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Phosphate fertilizers

Phosphate fertilizers are mineral substances that contain phosphorus in a form accessible to plants, or in a form that becomes accessible to plants when released into the soil, and serve to provide phosphorus to crops.

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Raw materials for the production of phosphate fertilizers

The raw material for phosphate fertilizers is natural phosphate-bearing ores – apatite and phosphorite. Waste products from the metallurgical industry are also used as phosphate fertilizers. The main global reserves of phosphate ores are located in Morocco, the USA and Russia.

According to the content of phosphorus ores are divided into rich ores up to 35% and poor ores containing 5-10%. Most often due to the large amount of impurities are subject to enrichment.

Apatite

Apatite is a mineral present in a dispersed state in soils and parent rocks. Deposits are rare. The world’s largest deposit was discovered in 1925 in the Khibiny on the Kola Peninsula. Smaller deposits of less valuable provenience are to be found in Russia, in the Urals and South Baikal region, in Brazil, Spain, Canada, the United States, and Sweden.

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

In the Khibiny, apatite is represented as apatitenepheline rock. Nepheline is an aluminosilicate of (K, Na)2O⋅Al2O3⋅SiO2 composition, containing up to 5-6% of K2O. Apatite and nepheline account for about 90% of the ore mass, the rest being feldspar, hornblende and other minerals.

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

The apatitenepheline ore is extracted by open-pit and underground mining methods. According to its external signs it is sorted, and in this case commercial ore is obtained with up to 30-31% P2O5 content. Further the ore is subjected to enrichment by flotation, due to which nepheline is almost completely removed. The resulting apatite concentrate contains 39-40% P2O5 and is used to produce phosphate fertilizers.

Phosphorites

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

Phosphorites are the result of the activity of marine plant and animal organisms in the past geological periods. The biological origin is confirmed by the content of organic matter (up to 0.5-1.0% carbon). Deposits are found in sedimentary rocks in the form of nodules of various sizes and shapes (nodule phosphorites), less often – in the form of beds of solid masses (layer phosphorites).

Phosphorites are characterized by greater particle strength than apatites; they can be amorphous and finely crystalline.

A distinction is made between nodular (nodule) phosphorites in the form of rounded stones and layered (massive) phosphorites, which are a fused mass. The latter are less common. There are also granular varieties of shell-forming rocks.

According to the geotectonic position, the phosphate deposits can be platform deposits, i.e. horizontally occurring in large areas of the crust with a low thickness of the layer, and geosynclinal, i.e. located in folded mountain areas. An example of a geosynclinal deposit is Karatau.

Most Russian phosphorite deposits are of the jugular type. Such phosphorites as a rule do not have a pronounced crystalline structure, are easier to decompose and therefore are of interest for direct (without chemical treatment) use as a fertilizer.

The crystalline structure is more pronounced in phosphorites of older geological age.

The disadvantage of most of the phosphate deposits is a low concentration of phosphorus with a high content of oxides such as R2O3 in the raw material, which complicates the processing and production of superphosphate. The impurity of oxides such as R2O3 leads to additional consumption of acid in the production of fertilizers and retrogradation, that is, the reverse transition of phosphate in low-soluble compounds. Thus, to obtain 1 ton of P2O5 in superphosphate, the decomposition of apatite concentrate requires 1.89 tons of sulphuric acid, whereas for phosphate rock with impurities – 2.5 tons.

Deposits of phosphate rock are quite widespread in the world, but, for example, in Western Europe, they are small and almost not suitable for development. The largest phosphorite deposits are in North Africa. In the United States, there are deposits in Florida, Tennessee and some other states.

Large reserves of phosphorites are concentrated in Russia, but most of them are poor in phosphorus and contain large impurities of oxides such as R2O3. Most of the deposits are concentrated in the European part of Russia.

Table. Chemical composition of phosphorites and apatites, % on dry matter[1]Yagodin B.A., Zhukov Yu.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.

Deposit
P2O5
CaO
R2O3
CO2
F
Insoluble residue
Phosphorites
Vyatsko-Kamskoe
23,5
37,2
5,4
4,5
2,5
15,6
Yegoryevskoye (Portland horizon)
27,1
42,0
5,4
5,2
3,3
9,4
Seshchinskoe
15,6
24,0
3,1
2,7
1,8
48,3
Shchigrovskoe
16,1
26,2
3,0
3,1
1,9
45,8
Apatity
Khibinskoe:
apatitenepheline rock
30,1
39,5
9,0
0,0
2,6
15,6
apatite concentrate
40,5
51,6
0,9
0,2
3,3
-

The Vyatsko-Kamskoe deposit is located in the northeast of European Russia, of the jugular type, with a phosphorus content of 24-26% P2O5.

The Egoryevskoye deposit is located in the Moscow region. The deposits are represented by two horizons, separated by a layer of loose glauconite sand: the upper is Ryazan and the lower is Portland. The quality of the latter is higher than the Ryazan sand: it contains 25-26% P2O5 and 4-5% semi-reductive oxides. The Ryazan layer contains, on average, 21-23% P2O5 and 10-12% semi-reactivated oxides.

The Seshchino deposit is located in the Dubrovskii district of the Bryansk region. Phosphorites lie in three horizons of sandy nodules, sometimes cemented into a plate. The upper layer is about 0.5 m thick and contains 14% P2O5, and the middle layer is 0.53 m thick and contains 16% P2O5. Between these horizons there is a layer of glauconite sand about 1 m thick.

The Shchigrovskoe deposit in the Kursk Region refers to sandy phosphorites. The crystals are of various sizes and shapes, cemented with sandy rock into a continuous slab (“nugget”). Sometimes these slabs contain clusters of gallstones, loosely embedded in loose sandy rock. This type of phosphorites occurs in the Voronezh, Tambov, Orel, Bryansk, Kaluga, and Smolensk Regions. The content of phosphorus is 14-19% P2O5, is of little use for processing and is used in the form of phosphate meal.

Phosphorites Karatau formed in the mobile areas of the earth’s crust, in place of which later emerged mountain formations. A distinctive feature of the deposit is the presence of thick phosphate-bearing layers of complex occurrence with high phosphorus content. The layers alternate with phosphate-siliceous and phosphate-carbonate rocks. In the main horizon of the deposit, the content of P2O5 is 26-29%. The greatest value are layer thicknesses up to 7 m, the content of P2O5 in them reaches 30-35% with 2-2.5% of semi-reductive oxides. A disadvantage of Karatau phosphorites is an increased content of magnesium, which gives them hygroscopicity. For elimination of this property additional processing is required that entails increase in cost of production.

Classification of phosphate fertilizers

Phosphate fertilizers, depending on solubility and availability to plants, are classified into three groups:

  • containing phosphorus in water-soluble form, include simple and double superphosphate, phosphorus is well available to plants;
  • containing phosphorus in a water insoluble form, but soluble in weak acids, such as 2% citric acid; these include precipitate, tomas slag, open-hearth phosphate slag, defluorinated phosphate; phosphorus is accessible to plants;
  • containing phosphorus that is insoluble in water, poorly soluble in weak acids, and soluble in strong acids; include phosphate meal, bone meal. These fertilizers are not available to most crops, but can be assimilated by some plants (lupine, buckwheat) under the influence of acidic root secretions.

