Classification and properties of mineral and organic fertilizers in agrochemistry

Direct and indirect action of fertilizers

The efficiency of plant nutrition depends on how fertilizers interact with the soil complex. Based on the nature of this effect, all agents are divided into direct and indirect. Direct fertilizers supply nutrients directly to plants, while indirect ones improve the physical and chemical properties of the soil itself. However, such a division is arbitrary, since almost any preparation exerts a bidirectional influence on the soil environment.

Direct mineral and organic fertilizers inevitably change the agrochemical indicators of the field, and this effect can be both positive and negative. At the same time, indirect fertilizers often work as direct ones as well. For example, lime and gypsum not only eliminate excessive acidity or alkalinity but also saturate the soil with calcium, which is necessary for plant nutrition.

Classification by origin and physical properties

According to the production method, fertilizers are divided into industrial and local. Industrial fertilizers include products of chemical synthesis and mineral processing, bacterial preparations, as well as organic and organo-mineral mixtures produced by factories. Local fertilizers are prepared directly on farms or in their vicinity. These include manure, liquid manure, poultry manure, excrement, composts, peat, ash, calcareous tuffs, pond or lake silt, green manure, as well as waste from metallurgical and hydrolysis industries.

According to their composition and origin, all fertilizers are divided into four key groups. They differ significantly in the concentration of active ingredients, their rate of absorption, and their influence on soil structure. The choice of a specific group depends on the agrochemical indicators of the field and the planned yield.

• Mineral — industrial products or mineral salts. They account for about 60% of the total balance of nutrients applied to the soil.
• Organic — fresh or biologically processed substances of plant and animal origin.
• Organo-mineral — chemical or adsorption compounds of organic matter with mineral salts.
• Bacterial — pure cultures of microorganisms that activate soil processes.

Mineral fertilizers are classified by the content of active ingredients. Depending on the volume of plant consumption, nutrients are divided into macro-, meso-, and micronutrients. A detailed classification of mineral components by these groups is provided below.

Organo-mineral fertilizers are obtained by treating humic-containing raw materials with ammonia, phosphoric acid, or by mixing them with dry mineral fertilizers. Their production utilizes humus (well-rotted manure), peat composts with liquid manure and poultry manure, as well as weathered lowland peat. Any forms of nitrogen, phosphorus, and potassium fertilizers, as well as lime materials, are used as the mineral component.

The physical form of fertilizers directly affects the choice of application machinery and the uniformity of distribution over the field area. In terms of aggregate state, mineral fertilizers are produced as solids, liquids, or suspensions. Regarding their internal structure, they can be powder, crystalline, or granular. The total amount of any fertilizer applied to a crop over its entire growing season constitutes the application rate.

Soil acidity and calculation of the active ingredient

Mineral fertilizers directly affect soil acidity. Based on the nature of this effect, they are divided into physiologically acidic, alkaline, and neutral. Physiologically acidic fertilizers acidify the soil solution, as plants absorb cations from them more actively, leaving anions behind. Conversely, with alkaline fertilizers, anions are absorbed faster, while the accumulating cations bind with the hydroxyl group and alkalinize the environment. Neutral fertilizers do not shift the pH level.

According to the concentration of nutrients, fertilizers are divided into four classes. The active ingredient is defined as the nutrient element itself, calculated in terms of dry matter or oxide. In nitrogen fertilizers, it is calculated based on nitrogen (N), in phosphorus fertilizers — based on phosphorus pentoxide (P₂O₅), in potassium fertilizers — based on potassium oxide (K₂O), in magnesium fertilizers — based on MgO, and in micro-fertilizers — based on the corresponding microelement.

• Low-concentration — less than 25% active ingredient
• Concentrated — from 25 to 60% active ingredient
• Highly concentrated — from 60 to 100% active ingredient
• Ultra-concentrated — more than 100% active ingredient

Special coefficients are used to convert values from oxides and salts into pure elements and back. This is necessary for accurate calculation of dosages when preparing tank mixtures and nutrient application plans. The table below lists all conversion coefficients.

Classification of compound fertilizers and physical properties of fertilizers

Based on their composition, mineral fertilizers are divided into single-component (simple, one-sided) and compound (multi-sided). Simple fertilizers contain only one limiting nutrient element — for example, nitrogen, phosphorus, potassium, or a specific meso- or microelement. Compound fertilizers combine at least two key nutrient elements.

In the labeling of compound fertilizers, the content of nutrient elements is indicated by digits separated by hyphens (e.g., 17-17-17-2(B)). The first digit reflects the percentage of nitrogen (N), the second — phosphorus pentoxide (P₂O₅), the third — potassium oxide (K₂O), and missing components are denoted by zero. If a mixture contains microelements, they are indicated as the fourth element. Active elements are abbreviated as NPK for three-component mixtures or NP, NK, PK for two-component mixtures, and their ratio is calculated by taking nitrogen as unity (for the 17-17-17 grade, the ratio is 1:1:1).

