Phosphorus fertilizers

Apatite

Apatite is a mineral; in a dispersed state it is present in soils and parent rocks. Deposits are rare. The world’s largest deposit was discovered in 1925 in the Khibiny Mountains on the Kola Peninsula. Small and less valuable in composition deposits occur in Russia in the Urals and the Southern Baikal region, in Brazil, Spain, Canada, the USA, and Sweden.

Apatites are rocks of endogenous origin. Pure apatite is a colorless, greenish, or yellow-green mineral; its phosphorus content is up to 42% calculated as P₂O₅. Apatite crystals are hexagonal prisms with high strength. The empirical formula is [Ca₃(PO₄)₂]₃⋅CaF₂. Fluorine can be replaced by chlorine, carbonate, or hydroxyl groups. Accordingly, fluorapatite, chlorapatite, carbonateapatite, and hydroxylapatite are distinguished.

In the Khibiny Mountains, apatite is represented as apatite-nepheline rock. Nepheline is an aluminosilicate of composition (K, Na)₂O⋅Al₂O₃⋅SiO₂, containing up to 5–6% K₂O. Apatite and nepheline make up approximately 90% of the ore mass; the rest is feldspar, hornblendes, and other minerals.

Nepheline on acidic soils can be used as a potassium fertilizer. It is insoluble in water, but in an acidic environment potassium passes into a plant-available form.

Apatite-nepheline ore is mined by open-pit and underground methods. By external signs it is sorted, yielding marketable ore with a content of up to 30–31% P₂O₅. Subsequently, the ore is enriched by flotation, owing to which nepheline is almost completely removed. The resulting apatite concentrate contains 39–40% P₂O₅ and is used for the production of phosphorus fertilizers.

Phosphorites

Phosphorites are sedimentary rocks, usually of marine origin, containing amorphous or crystalline calcium phosphate with an admixture of quartz, lime, clay particles, and other minerals.

Phosphorites formed as a result of the life activity of marine plant and animal organisms in past geological periods. The biological origin is confirmed by the content of organic matter (up to 0.5–1.0% by carbon). Deposits occur in sedimentary rocks in the form of nodules of various sizes and shapes (nodular phosphorites), less often in the form of beds of continuous mass (bedded phosphorites).

Phosphorites are characterized by greater particle strength than apatites; they can be amorphous and microcrystalline. Nodular (concretionary) phosphorites in the form of rounded stones and bedded (massive) phosphorites representing a fused mass are distinguished. The latter are less common. There are also granular shell types.

By geotectonic position, phosphorite deposits can be platform, that is, horizontally occurring over large areas of the Earth’s crust with a small layer thickness, and geosynclinal, that is, located in folded mountain regions. An example of a geosynclinal deposit is Karatau.

Most phosphorite deposits in Russia belong to the nodular type. Such phosphorites, as a rule, do not have a pronounced crystalline structure, are more easily decomposed, and therefore are of interest for direct (without chemical treatment) use as a phosphorus fertilizer.

The crystalline structure is more pronounced in phosphorites that are older by geological age.

The disadvantage of most phosphorite deposits is the low phosphorus concentration with a high content of sesquioxides in the raw material, which complicates processing and the production of superphosphate. An admixture of sesquioxides leads to additional acid consumption in the production of phosphorus fertilizers and to retrogradation, that is, the reverse transition of phosphates into poorly soluble compounds. Thus, to obtain 1 t of plant-available P₂O₅ in superphosphate for the decomposition of apatite concentrate, 1.89 t of sulfuric acid is required, whereas for phosphorites with impurities, 2.5 t is required.

Phosphorite deposits are quite widespread in the world, but, for example, in Western Europe they are small and almost unsuitable for development. The largest phosphorite deposits are in the countries of North Africa. In the USA, deposits are in Florida, Tennessee, and some other states.

In Russia, large reserves of phosphorites are concentrated, but most of them are poor in phosphorus and contain large impurities of sesquioxides (R₂O₃). Mainly, the deposits are concentrated in the European part of Russia.