Due to the fact that most soils have a near-neutral reaction, the most effective phosphorus fertilizers are considered water-soluble, which are widely used in the world. The technology for processing raw materials for the production of phosphate fertilizers is aimed at converting phosphorus into a form that is accessible to plants.

Fertilizers containing phosphorus in water-soluble form

Phosphate fertilizers containing phosphorus in water-soluble form include superphosphates.

According to the method of production and the content of P2O5 are divided into:

  • simple;
  • double;
  • triple.

By output form:

  • powdered;
  • granular.

Water-soluble forms are applicable on all types of soils, under all crops and in different methods. To increase their effectiveness, techniques are carried out aimed at reducing chemical absorption by the soil, i.e. introduction of granular forms, row and local introduction.

Simple superphosphate

Simple superphosphate, or calcium dihydroorthophosphate, one-substituted calcium phosphate, monocalcium phosphate, – Ca(H2PO4)2 – phosphate fertilizer, P2O5 content 16-20%. It is well soluble in water and weak acids.

The production technology was proposed by J. Libich. The first plant for the production was built in 1843 in England by Loose – the founder of Rotamsted agricultural experiment station.

Thanks to the simple and cheap production technology superphosphate is the main phosphate fertilizer used all over the world.

The technological scheme of production is of a continuous type. Raw materials are natural phosphates – apatite concentrate or phosphate rock. The treatment of phosphate rock with concentrated sulphuric acid produces one-substituted calcium phosphate and anhydrous calcium sulphate (gypsum):

[Ca3(PO4)2]3⋅CaF2 + 7H2SO4 + 3H2O = 3Ca(H2PO4)2⋅H2O + 7CaSO4 + 2HF.

The resulting gypsum remains as part of the fertilizer, accounting for up to 40%.

In addition to the formation of calcium dihydroorthophosphate, side reactions occur with the formation of free phosphoric acid:

[Ca3(PO4)2]3⋅CaF2 + 10H2SO4 = 6H3PO4 + 10CaSO4 + 2HF.

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

If there is a local deficiency of sulfuric acid in the reaction mixture, the formation of calcium hydroorthophosphate occurs:

[Ca3(PO4)2]3⋅CaF2 + 4H2SO4 + 12H2O = 6CaHPO4⋅2H2O + 4CaSO4 + 2HF.

Since the resulting gypsum is not separated, the phosphorus content in the product is about 2 times lower than in the feedstock. For this reason, phosphate rock with low P2O5 content is almost unsuitable for the production of superphosphate. Superphosphate with at least 19% of citrate-soluble phosphorus is obtained from apatite concentrate, and not less than 19.5% in the highest grade.

88-98% of phosphorus in superphosphate is contained in the form accessible to plants: water-soluble – calcium dihydroorthophosphate and phosphoric acid, and citrate-soluble – calcium hydrophosphate, which accounts for 10-25% of available phosphorus.

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

Free phosphoric acid in superphosphate prevents saturation of gypsum (CaSO4⋅2H2O) with water, so the calcium sulfate remains anhydrous or CaSO4⋅0,5H2O.

The end product is powdery superphosphate, which is a light gray substance. Free phosphoric acid leads to hygroscopicity and humidity, which should not exceed 12-15%. During storage and transportation powdered superphosphate cakes, loses its flowability and spreadability. When applied to soil, powdered superphosphate is subjected to rapid chemical absorption, and phosphorus becomes unavailable to plants.

These disadvantages are eliminated by granulating powdered superphosphate.

Granulated superphosphate does not clump, does not caked, has a reduced moisture content. Due to the slow dissolution of granules in the soil moisture and reducing the contact area of the fertilizer particles with the soil reduces the chemical binding, which is especially important when applied to acidic soils with a high content of H2O. Granulated superphosphate allows for more uniform dispersion.

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%, while the rotation of the drum is pelletized, taking the form of round small granules of different sizes. After drying, the granules are sorted to remove particles smaller than 1 mm and larger than 4 mm. A fraction of 1 to 4 mm in diameter is obtained. The larger pellets are crushed and together with the smaller ones (“retur”) are returned for re-pelletizing. The retur act as pelletizing centers.

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

The quality of superphosphate is estimated by the content of phosphoric acid and phosphorus soluble in water and citrate solution – aqueous solutions of ammonium citrate and ammonia.

Simple superphosphate is used on all types of soils. The main disadvantage is its low phosphorus content, which reduces its economic efficiency, especially in transportation.

Double superphosphate

Double superphosphate is a concentrated phosphate fertilizer that is obtained from apatite or phosphorite by treatment with phosphoric acid. It contains phosphorus in the form of calcium dihydroorthophosphate [Ca(H2PO4)2], like simple superphosphate with an admixture of up to 2.5% of free phosphoric acid. The main difference from simple superphosphate is the absence of gypsum.

There are two phases in the production process: the first is phosphoric acid, and the second is double superphosphate.

Two methods are used to produce phosphoric acid.

At the wet extraction method phosphoric acid is obtained by treatment of phosphorite, including with low phosphorus content, with sulfuric acid, with the formation of phosphoric acid:

Ca3(PO4)2 + 3H2SO4 + 6H2O = 2H3PO4 + CaSO4⋅2H2O.

Extraction of phosphoric acid is made with a 20-25% solution of sulfuric acid, so as not to dissolve a large amount of the contained semi-fluoric oxides. The phosphoric acid is then separated from the precipitate and concentrated by evaporation. The resulting phosphoric acid is used to process phosphorite with high phosphorus content and less contaminated with impurities:

Са3(РO4)2 + 4H3PO4 + Н2О = 3Ca(H2PO4)2⋅Н2О.

The second method of phosphoric acid production is the method of phosphorus sublimation from low-grade phosphorites at a temperature of 1400-1500 °C in electric furnaces or blast furnaces. In this case the separated elemental phosphorus is collected under water, burned and the resulting phosphorus oxide is neutralized with water:

P2O5 + 3H2O = 2H3PO4.

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

[Ca3(PO4)2]3⋅CaF2 + 14H3PO4 + 10H2O = 10Ca(H2PO4)⋅H2O + 2HF.

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

Available double superphosphate in the form of pellets of light gray color. The cost of 1 ton of P2O5 double superphosphate by 6-13% higher than in a simple one, but the high concentration of P2O5 causes savings in transportation and storage. The cost of using P2O5 double superphosphate is 8-13% lower than plain.

By the action of double superphosphate in an equivalent dose is not different from the simple. However, due to the lack of sulfur (in the form of gypsum), double superphosphate may be inferior to simple on soils with low sulfur content and under the crops that require sulfur nutrition, such as legumes and cruciferous plants. In these cases, the application of double superphosphate is combined with sulfur-containing fertilizers, such as ammonium sulfate, potassium sulfate, potassium magnesium sulfate.

