Characteristics and features of ammonium sulfate application in agriculture

Ammonium sulfate is a highly effective nitrogen-sulfur fertilizer. Its physical and chemical properties make it particularly in demand in regions with excessive humidity and on irrigated lands. This fertilizer, obtained from ammonia water, was first used in agricultural practice in 1860.

• Nitrogen in pure salt — 21.2 %
• Nitrogen in technical product — 20.5 %
• Sulfur content — 23–24 %
• Solubility at 20 °С — 76.3 g per 100 cm³ of water
• Fertilizer density — 0.8 t/m³

The fertilizer is obtained by neutralizing sulfuric acid with ammonia. The neutralization reaction proceeds according to the scheme: H2SO4 + 2NH3 = (NH4)2SO4. The precipitate formed in the saturated solution is separated by centrifugation and dried. The synthesis of coke-chemical ammonium sulfate from coal coking gases is cheaper than the synthetic version obtained from pure ammonia.

Domestic production of the fertilizer started in 1899 at the Shcherbinovsky mine in the Donbas. The technology was based on trapping and neutralizing ammonia with sulfuric acid. This gas was released during the coal coking process.

The synthetic product is white and contains from 0.025 % to 0.5 % of free sulfuric acid. The coke-chemical analog is colored gray, bluish, or reddish due to organic impurities — resin acids and phenol. Additionally, it contains small amounts of calcium, magnesium, and silicon.

Dry ammonium sulfate has low hygroscopicity, does not deliquesce in the air, and retains its friability perfectly. It practically does not cake during storage and is easily applied by fertilizer drills. The fertilizer can be mixed with most other fertilizers immediately before application to the field. Good results are obtained by preparing a mixture with phosphate rock, which effectively neutralizes the acidifying effect of ammonium sulfate.

Transformations of the fertilizer in the soil and neutralization processes

When applied to the soil, ammonium sulfate dissolves rapidly and enters into exchange reactions with the soil adsorbing complex (SAC). Ammonium cations are adsorbed by the SAC, transform into a less mobile form, and are reliably protected from leaching by precipitation or irrigation water. On soils saturated with bases, the mutual exchange reaction proceeds according to the following scheme:

[SAC] 4)2SO4 ⇄ [SAC]2NH4 + CaSO4

On soils with high potential acidity, exchange processes proceed differently. As a result of the reaction, free sulfuric acid is formed. This leads to the acidification of the soil solution:

[SAC]2H + (NH4)2SO4 ⇄ [SAC]2NH4 + H2SO4

Soil microorganisms convert a significant part of ammonium nitrogen into nitrates within 10–15 days. The rate of this process depends on temperature, humidity, aeration, and soil reaction. Biological oxidation of ammonium is accompanied by the formation of nitric and free sulfuric acids:

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

The resulting acids are neutralized by adsorbed bases, as well as by calcium or potassium bicarbonate according to the following schemes:

[SAC]Ca + 2HNO3 = [SAC]2 3)2

[SAC]Ca + H2SO4 = [SAC]2 4

2HNO3 3)2 3)2 + 2H2CO3

2HNO3 3)2 3)2 + 2H2CO3

H2SO4 3)2 = CaSO4 + 2H2CO3

H2SO4 3)2 = MgSO4 + 2H2CO3

Neutralization of mineral acids is accompanied by the destruction of bicarbonates in the soil solution and the displacement of bases from the soil adsorbing complex by hydrogen. This leads to a weakening of the soil's buffering capacity and an increase in its acidity. Soil acidification is also caused by the physiological acidity of ammonium sulfate. Its physiological acidity is due to a greater uptake of the NH4+ cation by plants than the SO42– anion. The latter, combining with hydrogen, forms sulfuric acid, which causes acidification of the soil solution. On buffered chernozem soils, especially on southern and ordinary ones, this circumstance has not a negative, but rather a positive significance, because the acidification caused by it contributes to the mobilization of a significant amount of soil nutrients.

On acidic sod-podzolic soils, ammonium sulfate increases not only acidity but also the content of mobile forms of aluminum, manganese, and iron, which, upon reaching a certain concentration, harm plants and reduce their productivity. The toxic effect of ammonium sulfate is amplified by its long-term use in the same place. Therefore, on poorly cultivated acidic sod-podzolic soils without liming, ammonium sulfate is significantly inferior in its effectiveness to nitrate forms of nitrogen fertilizers. The acidifying effect of ammonium sulfate most strongly affects crops sensitive to soil acidity: clover, wheat, barley, beet, and cabbage. For them, this fertilizer is less effective than nitrate. It can be applied on acidic soils only on the condition of their liming or neutralization of the fertilizer at the rate of 1.2 kg of crushed limestone or 2 kg of chalk per 1 kg of ammonium sulfate. On acidic soils, it is good to combine its application with phosphate rock.