The Vyatka-Kama deposit is located in the northeast of the European part of Russia, of nodular type, with a phosphorus content of 24–26% P₂O₅.

The Yegoryevskoye deposit is located in Moscow Oblast. The deposits are represented by two horizons separated by a layer of loose glauconitic sand: the upper one is the Ryazan horizon, the lower one is the Portland horizon. The quality of the latter is higher than that of the Ryazan: it contains 25–26% P₂O₅ and 4–5% sesquioxides. The Ryazan layer contains on average 21–23% P₂O₅ and 10–12% sesquioxides.

The Seshchinskoye deposit is located in Dubrovsky District of Bryansk Oblast. Phosphorites occur in three horizons of sandy concretions, sometimes cemented into a slab. For development, the upper layer about 0.5 m thick with a content of 14% P₂O₅ is important, as well as the middle layer 0.53 m thick with a content of 16% P₂O₅. Between these horizons there is an interlayer of glauconitic sand about 1 m thick.

The Shchigrovskoye deposit in Kursk Oblast belongs to sandy phosphorites. The nodules have different sizes and shapes and are cemented with sandy rock into a continuous slab (“samorod”). Sometimes such slabs contain accumulations of nodules freely occurring in loose sandy rock. This type of phosphorite occurs in Voronezh, Tambov, Oryol, Bryansk, Kaluga, and Smolensk Oblasts. The phosphorus content is 14–19% P₂O₅; they are poorly suitable for processing and are used as phosphorite flour.

The Karatau phosphorites formed on mobile areas of the Earth’s crust, in place of which mountain formations later arose. A distinctive feature of the deposit is the presence of thick phosphate-containing layers of complex occurrence with a high phosphorus content. The layers alternate with phosphate-siliceous and phosphate-carbonate rocks. In the main horizon of the deposit, the P₂O₅ content is 26–29%. The greatest value is represented by bedded layers up to 7 m thick; the P₂O₅ content in them reaches 30–35% with 2–2.5% sesquioxides. The disadvantage of Karatau phosphorites is their increased magnesium content, which gives them hygroscopicity. To eliminate this property, additional treatment is required, which entails an increase in the cost of production.

Simple superphosphate

Simple superphosphate, or calcium dihydrogen orthophosphate, monobasic calcium phosphate, monocalcium phosphate — Ca(H₂PO₄)₂ — is a phosphorus fertilizer with a P₂O₅ content of 16–20%. It is well soluble in water and weak acids.

The production technology was proposed by J. Liebig. The first production plant was built in 1843 in England by Lawes, the founder of the Rothamsted Agricultural Experiment Station.

Thanks to its simple and cheap production technology, superphosphate is the main phosphorus fertilizer used throughout the world.

The technological production scheme is continuous. The raw materials are natural phosphates — apatite concentrate or phosphorite flour. When phosphate raw material is treated with concentrated sulfuric acid, monobasic calcium phosphate and anhydrous calcium sulfate (gypsum) are formed:

The resulting gypsum remains in the fertilizer composition; it accounts for up to 40%.

Besides the formation of calcium dihydrogen orthophosphate, side reactions occur with the formation of free phosphoric acid:

The impurity of phosphoric acid in the final product can be 5.0–5.5%, which gives superphosphate an acidic reaction and hygroscopicity.

With a local deficiency of sulfuric acid in the reaction mixture, calcium hydrophosphate is formed:

Since the resulting gypsum is not separated, the phosphorus content in the product is approximately 2 times lower than in the initial raw material. For this reason, phosphorites with a low P₂O₅ content are almost unsuitable for superphosphate production. From apatite concentrate, superphosphate is obtained with a content of not less than 19% citrate-soluble phosphorus; in the highest grade, not less than 19.5%.

88–98% of the phosphorus in superphosphate is contained in a plant-available form: water-soluble — calcium dihydrogen orthophosphate and phosphoric acid, and citrate-soluble — calcium hydrophosphate; it accounts for 10–25% of the available phosphorus.