Superphos

Superphos is a new promising type of concentrated phosphate fertilizer with a long-lasting effect. It is produced by chemical enrichment and treatment with a mixture of sulphuric and phosphoric acids of phosphate meal.

The consumption of acids to produce 1 ton of P2O5 in superphos – 1-1.3 tons of sulfuric and 0.36 tons of phosphoric acid – 2 times less than the production of 1 ton of P2O5 double superphosphate. The use of P2O5 phosphate raw materials reaches 95%.

Superphos is produced in granular form, and contains 38-40% P2O5, 19-20% of which are in water-soluble form. The efficiency of action is not inferior to double superphosphate.

Fertilizers containing phosphorus that is insoluble in water but soluble in weak acids

Semisoluble (citrate-soluble) forms of phosphate fertilizers are used on all types of soils and for all crops, but their effectiveness can strongly depend on the type of soil. On acidic soils, the effect of fertilizers with an alkaline reaction, such as tomaslag and phosphate slag, may be higher than superphosphate.

Precipitate (dicalcium phosphate)

Precipitate, or dicalcium phosphate, calcium hydroorthophosphate, monocalcium phosphate, – CaНРO4⋅2H2O. It is obtained by interaction of orthophosphoric acid and milk of lime (calcium hydroxide solution) or a suspension of calcium carbonate:

2H3PO4 + 2Ca(OH)2 = 2(CaHPO4⋅2H2O);

H3PO4 + CaCO3 + H2O = CaHPO4⋅2H2O + CO2.

Quantities of initial substances are taken in the ratio corresponding to the chemical reaction equation.

The precipitate is separated from the liquid, dried at not more than 100 °C to prevent loss of crystallization water, which contributes to the solubility of the precipitate.

The technology of precipitate production as a fertilizer is not economically justified, so it is mainly used for fodder purposes. As a fertilizer, it is obtained by recycling weak solutions of orthophosphoric acid, which are wastes from other industries, such as the production of gelatin at bone-processing plants.

Fertilizer precipitate is a white or light gray powder that does not cake and is well dispersible. Depending on the initial raw material, it contains 25-35% of the citrate-soluble form P2O5. Feed precipitate contains 44% P2O5, no more than 0.2% P, 0.001% As, 0.002% Pb.

By its effect on the yield is similar to superphosphate, but it is used only for the main application for plowing in the same doses of P2O5 as superphosphate. On soils not saturated with bases and grey soils, the efficiency of precipitate is higher than superphosphate, due to stronger binding of phosphorus by superphosphate. On chernozems the effect of superphosphate is the same or slightly superior to that of precipitate.

Unfluorinated phosphate

Unfluorinated phosphate, or calcium orthophosphate, tricalcium phosphate – Ca3(PO4)2 contain 28-32% of citric-soluble P2O5. The content of P2O5 fertilizer refers to a concentrated phosphate fertilizer. 

It is produced by thermal treatment of phosphate raw materials. The process involves steaming water vapor mixture of apatite or phosphate with 2-3% silica (sand) at 1400-1550 °C. Fluorine, contained in apatite, is separated as hydrogen fluoride. The degree of defluorination reaches 94-96%.

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

n[Ca3(PO4)2]3CaF2 + mSiO2 + nH2O = 10nCaO⋅3nP2O5mSiO2 + nHF.

The resulting product contains up to 30-32% (from apatite) or up to 20-22% (from phosphorite) of citrate-soluble P2O5, depending on the raw material.

Unfluorinated phosphate has good physical properties. As a basic fertilizer on sod-podzolic and chernozem soils, it is as effective as superphosphate.

Unfluorinated phosphate is mainly used for mineral feeding of animals.

Томасшлак

Thomas slag, or fused magnesium phosphates, contains phosphorus in the form of tetracalcium phosphate (4CaO⋅P2O5 or Ca4P2O9) or silicocarnatite (Ca4P2O9⋅CaSiO3). According to technical specifications, the content of lemon-soluble P2O5 must be at least 14%. They occupy a small part among the used phosphate fertilizers.

It is obtained as a byproduct of the processing into iron and steel phosphoric iron by the method of S. Thomas. As the content of phosphorus reduces the quality of the metal, for its removal Thomas suggested in 1879 that phosphorus should be bound with freshly burnt lime. At 1800-2000 °C, phosphorus was oxidised to P2O5, and the binding of P2O5 resulted in the formation of calcareous salts of phosphoric acid. These compounds with calcium silica and other impurities float to the surface of molten metal in the form of slag, which is separated, after cooling is crushed, milled, and in this form used as phosphate fertilizer.

An excess of SiO2 produces a double salt of tetracalcium phosphate and silica calcium – silicocarnatite, and a lack of SiO2 produces tetracalcium phosphate. Both salts are soluble in 2% citric acid. Thomas slag also contains hard-soluble phosphates.

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

Used as a basic fertilizer. More effective on acidic soils, as it has an alkaline reaction.

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

Thomas slag is used on all types of soils, in which phosphorus fertilizers have a positive effect on yield, but its effectiveness on different soils is manifested differently. On chernozems – weaker than superphosphate, on soils of the Non-Black Earth zone, especially acidic peaty and sandy, tomas slag is more effective as it reduces acidity. Neutralizing ability of tomas slag is important when combining phosphorus fertilizers with physiologically acidic forms of nitrogen fertilizers.

Martin phosphate slag

Martin phosphate slag is produced as a by-product of the steelmaking process in open-hearth furnace steelmaking. Lime materials are also used for binding phosphorus.

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

It is used as a basic fertilizer. Has a strong alkaline reaction, so it is more suitable for acidic soils. Due to the low phosphorus content, it is advisable to use near production sites.

Thermophosphates

Thermophosphates contain 18-34% P2O5, produced by fusion or sintering of natural phosphates with carbonates or silicates of sodium or potassium, as well as metallurgical slags, lime, quartz. In this process, hard forms of phosphorus are converted to a citric-soluble form.

The melting temperature of thermophosphates is 1000-1200 °C. During high-temperature treatment the crystal lattice of phosphate raw materials is broken, fluorine is released in the form of hydrogen fluoride, and phosphorus passes into an amorphous form Ca3(PO4)2, which is accessible to plants. Amorphous form is obtained and remains stable at 1180 ° C. With decreasing temperature, it transforms into crystalline form, which is poorly assimilated. Therefore, the reaction mass is rapidly cooled to reduce this transition.

In composition and properties thermophosphates are similar to tomas slag and can be used on all soils. Thermophosphates obtained by fusion with alkaline salts, soluble in citric acid and ammonium citrate solution, have better accessibility to plants than tomas slag. The advantage of this method of phosphate fertiliser production is that low-percentage phosphate and apatite, which are unsuitable for superphosphate production, can be used as a feedstock.

