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Potassium fertilizers

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Potassium fertilizers are mineral fertilizers that meet the potassium requirements of crops.

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Raw materials for potassium fertilizer production

Natural potassium salts are used as raw materials for the production of potassium fertilizers, whose deposits are located in Russia, Germany, France, USA, Canada, Israel, Italy, Poland, England, Ukraine, Belarus, Kazakhstan and other countries.

Of the 120 potassium-containing minerals, only a small portion is of industrial importance.

Table. Minerals used to produce potassium fertilizers

Mineral
Approximate content K2O, %
Silvinite - nNaCl + mKCl
15-25
Carnallite - KCl⋅MgCl2⋅6H2O
17
Cainite - KCl⋅MgSO4⋅3H2O
19
Shenith - K2SO4⋅MgSO4⋅6H2O
23
Langbeinite - K2SO4⋅2MgSO4
23
Alunite - (K, Na)2SO4⋅Al2(SO4)3⋅4Al(OH)3
23
Polygalite - K2SO4⋅MgSO4⋅2CuSO4⋅2H2O
16
Nepheline - (K, Na)2O⋅Al2O3⋅Al2O3⋅2SiO2
6-7

One of the largest potash deposits in Russia is Verkhnekamskoye (over 12 billion tons), located near the cities of Solikamsk and Berezniki on the left bank of the Kama River on the western slope of the Northern Urals. It was formed as a result of the drying up of the ancient Perm Sea. The development of the deposit began in 1925 and the production of fertilizers began in 1929. The upper part of the deposit is represented by carnallite with an admixture of NaCl, CaSO4⋅2H2O, clay, K2O content – up to 17% (10-25%). Carnallite has a mottled color from a combination of yellow, orange, brown and red colors due to the admixture of iron oxide Fe2O3 (iron luster). Below the carnallite lies a thick layer of sylvinite, containing potassium and sodium chlorides in various ratios.

Sulfate potash is obtained from the minerals kainite, langbeinite, mixed langbeinite-kainite rocks and alunite. The deposits of polyhalite, kainite and glaserite (3K2SO4⋅Na2SO4) are available in the Saratov and Orenburg regions and in Bashkiria (Zavolzhskoe deposit).

There are large deposits of potassium minerals in Ukraine in Ivano-Frankovsk and Lvov regions. These deposits are dominated by langbeinite (K2SO4⋅2MgSO4), kainite (KCl⋅MgSO4⋅3H2O), polyhalite (K2SO4⋅MgSO4⋅2CaSO4⋅2H2O), schoenite (K2SO4⋅MgSO4⋅6H2O). The raw materials of these deposits are processed at the Stebnikovsky and Kalushsky combines. The share of impurities is up to 30%, mainly in the form of silt.

Belarusian deposits of potassium salts are located in Polesie (Soligorsk), which is probably an extension of the Carpathian deposits. They are represented by the minerals sylvinite, carnallite and halite.

Zhilyansk deposit in the Aktobe region of Kazakhstan is represented mainly by polyhalite. Carnallite, sylvinite, and glaserite are also present. Polyhalite minerals are the raw material for the production of potassium sulfate, potassium-magnesium sulfate and complex mineral fertilizers. After milling, it can be used as a sulfate form of potassium-magnesium fertilizer, containing 13-15% K2O and 6-7% MgO. 

Other minerals containing potassium can be used as fertilizer. For example, an aluminosilicate of potassium and sodium, nepheline (Na, K)2O⋅Al2O3⋅2SiO2, is found in apatite deposits, for example, in the Khibiny deposit. Its potassium content is 5-6%. It is poorly soluble in water, and in acidic soils, it partially decomposes. It also contains 10-13% Na2O and 8-10% CaO, therefore it has a neutralising effect in sour soils. As a rule, it is used as a local fertilizer on acidic and peaty soils.

In the production of aluminum from nepheline as a waste product is obtained potassium carbonate, containing 63-67% K2O, which is a valuable potash fertilizer.

Classification of potassium fertilizers

Potash fertilizers are classified into raw potassium salts and concentrated potassium fertilizers.

Raw potash salts are sylvinite and kainite.

Concentrated potash fertilizers – potassium chloride, potassium salt, potassium sulfate, potassium magnesium sulfate.

Raw potassium salts

Raw potassium salts (sylvinite, kainite) are produced by crushing and grinding natural potassium salts. As a rule, concentrated layers of the deposit are used for production, less concentrated ones are used for processing. Initially raw potassium salts were mainly used as fertiliser, later on they were substituted by concentrated potassium salts due to the fact that they contain a lot of ballast substances that increase transportation and application costs.