Ammonium sulfate is a highly effective nitrogen fertilizer for potatoes, as this crop is not sensitive to soil acidification. It is recommended for application under rice and other irrigated crops, where significant nitrogen losses are possible. As a basal fertilizer, ammonium sulfate can be used on heavy soils not only in spring, but also in autumn, without fear of nitrogen leaching. When applied in rows and as top dressing, ammonium sulfate is less effective compared to other nitrogen fertilizers.

Polymer-coated ammonium sulfate. It contains 20% nitrogen and 24% sulfur, and it takes more than three times longer to dissolve in water than uncoated ammonium sulfate. It is recommended for use on hayfields and pastures located on light soils, as well as in areas of irrigated agriculture.

Traditionally used in agriculture, ammonium sulfate, due to its high solubility, contributes to some extent to environmental pollution. It does not always provide plants with nitrogen during critical periods of their growth and development due to losses resulting from leaching. The use of polymer-coated ammonium sulfate virtually eliminates these drawbacks and reduces nitrogen leaching even at high application rates.

Ammonium chloride (NH4Cl) is a finely crystalline white or yellowish salt containing 24-25% nitrogen and 66% chlorine.

Ammonium chloride is a water-soluble fertilizer. 37.4 g of ammonium chloride dissolves in 100 cm3 of water at 20 °C. The density is 0.6 t/m3. It possesses good physical properties, is slightly hygroscopic, does not cake during storage, and is easily spread by machinery.

Ammonium chloride is obtained as a byproduct of the ammonia-soda process:

NH4HCO3 3 + NH4Cl or H2 3 + CO2 3 + NH4Cl.

A solution of NaCl is treated first with NH3, then with CO2. After centrifugation during product drying, NaHCO3 converts into Na2CO3. The remaining NH4Cl solution is evaporated until crystallization.

The amount of ammonium chloride available for fertilizer, after accounting for industrial needs, is determined by the scale of soda production. Thus, while it is present in the nitrogen fertilizer assortment, its quantity is limited compared to other fertilizers.

Ammonium chloride applied to the soil dissolves quickly and enters into exchange reactions just like ammonium sulfate. In chernozem soils, the exchange reaction follows this scheme:

[Soil Adsorption Complex]Ca + 2NH4Cl ⇄ [Soil Adsorption Complex]2NH4 + CaCl2, and in sod-podzolic soil:

[Soil Adsorption Complex]H + NH4Cl ⇄ [Soil Adsorption Complex]NH4 + HCl.

Consequently, under the influence of applied ammonium chloride, the acidity of the soil solution increases.

Ammonium chloride, like ammonium sulfate, undergoes nitrification in the soil. However, the oxidation of ammonium to nitric acid is somewhat slower, which is associated with the negative effect of the chlorine ion on the activity of nitrifying bacteria. Furthermore, ammonium chloride, like ammonium sulfate, possesses physiological acidity because plants absorb the NH4+ cation faster than the Cl– anion. The accumulation of acid residue in weakly buffered soil leads, as in the case of ammonium sulfate, to soil acidification, deterioration of its physical properties, and a decrease in biological activity. Therefore, all measures to increase the efficiency of ammonium sulfate apply equally to ammonium chloride. These include liming of the soil, pre-neutralization of the fertilizer (adding 1.4 centners of CaCO3 per 1 centner of NH4Cl), using it with alkaline salts, and combining it with organic fertilizers.

Ammonium chloride contains more nitrogen than ammonium sulfate, and its acidifying effect on the soil is the same. However, the significant amount of chlorine introduced when fertilizing with ammonium chloride necessitates caution regarding chlorine-sensitive crops (potatoes, tobacco, grapes, clover, buckwheat, sunflower, lupine, flax, hemp, vegetables). For example, potatoes may show a starch content reduction of 0.7–1.5%, and tobacco may suffer from decreased leaf burn quality. Such a significant reduction in harvest quality is not observed in other crops.

On light and acidic soils, the negative impact of chlorine is more pronounced than on heavier soils with a neutral or alkaline environment.

Ammonium chloride should not be applied at high rates to chlorine-sensitive crops. It should be applied well in advance, in the autumn. In this case, chlorine ions are easily leached from the root zone by autumn and spring atmospheric precipitation.