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

Bone meal

Bone meal is a by-product of bone processing. Skimmed bones are treated with steam at 1.5-2 atm, followed by a rinse with water to extract the glue. The result is a degreased and de-glued bone pulp, which is treated with hydrochloric acid. The minerals Ca3(PO4)2, Mg3(PO4)2 are dissolved and the soft bone structure consisting of ossein is separated. When heated with water, ossein yields high-quality gelatin.

A hydrochloric acid solution of phosphates is treated with “lime milk,” in which the phosphates precipitate with the formation of precipitate according to the equation:

Н3РО4 + Са(ОН)2 = СаНРO4⋅2Н2О.

Skimmed and deglazed bone meal contains 30-35% P2O5 and up to 1% nitrogen. The phosphorus in bone meal is in a water insoluble form, but is more available than phosphate meal. The effectiveness of bone meal is affected by the acidity of the soil. Even in slightly acidic environments, bone meal has a good effect on crop yields.

Fused magnesium phosphate

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

It is advisable to use on sandy loam soils, in which crops are well responsive to magnesium.

Red phosphorus

Red phosphorus contains 229% of phosphorus in terms of P2O5. A promising most highly concentrated phosphate fertilizer. To convert it into a plant-accessible form in the soil at the same time a catalyst, such as copper, about 1% of the weight of phosphorus is used.

On sod-podzolic soil, after 3 weeks after embedding, 20% of red phosphorus changes into compounds available for cereal crops. Its efficiency is equal to that of superphosphate, and later it surpasses it.

Fertilizers containing phosphorus that is poorly soluble in weak acids but soluble in strong acids

Hard-soluble fertilizers have a fairly good effect on acidic soils of the Non-Black Earth zone and on soils of the northern part of the Black Earth zone (leached and podzolized chernozems).

Phosphate meal

Phosphate meal (phosphorite flour) is finely ground phosphorite. It is used as a fertilizer on acidic sod-podzolic, gray forest and peaty soils, on podzolized and leached chernozem soils and red soils. On typical, common and southern chernozems the effect of phosphoric flour is weaker and unstable.

Phosphate meal is the cheapest fertilizer. It ranks second after superphosphate in terms of production and use. Its production consists of removing coarse impurities (sand, clay) from phosphate rock, cutting it into 1 to 3 cm pieces and grinding to a fine flour. Grinding fineness of phosphate meal affects its efficiency. According to the requirements of technical specifications at least 80% of the mass of phosphate flour should have a particle size of no more than 0.17 mm.

As raw materials, chelated phosphate rock, often of low percentages, without a pronounced crystalline structure, is used. Grinding of such raw materials yields a flour, suitable for direct application, but it is of little use for chemical processing. Such phosphorites include raw materials from the Egorievskoye, Shchigrovskoye, Seshchinskoye, Krylovetsky, and Kineshemskoye deposits.

Phosphate meal is a powder of gray, dark gray or brown color. The content of P2O5 in the fertilizer of the first grade is 28-30%, the second – 22-24%, the third – 19-21%. Fertilizer is non-hygroscopic, does not caked, well dispersed, a lot of dust.

Phosphorus in phosphate meal is contained in the form of fluorapatite [Ca3(РO4)2]3СaF2, which is insoluble in water and poorly soluble in weak acids, so it is inaccessible to most plants.

The efficiency of phosphate meal is influenced by the origin and composition of phosphate, the fineness of grinding, biological characteristics of crops, soil properties and the acidity of the accompanying fertilizers.

Phosphate meal interacts with the soil, which has actual and potential acidity, with the gradual decomposition of calcium phosphate, its transformation into dicalcium phosphate:

Ca3(PO4)2 +2H2O+ 2CO2 → 2CaHPO4 + Ca(HCO3)2.

According to studies, soils with a hydrolytic acidity of less than 2.5 mg⋅ eq/100 g, poorly decompose phosphorite, so its efficiency on such soils is low. On the contrary, the higher the hydrolytic acidity, the more effective action of phosphate meal. This explains the positive effect of phosphoric flour on degraded and leached chernozems, on which exchange acidity is low and hydrolytic acidity is high.

Soils with a small absorption capacity at hydrolytic acidity of 3-3.5 mmol/100 g of soil and saturation with bases of 50-60%, as a rule, have an acid reaction (5,0-5,5), which is caused by exchange acidity. At high absorption capacity, hydrolytic acidity equal to 6-7 mmol/100 g of soil and saturation degree of 75-85% the reaction is close to neutral (6.0-6.5). Therefore, high effect of phosphate meal will be manifested at high acidity and lower degree of saturation of soils with bases.

The effect of phosphate meal is influenced by the absorption capacity and the degree of saturation of the soil with bases. With the same hydrolytic acidity, the efficiency increases with decreasing absorption capacity.

The total interaction surface of phosphate meal with the soil increases with increasing fineness of grinding.

Table. Increase in potato yield from mineral fertilizers on different soils (All-Russian Institute of Fertilizers and Agrochemistry)

Average particle size, mm
Relative increase
in the number of particles
in the particle surface
in efficiency
0,510
1
1
1
0,220
12
2,3
2,2
0,092
170
5,5
3,6
0,041
1920
12,4
4,9
0,005
1061200
102,0
6,0

The effectiveness of phosphate meal depends on the biological characteristics of plants. The results of the experiments of D.N. Pryanishnikov, P.S. Kossovich and other scientists allowed to divide crops into groups according to their ability to assimilate phosphorus from hard-soluble phosphates. Plants with a good ability to assimilate difficultly soluble phosphates include lupine, buckwheat, mustard; somewhat less so peas, sainfoin, melil and hemp. All cereals, flax, beets, potatoes, vetch can assimilate phosphorus from phosphate meal only after interaction with acidic soils. Barley, spring wheat, flax, millet, tomato, turnipa do not assimilate phosphate flour.

The ability of plants to assimilate hard-soluble phosphates changes with age. Most plants are poor at assimilating the hard-to-digest forms during the first period of life; later in life, this ability increases.

Most scientists explain the ability to assimilate hard-to-dissolve phosphates by the amount and composition of acidic root excretions of plants. F.V. Chirikov explains this ability by the increased consumption of calcium: plants that absorb more calcium, better assimilate phosphorus.

The assimilation of phosphorus from phosphate meal, as established by D.N. Pryanishnikov, also depends on the accompanying fertilizers: physiologically acid fertilizers increase the availability of phosphorus, physiologically alkaline fertilizers and lime materials – reduce.

Phosphate meal was widely used in the former Soviet Union. Its use is likely to expand in the future. The disadvantage of phosphate meal is that it is very dusty. To reduce dusting, mixtures of phosphate meal can be used:

  1. Mixtures of phosphorite flour with ammonium chloride in the ratio N:P2O5 = 1:1; in this case the content of each nutrient is 14%, dustiness is eliminated, and the content of citric-soluble phosphorus increases by 1.5 times.
  2. The impact on the phosphate meal molten potassium disulfate at a temperature of 205-210 °C for 50-60 minutes in a screw mixer: dustiness is eliminated, the content of P2O5 is 16%, of which 70% in lemon soluble form, the content of K2O – up to 17%.