Finished superphosphate contains small impurities of calcium, iron, and aluminum phosphates.

Free phosphoric acid in superphosphate prevents the saturation of gypsum (CaSO₄⋅2H₂O) with water, therefore calcium sulfate remains anhydrous or as CaSO₄⋅0.5H₂O.

The final production product is powdered superphosphate, a substance of light gray color. Free phosphoric acid causes hygroscopicity and moisture, which should be not more than 12–15%. During storage and transportation, powdered superphosphate cakes, loses flowability and spreadability. When applied to the soil, powdered superphosphate undergoes rapid chemical absorption, and phosphorus becomes unavailable to plants.

These disadvantages are eliminated by granulation of powdered superphosphate.

Granulated superphosphate does not clump, does not cake, and has reduced moisture. Due to the slow dissolution of granules in soil moisture and the reduction of the contact area of fertilizer particles with the soil, chemical binding decreases, which is especially important when applied on acidic soils with a high content of sesquioxides. Granulated superphosphate allows more uniform spreading.

Granulated superphosphate contains up to 1–2.5% free phosphoric acid and up to 1–4% moisture.

The granulation process is carried out in long (7.5 m) rotating drums, in which powdered superphosphate is moistened to 16%; during drum rotation it is rounded, acquiring the form of round small granules of different sizes. After drying, the granulate is sorted to remove particles less than 1 mm and more than 4 mm. A fraction from 1 to 4 mm in diameter is obtained. Larger granules are crushed and, together with small ones (“return”), returned for repeated granulation. The return acts as centers of granule formation.

During granulation, free phosphoric acid is neutralized by adding ammonia, lime, or phosphorite. When ammonia is used in the process, ammoniated superphosphate is obtained, which contains 1.5–3% nitrogen. When neutralized with phosphorite flour, the phosphorus content in the finished fertilizer increases to 20–22%, but simultaneously the relative content of water-soluble phosphorus decreases.

The quality of superphosphate is assessed by the content of phosphoric acid and the proportion of phosphorus soluble in water and citrate solution — an aqueous solution of ammonium citrate and ammonia.

Simple superphosphate is used on all soil types. Its main disadvantage is its low phosphorus content, which reduces its economic efficiency, primarily during transportation.

Double superphosphate

Double superphosphate is a concentrated phosphorus fertilizer obtained from apatite or phosphorite by treatment with phosphoric acid. It contains phosphorus in the form of calcium dihydrogen orthophosphate Ca(H₂PO₄)₂, as does simple superphosphate, with an impurity of up to 2.5% free phosphoric acid. The main difference from simple superphosphate is the absence of gypsum.

The production technology includes two stages: in the first stage, phosphoric acid is obtained; in the second, double superphosphate.

Two methods are used to produce phosphoric acid.

In the wet extraction method, phosphoric acid is obtained by treating phosphorite, including low-phosphorus phosphorite, with sulfuric acid, with the formation of phosphoric acid:

Phosphoric acid is extracted with a 20–25% sulfuric acid solution so as not to dissolve large amounts of the contained sesquioxides. After that, phosphoric acid is separated from the precipitate and concentrated by evaporation. The resulting phosphoric acid is used to treat phosphorite with a high phosphorus content and less contamination by impurities:

The second method of producing phosphoric acid is the method of producing it by sublimation of phosphorus from low-percentage phosphorites at a temperature of 1400–1500 °C in electric or blast furnaces. Here the released elemental phosphorus is collected under water, burned, and the resulting phosphorus oxide is neutralized with water:

The second stage of double superphosphate production is the reaction of phosphoric acid with phosphate raw material with a high phosphorus content:

The initial raw material for the production of phosphorus fertilizers determines the composition of impurities. The best double superphosphate is obtained from apatite; its P₂O₅ content is 45–49%, free acid not more than 2.5%, and the proportion of water-soluble P₂O₅ not less than 85%.