In terms of efficiency, ammonium chloride is not inferior to ammonium sulfate when applied to chernozems for sugar beet, as well as when used at average application rates on sierozems for cotton.

Ammonium sodium sulfate (NH4)2SO4∙Na2SO4 is a yellow crystalline salt. It is a byproduct of caprolactam production and contains 16% nitrogen and 9% sodium (Na2O).

Ammonium sodium sulfate has fairly good physical properties: it is low-hygroscopic, almost non-caking, and spreads well. The density is 0.8 t/m3. This fertilizer is physiologically acidic, as plants consume the NH4+ cation from this salt more predominantly than the Na+ and SO42– ions. Due to the presence of sodium, the acidifying effect of this fertilizer on the soil is somewhat weaker than that of ammonium sulfate.

In the soil, ammonium sodium sulfate is bound into compounds that are low-mobility but accessible to plants, and it is insignificantly leached by atmospheric precipitation and irrigation water. In warm weather, as an ammonium fertilizer, it can nitrify, after which the nitrogen can be leached in the form of nitrates. When applying ammonium sodium sulfate to acidic soils, liming or neutralization of the fertilizer itself is necessary. To eliminate the acidifying effect, 1 kg of crushed limestone is added per 1 centner of this fertilizer.

Ammonium sodium sulfate serves as a good fertilizer for sugar beet and plants of the cabbage family, which are responsive to sulfur and sodium. It is also used for top dressing hayfields and pastures.

Ammonium carbonate* (NH4)2CO3 is a neutral ammonium carbonate salt containing 21–24% nitrogen. In appearance, it is a white crystalline substance. It is obtained in two ways: by saturating ammonia water with carbon dioxide followed by distilling off the ammonium carbonate at a temperature of 70–80 °C, and by the interaction of gaseous ammonia and carbon dioxide in the presence of water vapor.

Ammonium carbonate is a very unstable product; during storage in the open air, it easily decomposes with the release of volatile ammonia, gradually turning into an acidic ammonium carbonate salt – ammonium bicarbonate (NH4HCO3). The longer the storage of ammonium carbonate and the higher the air temperature, the greater the nitrogen losses. To reduce the volatility of ammonia, several storage methods have been proposed: compacting the fertilizer, coating it with resinous substances, and covering it with a sodium chloride solution.

Ammonium carbonate has an alkaline reaction, but when applied to the soil, it undergoes the process of nitrification, quite rapidly turns into a nitrate form, and slightly acidifies the soil solution. In its effect on plants, it is close to ammonium nitrate and urea. It has an advantage over ammonium sulfate as it possesses lower physiological acidity. Despite this, ammonium carbonate has not gained widespread use as a fertilizer due to its high instability during storage. It can be used on all types of soil and for all crops that need nitrogen. It is most effective against a background of phosphorus and potassium fertilizers, as well as on limed soils.

Ammonium bicarbonate (NH4HCO3) is an acidic ammonium carbonate salt containing 17.7% nitrogen. It is obtained through the adsorption of gaseous ammonia and carbon dioxide by a solution of ammonium carbonate:

Fertilizers marked with an asterisk are currently not produced.

(NH4)2CO3 + H2O + CO2 = 2NH4HCO3. White crystalline ammonium bicarbonate precipitates. The remaining ammonium bicarbonate in the solution is saturated with gaseous ammonia:

NH4HCO3 + NH3 = (NH4)2CO3.

The ammonium carbonate formed in the solution is reused for the production of ammonium bicarbonate.

Ammonium bicarbonate is used in agriculture as a nitrogen fertilizer in limited quantities, because this compound is unstable and easily decomposes at ordinary temperatures, which leads to significant losses of nitrogen. During 5–6 months of storage in the spring-summer period, nitrogen losses in the form of ammonia can reach 20–40%. Ammonium bicarbonate is applied to the soil surface and immediately incorporated to a depth of 10–12 cm. In the soil, it decomposes:

NH4HCO3 → NH3 + H2O + CO2.

Ammonium bicarbonate has an alkaline reaction, but as a result of the ammonium nitrification process, it acidifies the soil solution. Ammonium bicarbonate has extremely poor flowability, which hinders its mechanized application. In terms of efficiency, this fertilizer is close to ammonium nitrate and urea. It possesses slightly greater stability compared to ammonium carbonate. Its advantage over ammonium sulfate is the absence of ballast components and lower physiological acidity.