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

To increase availability, phosphorite meal is composted with topsoil peat or manure. In composts, the effectiveness increases with sour peat at a peat to flour ratio of 100:1. Phosphorite meal is used to prepare peat and manure-phosphate composts.

Phosphorite flour for better decomposition of phosphate meal is made in advance under deep plowing in a moist layer. Application in double or triple doses, has a prolonged effect.

In Russia acidic soils under arable land occupy about 50 million hectares. They are usually characterized by low content of mobile phosphorus.

Vivianite (bog ore)

Vivianite, or bog ore, is phosphoric acid iron oxide – Fe3(PO4)2⋅8H2O. It contains 28% P2O5. It is found under a layer of peat in the form of whitish mass.

A good source of phosphorus for crops on sod-podzolic, gray forest soils and leached chernozems. Vivianite is easily loosened by drying, well dispersed.

Phosphoritization

Phosphoritization is the application of hard-soluble phosphate (phosphorite flour) in the crop rotation for several years ahead and is one of the methods of increasing soil fertility, especially of acidic soils, the effectiveness of mineral fertilizers and increasing crop yields. Phosphate meal is applied in large doses to 1-1.5 t / ha, which provides phosphorus nutrition plants for 6-8 years, improves the nutrient regime and increases the productivity of the rotation.

Improvement of phosphate regime leads to increased efficiency of other fertilizers. Phosphoritization is a reclamation technique to improve the fertility of acidic soils, the effectiveness of which depends on the acidity and the provision of mobile phosphorus. First of all phosphoritization is carried out at pH below 5.5 and the content of mobile phosphorus to 5 mg/100 g soil.

Phosphoritization in the rotation is best done in a pair for winter crops and cereals with undersowing leguminous grasses, which are able to absorb hard-soluble phosphorus, better accumulate nitrogen and increase productivity of subsequent crops in the rotation.

Phosphoritization is used for radical improvement of meadows and pastures. Lime and phosphate meal are applied separately, for example, before and after plowing, in different soil layers.

Phosphoritization is a mandatory method of improving newly developed low fertility lands, drainage, development of peatlands and low fertility acidic meadows on mineral soils. Norms of phosphate meal is not less than 200 kg of P2O5, or 1 ton per meal. For more accurate calculation of the dose use rates of the nutrient to increase mobile phosphorus per 1 mg/100 g of soil.

Table. Fertilizer rates for increasing the content of mobile phosphorus by 10 mg P2O5/kg of soil (by Litvak Sh.I., 1990; Sychev V.G., Shafran S.A., 2013)

Soil
Granulometric composition
Method of determination
Fertilizer consumption, kg/ha
data variation
standard*
Sod-podzolicsandy and sandy loamby Kirsanov
47-90
50-70
light loam
60-108
70-80
medium loam
60-110
80-90
heavy loam
90-120
100-110
Grey Forestsandy and sandy loamby Kirsanov
70-80
70-80
loamy
80-110
90-110
heavy loam
120-140
120-140
Podzolized black earthlight loamby Chirikov
74-109
90-100
loamy
80-120
100-110
Leached black earthheavy loamby Chirikov
90-135
110-120
Typical black earthheavy loamby Chirikov
103-141
120-130
Common black earthloamyby Chirikov
94-122
100-110
heavy loam
100-140
120-130
Carbonate black earthsOn averageby Machigin
-
110-120
ChestnutOn averageby Machigin
-
90-110

Example. Initial data: sod-podzolic loamy sand soil; pH 4.5; content of mobile phosphorus 4.6 mg/100 g soil; planned – 9 mg/100 g soil.

The dose of phosphoritic flour is determined by the formula:

D = (P – F) ⋅ C, or

D = (9,0 – 4,6) – 60 = 264 kg P2O5/ha,

where D – dose, kg/ha P2O5; P – planned content, mg P2O5/100 g soil; F – actual content, mg P2O5/100 g soil; C – consumption of P2O5 to increase its content by 1 mg/100 g soil.

Interaction of phosphate fertilizers with soil

The solubility of phosphate fertilizers, including water-soluble ones, is lower than that of nitrogen and potassium fertilizers. When applied to the soil as the phosphate ion dissolves, it transforms into compounds characteristic of a particular soil type and due to the genetic features, physical, chemical and mineralogical properties, the degree of cultivation. The speed of this process is slow, so part of the applied phosphate fertilizers, especially in granular form or in semi-soluble and insoluble forms, remain unchanged for a long time.

Transformation of soluble fertilizer phosphorus can be due to:

  • chemical absorption of phosphate ions by cations of calcium, magnesium, oxides and hydroxides of iron, aluminum, manganese and titanium;
  • colloid-chemical (exchange) absorption on the surface of the soil solid phase;
  • biological absorption by plant root systems and soil microflora.

Exchange absorption (adsorption) of phosphate ions occurs on the surface of positively charged colloidal particles, such as colloids of oxide hydrates, or on positively charged areas of negatively charged colloids, such as the minerals of the kaolinite and montmorillonite groups and hydromica and colloids of the protein group. Exchangeable absorption is stronger in an acidic environment. For example, illite (a mineral of the hydromica group), bentonite (of the montmorillonite group) and kaolinite adsorbed at pH 4-4.5 from 7.7 to 9.7 mg-eq H2PO4 per 100 g of mineral. There were no significant differences in the absorption of anions by minerals of montmorillonite and kaolinite groups, as in the case of exchange absorption of cations. The reaction of the medium leads to a change in the electrical potential of the soil colloids. Acidification of the soil solution promotes better absorption of anions; alkalinization, on the contrary, causes a decrease in absorption. Therefore, for soils with weakly acidic and neutral reaction, exchange uptake is weaker (Antipov-Karataev et al:)

Soil
PO43- adsorbed from 0.05 n. H3PO4, mg⋅eq/100 g soil
Black earth
18,3
Podzolic
41,9
Red earth
74,0

The exchange absorption of phosphate ions on the common chernozem of the Stone Steppe is also confirmed by I.P. Serdobolsky.

According to the All-Russian Institute of Fertilizers and Agrochemistry, adsorption absorption of sod-podzolic soils accounts for 70-80% of the total amount of absorbed phosphates.

Exchange-absorbed phosphoric acid anions can be displaced into solution (desorption) by other mineral and organic acid anions, such as hydrocarbonate-ion, citric, malic, oxalic, formic and humic acids. These anions are always present in the soil solution as a result of plant respiration and root excretions as well as microbiological decomposition of plant residues and organic fertilizers. Thus, there is no shortage of anions for desorption of phosphate. This determines the good mobility and plant availability of exchange-absorbed phosphates. According to the results of studies, the availability of exchange-absorbed phosphate is close to water-soluble. However, the latter are few in the soil solution, so it is the exchange-absorbed phosphates that play an important role in the phosphorus nutrition of plants.