According to the production method, compound fertilizers are divided into four groups:

• Complex — chemical compounds containing two or more limiting elements, obtained in a single technological cycle through the chemical interaction of the source components.
• Combined — fertilizers where each granule contains separate chemical compounds of nutrient elements.
• Mixed — mechanical mixtures of simple fertilizers.
• Complex-mixed — mixtures obtained by adding one-sided fertilizers to complex ones.

In agrochemistry, it is important to distinguish between the concepts: "fertilizer type" defines the category by active ingredient, while "form" describes its chemical composition. When working in the field, one must consider the physicochemical and mechanical properties of the fertilizers. The application quality depends on their hygroscopicity, caking, granulometric (fractional) composition, average particle size, granule strength, angle of repose, water-holding capacity, true and bulk density, composition homogeneity of fertilizer mixtures, segregation, spreadability, salt composition, crystalline structure, solubility, vapor pressure, and thermodynamic characteristics.

Hygroscopicity is of particular importance — the ability of the fertilizer to absorb moisture from the air. It is evaluated by the hygroscopic point (h in %). For water-soluble salts, this value is calculated as the ratio of the partial vapor pressure over a saturated salt solution (P_a) to the water vapor pressure at the moment of air saturation (P) at the same temperature, multiplied by 100: h = (P_a / P) × 100.

The hygroscopic point corresponds to the equilibrium relative humidity, i.e., the relative humidity at which the substance neither absorbs nor loses moisture. At the maximum permissible moisture content, the fertilizer should have a hygroscopic point no higher than the average annual relative air humidity.

Hygroscopicity is assessed on a 10-point scale. Calcium nitrate (up to 9.5 points) and ammonium nitrate (9.3) possess high hygroscopicity; granular double superphosphate (4.7), urea (3.6), and potassium chloride (3.2–4.4 points) are medium to weakly hygroscopic. High hygroscopicity causes fertilizers to cake, granules become brittle, and flowability and spreadability deteriorate. Conditions for storage, transportation, and packaging of fertilizers depend on their hygroscopicity. Only weakly hygroscopic fertilizers (with a score of  3) are allowed to be transported and stored without containers, whereas highly hygroscopic fertilizers (7–10 points) are stored in polyethylene bags.

Caking is a property of a fertilizer to lose flowability and form agglomerates of various sizes and strengths. Caking manifests itself only after a certain period of fertilizer storage under static conditions. Fertilizer caking depends on their humidity, hygroscopicity, particle-size distribution, as well as storage conditions and duration. Caking is determined by the resistance to crushing of the caked fertilizer. The degree of caking is assessed on a seven-point scale (Table 61; Kochetkov V.N., 1982).

Table 61 – Conventional scale of fertilizer caking Resistance to crushing Score Degree of agglomerate caking, kPa

 I Slightly caking >98.1 II Weakly caking 98.1–196.2 III Somewhat caking 196.3–392.4 IV Moderately caking 392.5–686.7 V Significantly caking 686.8–981.0 VI Strongly caking 981.1–1471.5 VII Very strongly caking >1471.5

Highly caking fertilizers include fine-crystalline potassium chloride – 6 points. The caking of urea, ammonium sulfate, and ammonium nitrate (1–3 mm fraction) is estimated at 1–2, 2–3, and 3–4 points, respectively. Potassium sulfate and potassium magnesium sulfate practically do not cake.

The caking of water-soluble mineral fertilizers can be reduced by:

• granulation;
• reducing moisture content;
• increasing granule strength;
• cooling the product before storage or packing;
• conditioning it;
• using hermetic packaging;
• creating necessary storage conditions.

Flowability is the property of fertilizers to flow freely in a continuous stream under the influence of gravitational forces.

Spreadability is the ability of a fertilizer to enter the metering and spreading devices of application machinery and to be distributed evenly over the soil surface. The uniformity of fertilizer feed to the spreading devices is determined by their flowability. The quality of fertilizer distribution by modern machines during surface application depends on the fractional composition of the fertilizers. Spreadability depends on particle-size distribution, flowability, and granule strength. It is assessed on a 12-point scale: the better the spreadability, the higher the score (Table 62; Kochetkov V.N., 1982).

Table 62 – Fertilizer spreadability scale in points

The uniformity of fertilizer distribution over the soil surface depends on the fertilizer flowability and the design of the machines applying the fertilizers.

Particle-size (fractional) composition, or grinding fineness (particle size), is determined mechanically by sieving fertilizers through a set of screens. Fertilizer caking and spreadability depend on the density of coarse and fine fractions. When fertilizers with a uniform particle-size distribution are applied by centrifugal machines, they flow evenly into the metering device and are distributed across the machine's working width.

The average size of particles similar in size is determined by the equation: D 1, where: D is the average particle diameter; D1 is the diameter of the sieve opening through which the entire mass of material passes; D2 is the diameter of the sieve opening that retains the particles.

Granule strength determines the preservation of the fertilizer's particle-size distribution during transportation, storage, and application to the soil. It primarily depends on humidity, size, shape, and density of crystals, and the nature of the intergrowth contacts.