Double superphosphate is produced in the form of light gray granules. The cost of 1 t of P₂O₅ of double superphosphate is 6–13% higher than that of simple superphosphate, but the high concentration of P₂O₅ determines savings during transportation and storage. The cost of applying P₂O₅ of double superphosphate is 8–13% less than that of simple superphosphate.

In action, double superphosphate in an equivalent dose does not differ from simple superphosphate. However, due to the absence of sulfur (in the form of gypsum), double superphosphate may be inferior to simple superphosphate on soils with a low sulfur content and under crops demanding sulfur nutrition, for example, legumes and crucifers. In these cases, the application of double superphosphate is combined with sulfur-containing fertilizers, for example, ammonium sulfate, potassium sulfate, and potassium-magnesium sulfate.

Superphos

Superphos is a new promising type of concentrated phosphorus fertilizer with prolonged action. It is obtained by chemical enrichment and treatment with a mixture of sulfuric and phosphoric acids of phosphorite flour.

The acid consumption for the production of 1 t of P₂O₅ in superphos is 1–1.3 t of sulfuric and 0.36 t of phosphoric acid — 2 times less than for the production of 1 t of P₂O₅ of double superphosphate. At the same time, the use of P₂O₅ of phosphate raw material reaches 95%.

Superphos is produced in granulated form; it contains 38–40% P₂O₅, 19–20% of which is in water-soluble form. In effectiveness it is not inferior to double superphosphate.

Precipitate (dicalcium phosphate)

Precipitate, or dicalcium phosphate, calcium hydrophosphate, monocalcium phosphate — CaHPO₄⋅2H₂O. It is obtained by the interaction of orthophosphoric acid and milk of lime (calcium hydroxide solution) or a calcium carbonate suspension:

The amounts of the initial substances are taken in a ratio corresponding to the chemical reaction equation.

Precipitate is separated from the liquid and dried at a temperature not more than 100 °C to prevent losses of crystallization water, which promotes the solubility of precipitate.

The technology of precipitate production as a fertilizer is economically unjustified, therefore it is mainly used for feed purposes. As a fertilizer, it is obtained during the utilization of weak orthophosphoric acid solutions, which are wastes of other productions, for example, in gelatin production at bone-processing plants.

Precipitate for fertilizer is a white or light gray powder, non-caking and well spreadable. Depending on the initial raw material, it contains 25–35% citrate-soluble form of P₂O₅. Feed precipitate contains 44% P₂O₅, not more than 0.2% P, 0.001% As, and 0.002% Pb.

In its effect on yield it is analogous to superphosphate, but it is used only for main application under plowing in the same doses of P₂O₅ as superphosphate. On soils not saturated with bases and on sierozems, the effectiveness of precipitate is higher than that of superphosphate, due to the stronger binding of superphosphate phosphorus. On chernozems, the action of superphosphate is equal to or somewhat superior to the action of precipitate.

Defluorinated phosphate

Defluorinated phosphate, or calcium orthophosphate, tricalcium phosphate — Ca₃(PO₄)₂ — contains 28–32% citric-soluble P₂O₅. By P₂O₅ content, the fertilizer belongs to concentrated phosphorus fertilizers.

It is obtained by thermal treatment of phosphate raw material. The process consists in treating with water steam a mixture of apatite or phosphorite with 2–3% silica (sand) at a temperature of 1400–1550 °C. Here the fluorine contained in apatite is separated in the form of hydrogen fluoride. The degree of defluorination reaches 94–96%.

Chemical reaction of hydrothermal decomposition of apatite in the presence of silica:

In the obtained product, depending on the raw material, it contains up to 30–32% (from apatite) or up to 20–22% (from phosphorite) citrate-soluble P₂O₅.

Defluorinated phosphate has good physical properties. As the main phosphorus fertilizer on soddy-podzolic and chernozem soils, it is not inferior to superphosphate in effectiveness.

Defluorinated phosphate is mainly used for mineral feeding of animals.