Part of the phosphate of fertilizers dissolved in the soil solution is absorbed by the soil through chemical binding. Peculiarities of chemical absorption are determined by soil type and soil acidity.

The pH value of the soil determines the solubility of calcium, magnesium, aluminum, iron, manganese, titanium salts, which, when interacting with water-soluble phosphate, translate it into hard-soluble compounds. For example, at pH less than 5, the content of aluminum ions in the soil increases, and at pH less than 3 – iron ions. It is generally accepted that the lowest binding of phosphate and the highest mobility occurs in the pH range of 5.0-5.5. In more acidic soils, aluminum and iron oxides are absorbed; in less acidic soils, calcium and magnesium are absorbed.

Thus, in soils with a near-neutral reaction of water-soluble phosphate fertilizers, monophosphate [(Ca(H2PO4)2⋅H2O] after some time by chemical absorption into the two-substituted calcium and magnesium phosphates (CaНРO4⋅2H2O or MgНРO4) and remain long time in a form accessible to plants. Subsequently, the hydrogen of the two-substituted salt is gradually replaced with calcium or magnesium, forming three-substituted phosphates Ca3(РO4)2, Mg3(РO4)2, and subsequently the basic phosphate octacalcium phosphate [Ca4H(РO4)3⋅ЗН2O], with solubility constantly decreasing.

However, while these salts are in a freshly precipitated amorphous state, they retain the ability to dissolve in weak acids, which causes some accessibility to plants. Only as the three-substituted and basic phosphates crystallize (“ageing”), they lose their accessibility. The process of “aging” of phosphates is called phosphate retrogradation.

In sod-podzolic soils with acidic and weakly acidic reaction the chemical binding of phosphate ions is due to mobile semi-hazardous oxides:

Al(OH)3 + H3PO4 → AlPO4 + 3H2O;

Fe(OH)3 + H3PO4 → FePO4 + 3H2O.

Freshly precipitated amorphous aluminum and iron phosphates also remain available to plants for some time, but become insoluble as they “age”. Both water-soluble phosphates of fertilizers and phosphates transferred to the solution from the exchange-absorbed state as a result of desorption are subject to chemical absorption.

The intensity of chemical and colloidal-chemical absorption of phosphate fertilizers is in direct dependence on the content of mobile forms of oxides such as R2O3. Phosphoric acid as a result of biological absorption is able to be fixed in the soil, in the body of microorganisms. In terms of energy of absorption of phosphate of soluble fertilizers soils can be arranged in the following sequence: red soils > podzolic soils > chernozem > gray soils.

The process of phosphate uptake by the soil and further transformation is very slow. Experience of long-term application of high doses of phosphate fertilizers several times higher than P2O5 removal showed that most of the phosphorus is accumulated in soils in easily soluble form in amounts up to 600-1000 mg/kg of soil.

At the same time there is an excessive accumulation of phosphorus in soils. This phenomenon is observed in several European countries, which used phosphate fertilizers for over a century. In the late 80’s excessive accumulation of phosphorus occurred in Russia in the beet-growing zone and some farms of the Moscow region.

Field and vegetation experiments have shown that “residual”, i.e. previously unused phosphorus fertilizers remain available to plants. Thus, the effects of previously applied phosphorus fertilizers at Rotamsted experimental station have been observed for more than 50 years.

These experiments show that significant amounts of phosphate are not permanently fixed in the soil. There is information about the possibility of mobilization of phosphate resources of soils in conditions of deficit of phosphate fertilizers. In this case there is a gradual transformation of hard-soluble phosphates into soluble ones.

However, long-term cultivation of crops in conditions of phosphorus fertilizer deficiency leads to depletion of soil reserves and their gradual degradation.

Effectiveness of phosphate fertilizers

The effectiveness of phosphate fertilizers depends on:

  • soil and climatic conditions;
  • fertilizer properties;
  • zonal peculiarities of soils;
  • biological characteristics of crops;
  • agrochemical methods to optimize the use of phosphate fertilizers;
  • phosphate content;
  • moisture availability.

Features of the application of phosphate fertilizers, taking into account their solubility:

  1. water-soluble phosphates are used on all soils, under all crops and in different ways;
  2. the effectiveness of phosphates soluble in weak acids depends on soils, for example, on acidic soils their effect is higher than that of superphosphates;
  3. hard-soluble fertilizers are effective on acidic soils of the Non-Black Earth zone and on northern leached and degraded chernozems.

On all soils, superphosphate and precipitate have a more stable effect on the yield.

Influence of soil phosphorus content on efficiency

Phosphorus fertilizers have a greater effect on yield on soils with low and medium content of mobile phosphorus, while on soils with high and high content the effect is weak or absent.

On sod-podzolic and gray forest soils, the optimum content of mobile phosphorus by Kirsanov’s method is 10-15 mg/100 g. This level of security is considered sufficient to obtain under normal climatic conditions and nitrogen-potassium fertilizers on the background of high yields of field crops, such as grain – to 5.5 t/ha, hay perennial grass – 5,5-7,0 t/ha. The same value of the optimum content of mobile phosphorus according to Chirikov’s method is accepted for non-carbonate chernozems. On carbonate chernozems, chestnut and gray soils, the optimum content according to Machigin’s method is 3-4.5 mg/100 g.

With the content of mobile phosphorus in sod-podzolic soils, 10-12 mg/100 g, yield increases from making phosphorus fertilizers are unstable, and at 15 mg/100 g, the effect is usually absent. Complete rejection of phosphate fertilizers on these soils is not advisable, since it leads to depletion of soil mobile phosphates, so make compensating doses of fertilizers P2O5 removal by plants. Agrotechnically optimal can be considered a combination of making the main fertilizer hard-soluble forms with a row (start) introduction of soluble.

Phosphate fertilizers must be made at high doses of nitrogen-potassium fertilizers to avoid imbalances in the ratio of elements (N:P:K).

Application of fertilizers on soils with a low content of phosphorus should provide a gradual increase in content to optimum levels. To do this, the doses are calculated not only for the planned yield, but also to increase soil fertility. To increase the content of mobile phosphorus in the soil by 1 mg/100 g, you can use the developed All-Russian Research and Design Institute of Agricultural Chemistry nutrient rates.

Table. Consumption of nutrients to increase the content of mobile phosphorus in the soil by 1 mg/100 g[2]Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.

Soils
Granulometric composition
Consumption P2O5
Sod-podzolicSandy and sandy loam
50-60
Light and medium loamy
70-90
Clay and heavy loam
100-120
Grey forestsSandy and sandy loam
70-80
Light and medium loamy
90-110
Clay and heavy loam
120-140
Podsoled and leached black soilsSandy and sandy loam
80-90
Light and medium loamy
90-100
Clay and heavy loam
100-120

Influence of moisture availability

The effectiveness of phosphate fertilizers depends on the moisture availability of crops. As the continental climate increases, which leads to a decrease in moisture availability, the effectiveness decreases. However, phosphate fertilizers contribute to economical consumption of moisture by plants, so they mitigate the effects of moisture deficit.

