals, and freezes at temperatures: 25% at −56 °C, 20% at −33 °C. Density at 1 °C of the first grade is 0.910 kg/m³, of the second, 0.927 kg/m³.

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

When applied to the soil, ammonia is adsorbed by soil colloids and therefore migrates weakly. Over time, ammonium nitrogen nitrifies, and its mobility increases. The use of ammonia water is technically simpler and safer than liquid ammonia. The intensity of ammonia absorption by the soil is influenced by texture, humus content, moisture, and depth of incorporation. On heavy, well-cultivated soils with a high content of organic matter, ammonia absorption is higher than on light, dry, humus-poor soils, where losses from volatilization are significantly greater.

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

Urea

Urea, or carbamide, CO(NH₂)₂, contains 46.7% nitrogen; it is one of the most concentrated solid nitrogen fertilizers. 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. In the first stage of the process, ammonium carbamate is formed:

then, upon its dehydration, urea:

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

It is produced in granular form with granules 1–3 mm in size. During granulation it can be coated with hydrophobic additives. Granulated urea has good physical and mechanical properties, practically does not cake, and retains flowability and spreadability.

During granulation, under the action of elevated temperatures, an impurity is formed — biuret:

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

Biuret decomposition in the soil occurs within 10–15 days. Therefore, application of urea with a high biuret content 1 month before sowing does not have a negative effect on plants. Currently, granulated urea is produced with a biuret content of no more than 1%, which does not have an inhibitory effect on plants regardless of the time of application.

In the soil, urea dissolves in soil moisture; under the action of the enzyme urease of plant residues and microflora, it undergoes ammonification, turning into ammonium carbonate:

Under favorable conditions on cultivated soils, the transformation occurs within 1–3 days. On low-fertility sandy and waterlogged soils, the process takes up to 3 weeks. Urea dissolved in the soil solution before ammonification can be leached.

The resulting ammonium carbonate is unstable; in air it decomposes to form ammonium bicarbonate and gaseous ammonia:

Therefore, with surface application of urea without incorporation and with insufficient moisture, ammonia losses can occur. Losses increase on soils with neutral and alkaline reactions. Ammonium carbonate undergoes hydrolysis with the formation of ammonium bicarbonate, NH₃, and water, which leads to alkalinization of the medium:

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

Urea is used as a basic fertilizer on all soil types under any crops. Under rainfed agriculture, its effectiveness is equivalent to ammonium nitrate; under irrigated conditions, to ammonium sulfate. Under conditions of a leaching water regime of soils, urea is more effective than ammonium nitrate because amide nitrogen, rapidly turning into ammonium, is absorbed by the soil without leaching from the root-inhabited layer.

Urea is used for side-dressing winter crops in early spring with immediate incorporation by harrowing. According to experiments, incorporation of urea even to 1.5 cm sharply reduces ammonia losses. Urea is used for side-dressing row and vegetable crops with cultivator-fertilizer units. However, on hayfields and pastures, surface application of urea shows effectiveness 15–20% lower than ammonium nitrate due to significant ammonia losses from urea ammonification.

Urea is the best form for foliar feeding of plants, especially wheat, to increase grain protein content, because even at increased concentration (1% solution) it does not cause leaf burns and is well assimilated by plants. Urea can be absorbed by leaf cells as a whole molecule and assimilated by plants both as ammonia after ammonification and by direct involvement in the cycle of nitrogenous substance transformations. For foliar feeding, it is desirable to use the crystalline form, since its biuret content is lower than 0.2–0.3%.

Urea is a source of nitrogen nutrition and a physiologically active substance; it activates nitrogen metabolism processes, in particular the formation of sulfhydryl groups of the amino acids methionine, cysteine, and the tripeptide glutathione. Amino acids containing SH groups participate in metabolic processes, growth, and the initiation of reproductive organs. The effect of urea also affects the water regime of plants: late nitrogen side-dressing increases the hydration of colloids due to an increase in the total amount of nitrogen, water-soluble and non-extractable proteins. The amount of tightly bound water also increases and the water-holding capacity of leaves increases.

The use of urea as a starter fertilizer (in rows) can lead to delayed seed germination due to the inhibitory effect of excess free ammonia.

Due to the high nitrogen concentration, uniform application to the soil is especially important for urea. For uniform spreading, it is thoroughly mixed with other fertilizers immediately before application.

In the world assortment of nitrogen fertilizers, the share of urea use is constantly increasing. Its production technologies are also being improved, allowing higher-quality urea to be obtained while reducing cost.