Thomas slag

Thomas slag, or fused magnesium phosphates, contains phosphorus in the form of tetracalcium phosphate (4CaO⋅P₂O₅ or Ca₄P₂O₉) or silicocarnotite (Ca₄P₂O₉⋅CaSiO₃). According to technical specifications, the content of citric-soluble P₂O₅ should be not less than 14%. They occupy a small part among the phosphorus fertilizers used.

They are obtained as a by-product of processing phosphoric pig iron into iron and steel by the S. Thomas method. Since a phosphorus impurity reduces the quality of the metal, to remove it S. Thomas in 1879 proposed binding phosphorus with freshly burned lime. At a temperature of 1800–2000 °C, phosphorus oxidizes to P₂O₅; the binding of P₂O₅ leads to the formation of lime salts of phosphoric acid. These compounds, together with calcium silicate and other impurities, float to the surface of the molten metal in the form of slag, which is separated, after cooling crushed, ground, and in this form used as a phosphorus fertilizer.

With an excess of SiO₂, a double salt of tetracalcium phosphate and calcium silicate — silicocarnotite — is formed; with a deficiency of SiO₂, tetracalcium phosphate is formed. Both salts are soluble in 2% citric acid. Thomas slag also contains poorly soluble phosphates.

Thomas slag is a dark heavy powder; it contains from 7–8% to 16–20% citrate-soluble P₂O₅. As impurities, it contains calcium silicate, compounds of iron, aluminum, vanadium, magnesium, manganese, molybdenum, and others.

It is used as a main fertilizer. It is more effective on acidic soils, since it has an alkaline reaction.

When applied to the soil, as a result of interaction with soil moisture containing dissolved carbon dioxide (carbonic acid), it gradually decomposes with the formation of freshly precipitated tricalcium phosphate, which is available to plants.

Thomas slag is used on all soil types on which phosphorus fertilizers have a positive effect on yield, but its effectiveness on different soils manifests unequally. On chernozems it is weaker than superphosphate; on soils of the Non-Chernozem zone, especially acidic peaty and sandy soils, Thomas slag is more effective, since it reduces acidity. The neutralizing ability of Thomas slag matters when combining phosphorus fertilizers with physiologically acidic forms of nitrogen fertilizers.

Open-hearth phosphate slag

Open-hearth phosphate slag is obtained as a by-product in steel smelting from pig iron in open-hearth production. Lime materials are also used to bind phosphorus.

The phosphorus content in open-hearth slag is from 8 to 12% P₂O₅, almost all in citrate-soluble form. Phosphate slag contains a double salt of tetracalcium phosphate and calcium silicate, and impurities of iron, manganese, and magnesium compounds.

It is used as a main phosphorus fertilizer. It has a strongly alkaline reaction, therefore it is more suitable for acidic soils. Due to its low phosphorus content, it is advisable to use it near production sites.

Thermophosphates

Thermophosphates contain 18–34% P₂O₅; they are obtained by fusion or sintering of natural phosphates with carbonates or silicates of sodium or potassium, as well as metallurgical slags, lime, and quartz. Here hard-to-access forms of phosphorus pass into a citric-soluble form.

The melting temperature of thermophosphates is 1000–1200 °C. During high-temperature treatment, destruction of the crystal lattice of the phosphate raw material occurs, fluorine is released in the form of hydrogen fluoride, and phosphorus passes into the plant-available amorphous form Ca₃(PO₄)₂. The amorphous form is obtained and remains stable at a temperature of 1180 °C. With a decrease in temperature, it passes into a crystalline, poorly assimilable form. Therefore, to reduce such a transition, the reaction mass is rapidly cooled.

In composition and properties, thermophosphates are similar to Thomas slag and can be applied on all soils. Thermophosphates obtained by fusion with alkaline salts are soluble in citric acid and ammonium citrate solution and have greater availability to plants than Thomas slag. The advantage of this method of phosphorus fertilizer production is that low-percentage phosphorites and apatites unsuitable in superphosphate production can be used as the initial raw material.