Efficiency of phosphate fertilizers depending on soil type

On chernozem soils phosphate fertilizers show good efficiency, which is explained by the sufficient supply of nitrogen to the soil and the development of root systems of plants. On phosphorus fertilized background plants spend 10-15% less water to create a unit yield.

On gray forest soils, the effect of phosphorus decreases due to deterioration of nitrogen supply and mobility of organophosphorus compounds in them.

On sod-podzolic soils phosphate fertilizers show a fairly high efficiency if used in combination with other fertilizers, with observance of agronomic and meliorative measures.

Effect of fertilizer properties

To avoid nitrogen losses when applying phosphate and nitrogen fertilizers, observe the following rules:

  1. Do not mix alkaline forms of phosphate fertilizer with ammonium forms of nitrogen fertilizer.
  2. Dry superphosphate is mixed with ammonium nitrate before application, as their mixture dries out during long storage.
  3. Mixing superphosphate and ammonium sulfate leads to the formation of gypsum, the mixture hardens during prolonged storage.
  4. When you mix acidic superphosphate with nitrate fertilizer leads to the formation of free nitric acid, which quickly volatilizes:

Н3РО4 + NaNO3 = NaH2PO4 + HNO3.

  1. Before application, excessive acidity of superphosphate, which negatively affects young plants, is eliminated by mechanical mixing with phosphate rock (up to 15%), dolomite flour (up to 10%) or lime.

Methods of phosphate fertilizer application

Phosphate fertilizers, as a rule, are applied in two ways: pre-sowing and main. Given the low mobility of phosphate in the soil with poor root system development in the initial period of growth, the role of pre-sowing application of phosphate fertilizers is important in the formation of high yields.

Even on soils with a high content of mobile phosphorus the concentration of phosphate ions in the soil solution is not sufficient to provide plants with sufficient phosphorus in the early stages of growth. Row (starting) application of phosphate fertilizers is carried out in doses of 7-20 kg/ha of P2O5. At the same time only water-soluble easily accessible forms – granular superphosphates are used. Powdered superphosphates in the spring conditions quickly become damp, clumpy and clog the fertilizers.

According to CINAO data, 1 ton of granular superphosphate at row application gives an increase of 5-6 tons of grain, at main application – 1-2 tons. Phosphate fertilizers also affect the quality of production: increases the protein content of grain, sugar content of sugar beet roots, starchiness of tubers, accelerated ripening.

Water-soluble phosphate fertilizers, often granulated superphosphate, give a good effect, when applied when sowing crops in wells and nests. The fertilizer is applied by combined seeding machines. For sugar beet, potatoes, and other crops superphosphate is made by combined seeding machines simultaneously with nitrogen or nitrogen-potassium fertilizers. According to experimental data, 0.5 kg of granulated superphosphate, or 10 kg P2O5 per 1 hectare provides an additional 250-300 kg/ha of grain. When there is a deficit of phosphate fertilizers, superphosphate application under grain crops during sowing shows good efficiency.

Feeding with superphosphate may be effective:

  • when inadequate doses of phosphate fertilizer are applied in the main application, under autumn plowing;
  • in areas of sufficient moisture or irrigation;
  • on soils with strong chemical absorption in case of prolonged contact of superphosphate with soil, especially on acidic ones with high R2O3-type oxide content.

In other cases, top dressing is less effective than the application of similar doses before sowing or in the rows.

A wide variety of soil types in Russia allows the successful use of all types of phosphate fertilizers for the main fertilizer.

Timing of phosphate fertilizer application

The timing of application is important for hard-to-remove phosphates. They are applied well in advance, in the fall, so that some of the calcium phosphate has had time to transform into more accessible forms by the growing season.

Depth of phosphate fertilizers embedding in the soil

Due to the low mobility of phosphate in the soil, the depth of embedding of the main phosphorus fertilizer is important. Therefore, it is sought to create a supply of available phosphorus in the zone of the location of the active part of the root system of plants. This is especially important in arid conditions, where the upper part of the arable layer dries up in summer. Thus, in the experiment with 32P, surface application of superphosphate on pasture at a dose of 450 kg/ha P2O5 did not lead to penetration of phosphorus deeper than 2.5 cm.

The depth and location of fertilizer depend on the method of embedding.

Table. Placement of fertilizers in the arable soil layer depending on the method of their embedding, %[3]Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. - Moscow: Kolos, 2002. - 584 p.: ill.

Depth of arable layer, cm
Method of fertilizer embedding into the soil
light harrow
heavy harrow
heavy cultivator
plow
with a skimmer plow
0-3
98
75
55
11
3
3-6
2
22
21
12
4
6-9
3
23
16
12
9-12
1
16
14
12-15
23
20
15-20
22
47

From the above data, we can see that the main part of the fertilizer concentrates in the upper (0-9 cm) layer by harrows or cultivators. More uniform embedment is achieved by the plough without skimmer, deeper – plough with skimmer, but in this case in the upper layer of fertilizer remains little. In the latter case, there is a need for row-before-sowing fertilizer. Applied phosphate fertilizers do not migrate along the soil profile and remain in the places of embedding. Only subsequent tillage changes their location in the arable layer.

Therefore, the depth of plowing for a particular crop determines the depth of embedding of the main phosphorus fertilizer.

Optimization of phosphate fertilizer doses

Soils with sufficient reserves of phosphorus through systematic fertilization are able to provide crops with optimal phosphorus nutrition for a long time. Phosphorus mitigates the effect of extreme weather conditions on plants, high yields can be formed even in conditions of drought, low or high temperatures.

In the practice of world farming, especially in Europe, the increase of phosphorus content in the soil in the rotation is achieved by periodic application of high doses of phosphorus fertilizers. Due to the conservation of phosphorus in a form accessible to plants, the weak migration along the soil profile and the absence of losses, as well as data on the optimum levels of mobile phosphorus for crops allows you to calculate the rates of phosphorus fertilizers, which are necessary to achieve optimum phosphate nutrition. The main way to maintain an optimal diet of phosphorus is to apply mineral and organic fertilizers.

Agrochemistry has accumulated enough knowledge about this biogenic element, there are still a number of unresolved problems:

  1. Low utilization rate of phosphorus fertilizers by individual crops and, in general, in the agrocenosis.
  2. Systematic application of high doses of phosphorus fertilizers and over-phosphating of soils leads to a violation of the balance of other biogenic elements, which worsens the nutrient regime.
  3. Various substances contained in phosphate fertilizers in the form of impurities, including heavy metals, have a negative impact on the environment and by getting into plants and agricultural products.
  4. Immobilization (retrogradation) of phosphorus in the soil as a result of chemical absorption. These processes are particularly intense in carbonate chernozems, red soils, acidic sod-podzolic soils with a high content of aluminum and iron oxides.
  5. Mobilization of soil phosphates. It is especially important for those farming areas and soils where as a result of systematic application of large doses of phosphate fertilizers created stocks that exceed the optimal phosphate level.