Urea is used in the production of compound and slow-release nitrogen fertilizers. Due to the higher economic efficiency of urea and other high-analysis nitrogen fertilizers, fertilizers with low nitrogen content are losing their importance in the balance of nitrogen fertilizer consumption.

Calcium cyanamide

Calcium cyanamide, CaCN₂, contains 20–21% nitrogen. It is a light powder of black or dark gray color, dusty when spread; if it gets into the eyes and respiratory tract, it can cause inflammation. It is a physiologically alkaline fertilizer, since it contains up to 20–28% CaO. Technical factory calcium cyanamide contains impurities of coal — 9–12%, silicic acid, iron and aluminum oxides.

Its systematic use on acidic soils improves their physicochemical properties due to its acid-neutralizing ability and calcium enrichment. It is applied 7–10 days before sowing or in autumn under fall treatment. It is not recommended for side-dressing, since in the soil calcium cyanamide undergoes hydrolysis and interacts with the absorbing complex to form cyanamide H₂CN₂, which is toxic to plants.

It is almost not used as a fertilizer; it is more often used for pre-harvest defoliation of cotton and sunflower when harvesting for seed.

Ammoniates

Ammoniates are nitrogen fertilizers representing an aqueous solution of ammonia and ammonium nitrate, ammonium and calcium nitrates, urea, or ammonium nitrate and urea. They contain from 30 to 50% nitrogen. Ammoniates are produced in special mixers by introducing a hot solution of ammonium nitrate (urea or calcium nitrate) into a 10–15% ammonia solution.

Ammoniates are light-yellow liquids; depending on composition, the ammonia vapor pressure at 32 °C is from 0.2 to 3.6 atm. By ammonia vapor pressure, ammoniates are divided into two groups:

• with moderate vapor pressure — 0.2–0.7 atm, with a nitrogen content of 35–40%;
• with elevated vapor pressure — 0.7–3.6 atm, with a nitrogen content of 40–50%.

Ammoniates differ in crystallization onset temperature: from 14 to 70 °C. In winter, ammoniates with a low crystallization temperature are produced for storage; in summer, with a higher one.

Ammoniates are capable of corroding copper alloys and ferrous metals; therefore, tanks and equipment are made of alloy steels, aluminum and its alloys, or steel tanks with a protective anticorrosion coating (epoxy resins) are used, as well as tanks made of polymer materials. They are transported and stored in special hermetic tanks designed for low pressure. 20–40% of the nitrogen in ammoniates is in the form of ammonia and 60–80% in the form of an ammonium salt or urea.

The use of ammoniates requires the same application conditions as liquid ammonia fertilizers, that is, observance of incorporation depth depending on texture. In the soil, ammonia diffusion, as a rule, is no more than 8–10 cm; therefore, the distance between openers when applying ammoniates should be no more than 20–25 cm. When applying ammoniates as side-dressing for row crops, the distance between openers is set equal to the row spacing.

In their effect on agricultural crops, ammoniates are equivalent to solid nitrogen fertilizers. In Russia, ammonium carbonates — ammonia solutions of ammonium carbonate and bicarbonate and urea, containing 4–7% ammonia and 18–35% total nitrogen — have become most widespread.

UAN

UAN are nitrogen fertilizers representing an aqueous solution of urea and ammonium nitrate.

UAN with a nitrogen content of 28–32% has a number of advantages over solid and liquid nitrogen fertilizers: it does not contain free ammonia, therefore it is more technological and convenient to use; it can be stored in open tanks without nitrogen losses. Urea and ammonium nitrate in solutions create a mutual dissolution effect, which allows more concentrated fertilizers to be obtained without the risk of crystallization. They are transparent or yellowish liquids with a density of 1.26–1.33 g/cm³, with a neutral or slightly alkaline reaction.

UAN solutions are obtained under industrial conditions from unevaporated melts of urea and ammonium nitrate. By excluding evaporation, granulation, conditioning, and packaging stages, their production cost is reduced.

Changing the ratio of urea and nitrate allows the crystallization (salting-out) temperatures to be regulated, which makes it possible to use them in different regions, times, and seasons.

Grades of UAN solutions are selected taking into account storage and use temperature to prevent crystallization.

On the basis of UAN solutions, it is possible to use compound fertilizers containing macro- and micronutrients, for example, salts of cobalt, boron, copper, molybdenum, herbicides, and growth retardants.

UAN solutions are transported in railway tanks made of carbon steel or in road tankers using corrosion inhibitors. With an equal volume, UAN-32 contains 1.3 times more nitrogen than granulated urea and 1.5 times more than ammonium nitrate, which reduces transportation and storage costs. For on-farm transportation and application of UAN to the soil, the same equipment can be used as for other liquid fertilizers, ammonia water, or herbicides.