On acidic soils they have a stronger action than superphosphate, especially on podzolic soils.

Bone (bone) meal

Bone meal is a by-product of bone processing. Defatted bones are treated with steam under a pressure of 1.5–2 atm, followed by washing with water to extract glue. Defatted and deglued bone mass is obtained, which is treated with hydrochloric acid. Here the mineral substances Ca₃(PO₄)₂ and Mg₃(PO₄)₂ dissolve, and the soft bone skeleton consisting of ossein separates. When heated with water, ossein gives high-quality gelatin.

The hydrochloric acid solution of phosphates is treated with “milk of lime”; here the phosphates precipitate with the formation of precipitate according to the equation:

Defatted and deglued bone meal contains 30–35% P₂O₅ and up to 1% nitrogen. Phosphorus in bone meal is in a water-insoluble form, but it is more available than the phosphorus of phosphorite flour. The effectiveness of bone meal is influenced by soil acidity. Even in a weakly acidic environment, bone meal has a good effect on crop yield.

Fused magnesium phosphate

Fused magnesium phosphate contains 20% P₂O₅ in citric-soluble form and 12% MgO. It is obtained by fusion of phosphorite with silicate olivinite or serpentinite.

It is advisable to use it on sandy loam soils, where crops respond well to magnesium.

Red phosphorus

Red phosphorus contains 229% phosphorus calculated as P₂O₅. It is a promising high-concentrated phosphorus fertilizer. To convert it into a plant-available form in the soil, a catalyst is used simultaneously, for example, copper, about 1% of the phosphorus mass.

On soddy-podzolic soil, 3 weeks after incorporation, 20% of red phosphorus passes into compounds available to cereal crops. In effectiveness it is not inferior to superphosphate; in aftereffect it is superior to it.

Phosphorite flour

Phosphorite flour is finely ground phosphorite. It is used as a fertilizer on acidic soddy-podzolic, gray forest, and peaty soils, on podzolized and leached chernozem soils, and on red soils. On typical, ordinary, and southern chernozems, the action of phosphorite flour is weaker and unstable.

Phosphorite flour is the cheapest fertilizer. By volume of production and use, it occupies second place after superphosphate. Production is reduced to separating coarse impurities (sand, clay) from the phosphate raw material, crushing into pieces 1–3 cm, and grinding to fine flour. The fineness of grinding of phosphorite flour affects its effectiveness. According to technical specifications, not less than 80% of the mass of phosphorite flour should have a particle size less than 0.17 mm.

As raw material, nodular, often low-percentage phosphorites, without a pronounced crystal structure, are used. When grinding such raw material, flour is obtained suitable for direct application but poorly suitable for chemical processing. Such phosphorites include raw material obtained at the Yegoryevskoye, Shchigrovskoye, Seshchinskoye, Krylkovetskoye, and Kineshma deposits.

Phosphorite flour is a powder of gray, dark gray, or brown color. The P₂O₅ content in fertilizer of the first grade is 28–30%, of the second, 22–24%, and of the third, 19–21%. The fertilizer is non-hygroscopic, non-caking, well spread, and strongly dusty.

Phosphorus in phosphorite flour is contained in the form of fluorapatite — [Ca₃(PO₄)₂]₃CaF₂, which is insoluble in water and poorly soluble in weak acids, therefore poorly available to most plants.

The effectiveness of phosphorite flour is influenced by the origin and composition of phosphorites, fineness of grinding, biological features of crops, soil properties, and the acidity of accompanying fertilizers.

Phosphorite flour interacts with soil having actual and potential acidity, with gradual decomposition of calcium phosphate and its transformation into dicalcium phosphate:

According to studies, soils with hydrolytic acidity less than 2.5 mg⋅eq/100 g poorly decompose phosphorite, therefore its effectiveness on such soils is low. Conversely, the higher the hydrolytic acidity, the more effective the action of phosphorite flour. This explains the positive action of phosphorite flour on degraded and leached chernozems, where exchangeable acidity is small but hydrolytic acidity is high.

