Application and storage specifics of the nitrogen-calcium fertilizer calcium cyanamide

Calcium cyanamide is a nitrogen-calcium fertilizer that is optimal for application on acidic soils. Unlike standard rapidly soluble nitrogen forms, it acts gradually, preventing the leaching of nutrients and providing the crop with nutrition throughout the entire growing season. However, due to the high toxicity of the intermediate decomposition products, working with this fertilizer requires strict adherence to the application procedure and safety measures.

• Nitrogen content — 20–22 %
• Free calcium oxide content — up to 28 %
• Interval before sowing — at least 7–10 days
• Decomposition period in soil — 1–2 weeks
• Fertilizer bulk density — 0.6 t/m³

How calcium cyanamide works in the soil

When applied to the soil, calcium cyanamide undergoes hydrolysis and interacts with the soil absorption complex (SAC). During this process, free cyanamide is formed, which in moist, slightly acidic soil quickly turns into urea accessible to plants without the participation of microorganisms. On acidic sod-podzolic soils, the fertilizer significantly improves agrochemical and physicochemical properties, as the free calcium oxide it contains neutralizes excess acidity.

The agronomist must take into account the high toxicity of free cyanamide to germinating seeds and seedlings. If the fertilizer is unevenly distributed over the field, a strongly alkaline reaction occurs in areas of its accumulation, triggering a polymerization process with the formation of dicyandiamide. This stable compound is toxic to seedlings, and the nitrogen it contains becomes practically unavailable for uptake by the crop. To avoid the harmful effects of undecomposed compounds on seedlings, strictly follow the application procedure.

1. Apply the fertilizer to the soil in advance — best of all in the autumn for winter ploughing or in the spring at least 7–10 days before sowing.
2. Ensure high-quality incorporation into the moist soil layer to initiate hydrolysis processes.
3. Maintain an interval of 1–2 weeks before sowing for the complete conversion of toxic cyanamide into safe, assimilable forms.

Storage rules and the advantages of slow-release nitrogen

Technical calcium cyanamide is produced at temperatures of 1000–1100 °С by passing gaseous nitrogen through crushed calcium carbide. The result is a dark gray or black powder with a faint smell of kerosene. To reduce dusting during production, 1 to 3 % of mineral oil is added to the product.

Under normal conditions, dry calcium cyanamide has low hygroscopicity, does not cake, and is easy to spread. However, when stored in conditions of high humidity and free access to carbon dioxide, the fertilizer decomposes into free cyanamide and calcium carbonate. This process is accompanied by an increase in mass volume and nitrogen loss, so the product can only be stored in dry rooms and in airtight, waterproof packaging.

Unlike standard rapidly soluble nitrogen fertilizers, which at high doses can sharply increase the osmotic pressure of the soil solution and be leached by precipitation or irrigation water, calcium cyanamide belongs to the group of slow-release fertilizers. The gradual conversion of nitrogen into an assimilable form prevents its losses from leaching into deep soil layers and reduces gaseous losses resulting from nitrification and denitrification. This allows the entire required nitrogen application rate to be applied in one go for the entire growing season without the risk of scorching the root system.

Types and characteristics of slow-release nitrogen fertilizers

Low-solubility nitrogen forms are the most promising in rice farming and in regions with excessive moisture. Such fertilizers are indispensable for crops sensitive to high salt concentrations in the soil solution and increased osmotic pressure. In agrochemistry, the development of this direction is proceeding in three ways: synthesis of compounds with limited solubility (urea-forms), encapsulation of standard granules with protective coatings, and the introduction of nitrification inhibitors.

The main group of slow-release nitrogen fertilizers is produced by the condensation of urea with aldehydes. The most common among them is urea-formaldehyde fertilizer (UFF, carbamide-form, or ureaform). It is synthesized in acidic solutions at a pH of 3 and a temperature of 30–60 °C. During the reaction, urea molecules are linked by methylene groups, forming methoxyl groups. The process occurs sequentially: first, monomethyl urea is formed, then it condenses into methylenediurea, which converts into polymethylene urea. The resulting condensate is filtered, dried, and ground.