UAN solutions are used as a basic fertilizer and for side-dressing. For basic application, direct application into the soil or surface application with subsequent incorporation can be used. They can be used for root side-dressing of row crops and for foliar side-dressing of cereals. UAN solutions can be applied together with irrigation water by sprinkling systems.

Encapsulated nitrogen fertilizers

The development of encapsulated nitrogen fertilizer production is promising. Encapsulated fertilizers are ordinary water-soluble forms, but their granules are coated with films that slow dissolution. Encapsulated fertilizers have good physical and mechanical properties: they are less hygroscopic, the granules are stronger, and they do not cake. When they enter the soil, nitrogen is gradually released from the granules and assimilated by plants as the capsules break down. Depending on the composition and thickness of the capsules, it is possible to regulate the dissolution rate of the fertilizer in accordance with the biological needs of crops and the periodicity of nutrition.

For encapsulation, paraffin, polyethylene emulsion, sulfur compounds, acrylic resin, and polyacrylic acid are used.

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

Nitrification-inhibiting fertilizers

Among nitrification inhibitors, cyanoguanidine (dicyandiamide), the American preparation N-serve, or nitrapyrin (2-chloro-6-(trichloromethyl)pyridine), or the Japanese preparation AM (2-amino-4-chloro-6-methylpyrimidine) are used. In Russia, the inhibitors picochlor and jakos — derivatives of nitrapyrin — are produced. Introduction of these inhibitors in a mixture with solid or liquid ammonium fertilizers at doses of N-serve 0.5–1%, AM 1–3% of the nitrogen content inhibits nitrification processes for up to 1.5–2 months, that is, for the period of intensive nitrogen consumption by plants.

The rate of decomposition of inhibitors in the soil and, accordingly, the duration of their action are influenced by soil texture, moisture, reaction of the medium, temperature, and humus content.

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

The use of inhibitors has a positive effect on product quality, since it prevents nitrate accumulation and reduces the incidence of certain plant diseases. Due to the increased nitrogen utilization coefficient, doses of nitrogen fertilizers are reduced, and split application is replaced by application of the entire dose at once.

Urea-formaldehyde fertilizers (UFF)

Urea-formaldehyde fertilizers (UFF), or carbamideform, ureaform, are products of chemical condensation of urea CO(NH₂)₂ and formaldehyde (CH₂). Condensation proceeds in concentrated solutions at equivalent ratios of urea and formaldehyde in a medium acidified to pH 3, at a temperature of 30–60 °C. This produces monomethylolurea CONHCH₂NH₂OH, which interacts again with urea and turns into methylenediurea NH₂CONHCH₂NHCONH₂ with the release of water. The resulting condensate is filtered, dried, ground, and, if necessary, granulated. The reaction product is usually a white friable powder that does not cake and retains flowability even at high humidity.

The nitrogen content in UFF is 38–40%; the water-soluble fraction accounts for 8–10%; the insoluble part remains available to plants.

One of the main indicators of UFF is the availability index — the amount of water-insoluble nitrogen that dissolves upon boiling for 1 h. It is expressed as a percentage of water-insoluble nitrogen. The availability index depends on the reaction, temperature, molar ratio of urea and formaldehyde, and duration of condensation. It varies from 15 to 55%.

In some foreign countries, the availability index is conventionally taken to be equal to the amount of nitrogen that is nitrified within 6 months of the fertilizer being in the soil. The degree of nitrification of UFF — an indicator of their effectiveness — depends on the availability index and soil properties. UFF with a high availability index corresponds to greater and faster accumulation of nitrate nitrogen in the soil.

The acidic reaction of the soil reduces the rate of transformation of UFF; therefore, liming increases the nitrification rate. High doses of UFF alkalinize the soil; as they mineralize, gradual acidification occurs.

Under certain conditions of the condensation reaction, for example, at a temperature of 30–40 °C, UFF are obtained with a high content of plant-available nitrogen, approaching soluble nitrogen fertilizers. In this case, they lose their purpose as a slow-release fertilizer.

The production of UFF is promising because all nitrogen fertilizers are highly soluble; however, their application in large doses creates a high concentration and osmotic pressure of the soil solution, which negatively affects plants in the initial stages of growth, especially crops sensitive to high salt concentrations. In addition, in areas of sufficient moisture, primarily on light soils, and under irrigation, nitrogen losses from leaching are possible.

On sod-podzolic soils with different degrees of cultivation and in different links of field crop rotations, no advantages of UFF over soluble nitrogen fertilizers were found either in yield size or product quality. On heavy sod-podzolic soils, the effectiveness of UFF on the yield of green mass of maize was lower.