Optimization of phosphorus nutrition of crops depends on the specification of crop rotations in specific soil and climatic conditions. The complexity of optimizing phosphorus nutrition of plants is associated with the binding of a number of biogenic elements, such as zinc, copper and the imbalance of nutrients in the soil.

Development and application of optimal doses of phosphorus is associated with a set of agrotechnical, chemical and biological methods of mobilization of phosphorus accumulated as a result of systematic application of phosphorus fertilizers. Thus, the use of physiologically acidic nitrogen and potassium fertilizers in combination with trace elements mobilizes phosphorus on chernozems, gray and chestnut soils, which were introduced excessive amounts of phosphorus. In this case, it is possible to obtain high crop yields for a long time without making phosphorus fertilizers. Liming of acidic sod-podzolic soils also contributes to the mobilization of soil phosphates associated with halved aluminum and iron oxides.

When solving the problems of optimization of phosphorus fertilization, taking into account the phosphate regime of the soil must be taken into account:

  1. Objective evaluation of the effectiveness of phosphorus fertilizers is carried out not only on the productivity of individual crops, but also on the crop rotation.
  2. Methods for assessing the phosphate level and optimization of phosphorus fertilization depend on the ways of determining mobile phosphorus in the soil.
  3. For objective assessment you should consider both the content of mobile phosphorus by the method adopted for this type of soil, and its mobility in weak salt suspensions.

On sod-podzolic light loamy soils, the optimum content of mobile phosphorus in the arable layer is considered to be 10-15 mg/100 g of soil. On these soils, provided a good agricultural practices and the provision of plants with nitrogen and potassium, the average annual productivity of the field rotation is 45-50 centners of grain units of the main products. A higher content of mobile phosphorus reduces the payback period of phosphorus fertilizers.

Optimal phosphate regime on gray forest soils is close to the regime of sod-podzolic soils when using Kirsanov method. The same value of the optimum content of mobile phosphorus is established for chernozems, when determined by the Chirikov method. On carbonate chernozem, gray and chestnut soils the optimal level is 3-4.5 mg/100 g of soil by Machigin’s method.

To optimize the phosphorus fertilizer in addition to the optimal content use:

1. balance coefficient of utilization, or balance coefficient, removal coefficient. Shows the proportion of nutrient removal from the nutrients applied with fertilizer, calculated by the formula:

where Kb – balance coefficient; R – phosphorus removal with the crop; D – the dose of phosphorus applied.

2. Compensation compensation coefficient, or intensity of balance (Kc), the value inverse to Kb, is equal to:

Balance coefficient – a measure of fertilizer efficiency at the appropriate content of nutrients for given soil conditions.

Increase or decrease in fertilizer dose (Kopt, %) in accordance with the nutrient removal is calculated by the formula:

The optimal dose of fertilizer is calculated by the formula:

Dopt = Ropt ⋅ V%.

Then, the degree of soil supply of phosphorus depending on the content of its mobile form (K) is equal:

К = Dopt – Ropt.

At a low content of soil mobile phosphorus K is 48-55 kg P2O5/ha, at medium – 17-20 kg P2O5/ha, at high – 3-6 kg P2O5/ha.

3. Doses of phosphorus and potassium fertilizers are calculated according to the formula:

DР(К) = R – SO + CР(К),

where DP(K) – the dose of phosphorus or potassium fertilizers, kg a.s./ha; R – phosphorus or potassium removal with the planned yield, kg/ha; SO – phosphorus or potassium content in organic fertilizers, kg/ha; CP(K) – amount of phosphorus or potassium increasing these elements by 10 mg/kg in soils with low content and 5 mg/kg in soils with average nutrient content, kg/ha.

For the optimum level is taken the content of mobile phosphorus in the soil, which achieves at least 90-95% of the maximum yield, and the missing 5-10% is replenished by phosphorus fertilizer to compensate the removal of the planned yield.

Generalization of the results of long-term experiments allowed to develop general principles of differentiation of doses of fertilizers, taking into account specific conditions.

Table. Differentiation of doses of phosphorus fertilizers and phosphorus removal by plants depending on the provision of soils with mobile phosphorus[4]Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. - M.: Publishing house of the All-Russian Scientific Research Institute named after D.N. Pryanishnikov, 2017. - 854 p.

Content of mobile P2O5 in soil, mg/100 g
Doses of P2O5, kg/ha
Dose differential coefficient*
Possible removal P2O5, kg/ha
The coefficient of change of removal
Residual phosphorus in the soil, kg/ha
< 5
120
2,0
30 - 35
0,75
85 - 80
5,1 - 10,0
90
1,5
35 - 40
0,85
55 - 50
10,1 - 15,0
60
1,0
40 - 50
1,00
15 - 5
15,1 - 25,0
30
0,5
45 - 50 и более
1,15
- (25 - 30)
> 25
10**
0,2
45 - 50 и более
1,15
- (45 - 50)

*A single dose (differentiation coefficient of 1) is taken as a dose of P60.

**In a row at sowing.

Doses of P2O5 for pre-sowing application are determined by the crop. Some of them, such as corn and sunflowers, can be inhibited by direct contact of seeds with superphosphate. Therefore you need to create a soil layer between the seeds and the fertilizer; the doses of P2O5 in this case are 7-10 kg/ha.

Cereals and vegetable crops, flax, hemp are less sensitive and respond positively to granulated superphosphate at a dose of about 10 kg/ha, it is acceptable to mix it with the seeds before sowing with an ordinary row seeder. In this case, the seeds and fertilizer must be dry, granules must have good mechanical strength, not to be crushed in the sowing unit and not to clog it. Superphosphate must have a neutral or slightly acidic reaction. Acidic superphosphate even in short contact with seeds (up to 2 hours) reduces the germination of winter rye, barley, spring wheat, flax and beet seeds. If its acidity is less than 1%, it may be mixed with rye and beet seeds not earlier than 2 hours before sowing; with other listed crops – 4-8 hours. Neutralized superphosphate can be mixed with the seeds of these crops one day before sowing.

When sowing sugar beets and potatoes, 20 kg/ha of granulated superphosphate or the same dose of complex fertilizer is applied. The remainder of the total calculated dose of phosphorus (minus pre-sowing dose) is applied in the main fertilizer.

On average, phosphate fertilizer doses vary from 30-45 kg/ha to 90-120 kg/ha of P2O5 and depend on soil fertility, soil and climate conditions, planned yields, forecrop and related fertilizers.

Sources

Yagodin B.A., Zhukov Y.P., Kobzarenko V.I. Agrochemistry / Edited by B.A. Yagodin. – Moscow: Kolos, 2002. – 584 p.: ill.

Agrochemistry. Textbook / V.G. Mineev, V.G. Sychev, G.P. Gamzikov et al. – M.: Publishing house of the All-Russian Scientific and Research Institute named after D.N. Pryanishnikov, 2017. – 854 с.

Fundamentals of Agronomy: Tutorial/Y.V. Evtefeev, G.M. Kazantsev. – M.: FORUM, 2013. – 368 p.: ill.