In addition to UFF, other slow-release nitrogen compounds are used in practice. They differ in their raw materials, content of active ingredient, and dissolution rate. Detailed characteristics of these fertilizers are provided in the table:

Encapsulated nitrogen fertilizers form a separate group. To produce them, granules of standard water-soluble forms are coated with thin films of wax, paraffin, oils, resins, or polymers. Water penetrates these coatings slowly, turning a fast-acting fertilizer into a slow-release one.

Behavior in the soil and application efficiency

Nitrogen from poorly soluble compounds converts into available forms gradually, ensuring the nutrition of crops over several years. Under the influence of soil biochemical and physicochemical processes, the molecules break down into ammonium, which is then converted into nitrates. The rate of this transition depends on both soil properties and the quality of the fertilizer itself.

The main indicator of UFF quality is the digestibility index — the percentage of water-insoluble nitrogen that passes into solution when the fertilizer is boiled for 1 hour. Depending on the synthesis parameters, this index ranges from 15 to 55%. In some countries, instead of this, they evaluate the share of nitrogen that nitrifies in the soil over 6 months. The higher the UFF digestibility index, the faster and more actively nitrate nitrogen accumulates in the soil.

The use of slow-release nitrogen forms provides the agronomist with three key advantages:

• The ability to apply the entire calculated nitrogen rate for the planned harvest in a single pass, significantly reducing the costs of repeated machinery passes.
• Protection of nitrogen from losses: due to its low solubility, it does not volatilize, does not leach into deep soil layers, and converts more slowly into unavailable organic matter.
• Increasing the nitrogen use efficiency by crops and preventing the excessive accumulation of nitrates in the final product.

• paraffin;
• polyethylene emulsion;
• sulfur compounds;
• acrylic resin;
• polyacrylic acid and other substances.

Such granular fertilizers, coated with films, possess improved physical and mechanical properties: they are less hygroscopic, mechanically stronger, and do not cake during storage. By adjusting the composition and thickness of the coatings, it is possible to obtain fertilizers with different rates of nitrogen release, i.e., slow-release, taking into account the biological requirements and the periodicity of nitrogen nutrition of agricultural crops. Encapsulated nitrogen fertilizers are utilized by plants more evenly during the growing season, which positively affects crop yields and product quality, such as the protein content in the grain of cereals.

Nitrogen-sulfur-calcium fertilizer. Obtained by pressing urea with phosphogypsum. Nitrogen content is at least 30%, sulfur – at least 7%.

Urea with a humic coating. Contains 46% nitrogen. The static strength of the granules is somewhat higher than that of standard fertilizer without humins. The positive effect of the fertilizer is enhanced by including a biological growth stimulant – oxyhumate or hydrohumate – in the composition of the coating.

Phosphate-coated urea. A slow-release nitrogen fertilizer containing at least 39% nitrogen and over 7% P2O5. It dissolves in water 1.5 times slower than uncoated urea.

Polymer-coated urea. Contains at least 42% nitrogen; it dissolves in water twice as slowly as standard urea.

Nitrification inhibitors most commonly include cyanguanidine (dicyandiamide), the American product 2-chloro-6-(trichloromethyl)pyridine, and the Japanese product AM (2-amino-4-chloro-6-methylpyrimidine). When applied in a mixture with urea at rates of 0.5–1% and 1–3% AM of the fertilizer nitrogen amount respectively, these inhibitors suppress nitrification processes for 1.5–2 months, i.e., during the period of intensive nitrogen uptake by plants. The decomposition rate of inhibitors in the soil, and consequently the duration of their action, depends most significantly on the particle-size distribution of the soil, its humidity, reaction, temperature, and humus content.