Granule strength is characterized by dynamic strength (Pd), abrasion resistance (Ri), and static strength (Pc). Dynamic strength mainly characterizes granule brittleness. It is assessed by the number of crushed granules after dropping them from a certain height onto a hard surface; abrasion resistance is characterized by the proportion of surviving granules during testing. Static strength is characterized by the average ultimate strength under uniaxial compression. Static strength varies within 0.5–8.0 MPa; it depends actively on humidity, i.e., it is significantly more sensitive (compared to Pd and Ri) to changes in the physical and chemical structure of the fertilizer granules.

The angle of repose is the angle formed between the horizontal plane and the slope plane of a pile of bulk fertilizer. This indicator is taken into account when constructing warehouses where fertilizers are stored in bulk, designing hoppers, and choosing transport vehicles. The angle of repose is also an indicator of fertilizer spreadability.

Leaching of fertilizers is the leaching of water-soluble components from fertilizers during storage in the open air. In laboratory conditions, leaching is used when it is necessary to extract plant nutrients from solid fertilizers.

Fertilizer moisture capacity is the maximum amount of moisture that a fertilizer is capable of retaining. It corresponds to the maximum humidity that allows the fertilizer to be satisfactorily spread by fertilizer drills. Sorption moisture capacity of fertilizers is determined by the maximum mass of moisture absorbed from the air at a given humidity and temperature. "Buffer" moisture capacity determines the mass of moisture that can be added to a standard (in terms of moisture content) fertilizer without impairing its spreading ability.

Bulk density is the mass per unit volume of a bulk substance, determined in t/m3. It is a function of the substance density, particle size, fractional composition, moisture, and the pressure of overlying layers. The volume per unit mass (Y) is the reciprocal of the bulk density:

The average mass (X) of a substance is determined for bins and silos with vertical walls using the following equation: a+b

2, where: a is the bulk density of the freely poured product; b is the bulk density of the bottom (compressed) layer.

Bulk density is taken into account when designing storage facilities and bins.

Bulk density, leaching, angle of repose, tendency to segregate (for mixed fertilizers), and viscosity must be considered to assess the quality state of fertilizers during storage. When organizing fertilizer storage, one should also know their characteristics such as fire and explosion hazards, as well as chemical properties – free acidity, tendency to retrogradation, and emission of ammonia.

Acidity is the free acid content in a fertilizer, expressed as a percentage.

Alkalinity is the free base content in a fertilizer, expressed as a percentage.

Fertilizer solubility is the mass of fertilizer in kilograms that can be dissolved in 100 l of water at a specific temperature.

Retrogradation is a decrease in the content of water- or citrate- and citric-soluble forms in a fertilizer during storage or neutralization. Phosphorus fertilizers are primarily subject to retrogradation. Retrogradation is caused by the presence of sesquioxides, R2O3 (Fe2O3 + Al2O3), and lime in phosphorus fertilizers, and it is reduced during granulation.

Deflagration (cigar-like burning) of fertilizers is the self-propagating decomposition of fertilizers. A fertilizer is considered prone to deflagration if a small reaction zone within it moves at a certain speed through the entire decomposing mass. This process occurs without a flame and does not require oxygen from the air for the reaction. Decomposition begins as a result of local overheating and is accompanied by an exothermic reaction with the release of gases and an increase in temperature in the reaction zone to 250–500 °C. The heat released during this process is transferred by convection, thermal conductivity, and radiation to the nearby, not yet decomposed part of the fertilizer, where heating and initiation of the decomposition process also occur. The higher the temperature, the more intense the decomposition process.

Self-sustaining exothermic decomposition of fertilizers is possible with the simultaneous presence of NO3–, NH4+, and Cl– ions. In this case, the Cl– anion acts as a catalyst. Even a 0.05% content of chlorine anions in the mixture leads to nitrogen losses, which reach a maximum at its concentration of 0.5%. Trace elements – Co, Cr, Cu – possess catalytic action. The introduction of copper in the form of a 0.01–0.5% CuSO4∙5H2O suspension into fertilizers such as nitrophoska before granulation leads to an increase in the deflagration rate from 6 to 24 cm/h. A similar pattern is observed when dusting granules with CuO, Cu2O 3)2. The simultaneous addition of MgSO4·H2O (1.2% Mg and 1.6% S) and sodium tetraborate reduces the ability of complex fertilizer to self-propagating decomposition. Conditioning additives increase the burning rate of fertilizers. The burning temperature of conditioned fertilizer is approximately 410°C, and 350–365 °C without conditioning.

A reduction in spontaneous decomposition of fertilizers is achieved by introducing neutralizing substances – ammonia, hydrated lime, cyanamide, calcium or magnesium oxide, cement dust; by manufacturing fertilizers with a high content of ammonium phosphate, as its decomposition occurs with heat absorption, which reduces the total thermal effect. As the moisture of complex fertilizers increases, the tendency for their decomposition disappears. Humidity from 5 to 15% reduces or eliminates the ability of fertilizers to self-sustaining decomposition. The presence of the SO42– ion in the form of (NH4)2SO4 and K2SO4 has a stabilizing effect on nitrophosphates, which is explained by the formation of stable phosphate forms in these fertilizers.