Soils with a small absorption capacity at hydrolytic acidity of 3–3.5 mmol/100 g soil and base saturation of 50–60%, as a rule, have an acidic reaction (5.0–5.5), which is due to exchangeable acidity. At a large absorption capacity, hydrolytic acidity equal to 6–7 mmol/100 g soil, and degree of saturation of 75–85%, the reaction is close to neutral (6.0–6.5). Therefore, a high action of phosphorite flour will manifest at high acidity and a lower degree of soil saturation with bases.

The action of phosphorite flour is influenced by the absorption capacity and degree of saturation with bases of the soil. At the same hydrolytic acidity, effectiveness increases with a decrease in absorption capacity.

With an increase in the fineness of grinding, the total surface of interaction of phosphorite flour with the soil increases.

The effectiveness of phosphorite flour depends on the biological features of plants. The results of experiments by D. N. Pryanishnikov, P. S. Kossovich, and other scientists allowed agricultural crops to be divided into groups by their ability to assimilate phosphorus from poorly soluble phosphates. Plants with a good ability to assimilate poorly soluble phosphates include lupine, buckwheat, and mustard; somewhat less so pea, sainfoin, sweet clover, and hemp. All cereals, flax, beet, potato, and vetch can assimilate phosphorus from phosphorite flour only after interaction with acidic soils. They do not assimilate phosphorus of phosphorite flour: barley, spring wheat, flax, millet, tomato, and turnip.

The ability of plants to assimilate poorly soluble phosphates changes with age. Most plants poorly assimilate hard-to-access forms in the first period of life; later this ability increases.

Most scientists explain the ability to assimilate poorly soluble phosphates by the quantity and composition of acid root exudates of plants. F. V. Chirikov explains this ability by increased calcium consumption: plants absorbing more calcium better assimilate phosphorus.

The assimilation of phosphorus from phosphorite flour, as established by D. N. Pryanishnikov, also depends on accompanying fertilizers: physiologically acidic fertilizers increase phosphorus availability; physiologically alkaline fertilizers and lime materials reduce it.

Phosphorite flour became widespread in the countries of the former USSR. Probably in the future its use will expand. The disadvantage of phosphorite flour is that it is very dusty. To reduce dusting, the following may be used:

• mixtures of phosphorite flour with ammonium chloride in a ratio N:P₂O₅ = 1:1; here the content of each nutrient is 14%, dustiness is eliminated, and the content of citric-soluble phosphorus increases 1.5 times;
• treatment of phosphorite flour with molten potassium disulfate at a temperature of 205–210 °C for 50–60 minutes in a screw mixer: dustiness is eliminated, the P₂O₅ content is 16%, of which 70% is in citric-soluble form, and the K₂O content is up to 17%.

The effectiveness of phosphorite flour is influenced by the geological age and mineralogical composition of phosphorite. Phosphorites of ancient origin with a crystalline structure are characterized by poor availability to plants, especially apatite.

To increase availability, phosphorite flour is composted with high-moor peat or manure. In composts, effectiveness increases with acidic peat at a ratio of peat to flour of 100:1. Phosphorite flour is used for preparing peat-manure and manure-phosphorite composts.

Phosphorite flour, for better decomposition, is applied in advance under deep plowing into a moistened layer. Application in double or triple doses has a long aftereffect.

In Russia, acidic soils under arable land occupy about 50 million ha. They, as a rule, are characterized by a low content of mobile phosphorus.

Vivianite (bog ore)

Vivianite, or bog ore, ferrous phosphate — Fe₃(PO₄)₂⋅8H₂O. It contains 28% P₂O₅. It occurs under a peat layer in the form of a whitish mass.

It is a good source of phosphorus for crops on soddy-podzolic, gray forest soils, and leached chernozems. Vivianite easily loosens when drying and spreads well.