By suppressing the nitrification of fertilizer nitrogen, inhibitors reduce its losses in gaseous form, via surface water runoff, and as a result of nitrate leaching. This leads to a significant increase in the efficiency of nitrogen fertilizers and crop yields under irrigation or in areas with high humidity. The use of inhibitors helps improve product quality by preventing the accumulation of nitrates in agricultural produce in quantities toxic to humans and animals, reducing the risk of certain plant diseases, and providing the possibility to lower nitrogen fertilizer rates due to an increased nitrogen utilization coefficient. In this case, it is possible to replace split nitrogen fertilizer application with a single application, which increases the economic efficiency of their use.

Liquid ammonia (NH3). The most concentrated nitrogen fertilizer. It contains 82.3% nitrogen, with the remainder being hydrogen. This fertilizer is a gas liquefied under a pressure of 1.6 MPa at a temperature of 40 °C. It is explosive when mixed with air or oxygen, and causes severe burns upon contact with the skin. In appearance, it is a colorless liquid. Inhalation of air containing ammonia in a ratio of 1:10000 is fatal. In the air, ammonia boils and evaporates rapidly. It is stored in special steel tanks and produced by liquefying gaseous ammonia. At atmospheric pressure, ammonia is a volatile gas. Therefore, it must be incorporated into the soil to a depth of at least 15 cm using injectors. Once applied to the soil, anhydrous ammonia changes from liquid to gas, which is adsorbed by the colloidal fraction and absorbed by soil moisture, forming ammonium hydroxide. By interacting with anions of the soil solution, ammonium forms various salts and, entering into physico-chemical interaction with soil colloids, is absorbed by the solid part of the soil. Simultaneously with physico-chemical transformations, ammonia undergoes nitrification:

[SEC]H + NH3 → [SEC]NH4,

NH3 + H2O → NH4

2NH3 + 3O2 = 2HNO2 + 2H2O,

2HNO2 + O2 = 2HNO3.

The rate and degree of ammonia absorption by the soil depend on its humus content, particle-size distribution, and humidity, as well as the method and depth of placement of the fertilizer. The higher the content of silt fraction and humus in the soil, the faster ammonium ions are absorbed. In the first days after the application of liquid ammonia, the soil reaction shifts towards alkalization as a result of ammonium hydroxide formation. In the zone of fertilizer application (injections), a high concentration of ammonia causes temporary soil sterilization, which, in particular, halts the nitrification process of ammoniacal nitrogen. However, after 1–2 weeks, the activity of microorganisms resumes, facilitated by the abundance of nitrogen in the soil. Approximately half of the ammonia is converted into nitrates at a temperature of 10 °C within 15–30 days. This contributes to a decrease in pH, which is why the fertilizer will have a physiologically acid reaction. Fluctuations in pH have a positive effect on the availability of phosphates and trace elements in the soil to plants. Application of ammonia into the root-inhabiting soil layer can lead to partial destruction of root hairs and root tissues. Its negative effect persists until it is converted into ammonium. This lasts from several hours to 10–15 days, after which it begins to be absorbed by the root system of plants. The amount of absorbed ammonium depends on soil and climatic conditions, so it is advisable to apply liquid ammonia before the sowing of crops. In sandy and sandy-loam soils, the formation of ammonium salts from ammonia and the adsorption of the ammonium ion occur more slowly than in loamy soils. In this regard, in light soils, the fertilizer persists in the form of NH3 for a long time and may volatilize. Ammonia volatilizes less from moist soil than from dry soil. To avoid nitrogen losses, anhydrous ammonia is placed at a depth of 12–14 cm in sod-podzolic loamy soils and at a depth of 16–18 cm in sandy-loam soils. For application, special machines such as ABA-0.5M, ABA-1, ASHA-2, and others are used in an assembly with a KRN-4.2 cultivator. Anhydrous ammonia can be used as a basal fertilizer and for top dressing, with mandatory incorporation into the soil.

The agrochemical value of ammonia is practically equivalent, and in some cases higher than an equivalent amount of nitrogen applied with solid nitrogen fertilizers. The main advantage of liquid ammonia over other forms of nitrogen fertilizers is the lower cost per unit of nitrogen (40% lower) and the possibility of complete mechanization from transport to application into the soil. The value of this fertilizer is reduced due to higher equipment costs, as well as limited application.

Aqueous ammonia (ammonia water) is a solution of ammonia in water. It is a colorless or yellowish liquid. The industry produces two grades: the 1st grade contains  20.5% nitrogen or 25% NH3, freezing point -56 °C; the 2nd grade contains  18% nitrogen or 22% NH3, freezing point -33 °C.

Aqueous ammonia is characterized by low ammonia vapor pressure (25% aqueous ammonia – 0.15 kg·f/cm² at 40 °C), does not corrode ferrous metals, and freezes only at a very low temperature. Therefore, it can be stored and transported in sealed tanks made of ordinary carbon steel, designed for low pressure (0.03–0.05 MPa). In aqueous ammonia, nitrogen is present in the form of free ammonia (NH3) and ammonium (NH4OH). The equilibrium constant between NH3 and NH4OH in an aqueous ammonia solution [(NH4 3+H2O] = 10–5 shows that it contains significantly more free ammonia than ammonium. This accounts for the possibility of nitrogen losses during transportation, storage, and fertilizer application due to ammonia volatilization.

To avoid damaging plants, aqueous ammonia is applied to a depth of at least 8–10 cm, at a distance of 10–12 cm from plants, as it is a strong alkali. When applying equal nitrogen application rates, the effectiveness of ammonia and aqueous ammonia is equal to the effectiveness of solid nitrogen fertilizers.

On soils with a heavy particle-size distribution, aqueous ammonia, like anhydrous ammonia, can be applied in the fall as a basic fertilizer for almost all crops when the soil temperature is below +10 °C, as well as in the spring before sowing. On soils with a light particle-size distribution, it is desirable to apply these fertilizers in the spring.

When aqueous ammonia is applied to the soil, ammonia is adsorbed by colloids and therefore moves poorly within it. Over time, ammoniacal nitrogen undergoes nitrification and gains greater mobility. Using aqueous ammonia as a fertilizer is technically simpler and safer compared to liquid ammonia, but its major drawback is the low nitrogen content, which increases the costs associated with transporting, storing, and applying the fertilizer into the soil. Therefore, the use of aqueous ammonia is advisable only in farms located near enterprises that produce this fertilizer.

Aqueous ammonia is applied using special machines (POU) equipped with a ULP-8 device for incorporation into the soil. Considering that ammonia moves 10–12 cm in the soil, for crops sown in continuous rows, the distance between openers when applying ammonia fertilizers should be no more than 20–25 cm, and for row crops it should be equal to the width of one row spacing. It is most effective to apply these fertilizers together with organic ones. Ammonia fertilizers should not be applied to the same plot for several years in a row, as they increase the mineralization of organic matter, which can lead to a decrease in its content in the soil.

Urea-ammonia liquor is an ammonia-water solution of carbonate [(NH4)2CO3 and CO(NH2)2], containing 20–25% NH3, 19–28 2)2, 7–12% CO2, at least 29% nitrogen; crystallization temperature – 10 °C.

Urea-ammonium nitrate fertilizer (UAN) is a solution (mixture) of urea and ammonium nitrate containing 28–32% nitrogen. It is one of the promising fertilizers. The industry produces three forms of the preparation:

Application and storage of urea-ammonium nitrate (UAN)

The use of urea-ammonium nitrate (UAN) allows for the optimization of costs for plant nitrogen nutrition. The cost of a unit of nitrogen in UAN is lower than in solid fertilizers due to the elimination of energy-intensive stages of evaporation, granulation, and condensation. The fertilizer is produced from urea and ammonium nitrate melts using a traditional or more economical integrated scheme with simplified production of urea melt and neutralization of residual ammonia with nitric acid. The finished product is a clear or yellowish liquid with a density of 1.26–1.33 g/cm³ and a neutral or slightly alkaline reaction. Since there is practically no free ammonia in the mixture, it can be applied without immediate incorporation into the soil, and can also be distributed with irrigation water.

Due to the low crystallization and freezing temperature (from -2 to -18 °C), UAN solutions can be transported and stored all year round. Regular road and rail tankers (provided there are corrosion inhibitors), pipelines, or water transport are suitable for this purpose. On farms, liquid fertilizer is stored for up to 6 months in metal containers or in sunken concrete and asphalt storage facilities with an internal lining. During production, a small amount of phosphates is also added to UAN to protect equipment from corrosion.

UAN is suitable for all agricultural crops for both basal application and top dressing. The fertilizer can be applied to the surface, locally in bands with incorporation, or into the soil. For surface broadcast distribution, sprayers such as OPSH-15, POM-630, POM-2000, or PZhU-9 are used. For local subsurface application, PZhU-2.5 and PZhU-5 machines are utilized. The liquid form guarantees high dosing accuracy and uniform nitrogen distribution across the field.

Fractional application of UAN is particularly effective on winter and spring grain crops according to the growing season phases:

1. First early spring top dressing of winter crops. Carried out at the beginning of the growing season on thawed soil. At this stage, UAN can be used in concentrated form, without dilution with water.
2. Second top dressing during the stem elongation phase. The optimal air temperature for application is 16–18 °C. The fertilizer dose should not exceed 20–30 kg/ha. To avoid chemical leaf burns, be sure to dilute UAN with water in a 1:2 or 1:3 ratio, especially if herbicides are added to the tank mixture.
3. Top dressing at the beginning of the grain heading phase. Nitrogen is applied at a dose of no more than 15–20 kg/ha. A concentrated UAN solution, especially when applied together with fungicides, must be diluted with water in a 1:2 or 1:3 ratio.

Advantages of using UAN compared to solid nitrogen fertilizers:

• full mechanization of all loading and unloading operations in the warehouse and in the field;
• minimization of active ingredient losses during storage and application;
• reduction of total costs for fertilizer production and application;
• improvement of working conditions for personnel;
• elimination of fertilizer caking during long-term storage;
• high uniformity of nitrogen distribution across the field area;
• ease of preparing tank mixtures with microelements and pesticides.

Ammoniates: rules for working with volatile liquid fertilizers

Ammoniates are liquid nitrogen fertilizers of light yellow or yellow color with a nitrogen content of 30 to 50%. They are produced in factories by introducing ammonium nitrate solutions, urea, or their mixtures into ammonia water. Nitrogen in ammoniates exists in two forms: 20–40% is represented by free ammonia, and 60–80% by nitrogen from ammonium nitrate or urea. Due to the presence of free ammonia, these liquids have high volatility. At a temperature of 32 °C, the vapor pressure of ammonia, depending on the fertilizer grade, ranges from 2.0 to 3.6 atm.

The crystallization onset temperature of ammoniates varies significantly — from 14 to 70 °C. For this reason, factories produce grades with a low crystallization temperature for winter storage and higher ones for summer use. Ammoniates cause severe corrosion of copper alloys and ferrous metals. Only tanks made of alloyed steels, aluminum and its alloys, or standard steel reservoirs with a special internal protective coating are suitable for their transportation and storage.

When applying ammoniates into the soil, it is critically important to maintain the correct depth of incorporation. Free ammonia evaporates rapidly, and its diffusion in the soil layer usually does not exceed 8–10 cm. For efficient nitrogen distribution, the distance between the openers during application should not exceed 20–25 cm. When top-dressing row crops, the openers must be adjusted strictly according to the row spacing. When the application technology is followed, ammoniates are fully equivalent to solid nitrogen fertilizers in terms of effectiveness and impact on the harvest.

• Nitrogen content in ammoniates — 30–50%
• Ammonia diffusion in soil — up to 8–10 cm
• Distance between openers during application — up to 20–25 cm
• Vapor pressure at 32 °C — from 2.0 to 3.6 atm
• Crystallization temperature — from 14 to 70 °C