Because of the expensive transportation of raw potassium salts are used in the areas of their extraction on a limited scale. The main part is used to produce concentrated potash fertilizers.

Sylvinite

Sylvinite is a mineral that is a mixture of potassium and sodium chlorides and contains 12-18% K2O and 35-40% Na2O. According to the technical specifications, sylvinite from the Solikamsk deposit must contain 15% K2O. It is hygroscopic and cakes when stored.

It is offered in rough milling with the size of crystals of 1-5 mm. It has a pinkish-brown color with inclusion of blue crystals. It is transported in bulk. It is applied to sodium-loving crops.

Kainite

Kainite (KCl⋅MgSO4⋅3H2O) is a mineral of cainite-langbeinite rock, representing large pinkish-brown crystals, with mechanical admixtures of rock salt (NaCl), CaSO4, MgSO4, etc.  It contains about 10-12% K2O, 6-7% MgO, 32-35% Cl–, 22-25% Na2O, 15-17% SO42-. When potash and potassium chloride are mixed, they make potassium salt, which contains 30-40% K2O. Humidity not more than 5%. It does not cake, it is transported in bulk.

It is a good fertilizer for sugar beets on chernozems. It is extracted in Stebnik (Western Ukraine), the composition of kainite of these deposits is close to Solikamsk sylvinite.

Concentrated potassium fertilizers

In terms of potassium content, the most concentrated fertilizer is potassium chloride, the most used potassium fertilizer in Russia.

Potassium chloride

Potassium chloride (KCl) is the main potassium fertilizer. Production accounts for 80-90% of the total production of potash fertilizers. It is received from sylvinite. Chemically pure potassium chloride contains 63% K2O. Depending on the method of production, the potassium chloride used as fertilizer contains 50-60% K2O. It is a fine crystalline powder of pink or white color with a grayish tint. It has a small hygroscopicity, often caking.

In the industry different methods of production are used, for example, halurgical, flotation, gravitational.

The halurgical method splits potassium and sodium chlorides on the basis of their different solubility. Solubility of KCl doubles when the temperature rises from 0 °C to 100 °C, while the solubility of NaCl almost does not change. Grinded sylvinite is dissolved at 110°C in a solvent lye – saturated NaCl solution, and only KCl of sylvinite dissolves, while NaCl remains insoluble as a precipitate.

When the resulting solution is cooled, a crystalline precipitate of KCl falls out, and the mother saturated solution of NaCl is used to process new batches of sylvinite. The production waste is up to 95% NaCl, which is used to produce soda, technical and table salt.

The flotation method of separating the minerals sylvin (KCl) and halite (NaCl) on the basis of the different ability of the surface of the particles of these minerals to wetting by water. Preliminary crushed ore is agitated in an aqueous solution with addition of alkylsulfates as a reagent-collector at the rate of 100-200 g of the reagent for 1 ton of ore. The reagent is adsorbed on the surface of potassium chloride particles. Then air is blown through the pulp in the form of small bubbles. Particles of hydrophobized sylvin are carried with the air bubbles to the surface in the form of foam. The KCl foam concentrate is dehydrated by centrifugation and dried. The halite particles are collected at the bottom of the flotation machine.

The flotation potassium chloride has larger pink crystals. Hydrophobic additives reduce hygroscopicity and caking. The advantage of the method is that no high temperatures are required and the product has better physical properties. The flotation method is used at Berezniki potassium plant; the produced potassium chloride contains 60% K2O. 

The flotation method is the most widespread in Russia.

The gravity method is relatively new in France and other countries, based on the different densities of KCl (1.987 g/cm3) and NaCl (2.17 g/cm3). In Russia the method has been improved. Hydrocyclones are used to separate small particles of KCl and NaCl. The method is used at Solikamsk combine.

Methods of in-situ leaching of ore (salvinite) with subsequent processing of solution by evaporation and crystallization are also used.

The use of coarse-crystalline and granular potassium is more preferable, as fine-crystalline has poor physical properties, is not convenient for the preparation of fertilizer mixtures with granular superphosphate and granular ammonium nitrate. Introduction of such fertilizer mixtures with centrifugal spreaders leads to separation (segregation) of fertilizers and uneven application. Coarse-crystalline potash is 30% less absorbed by the soil and remains in a form accessible to plants for a longer time, which increases the effectiveness of coarse-crystalline potash.

Potassium salt

Potassium salt contains 40-44% K2O, 20% Na2O and 50% Cl. It is obtained by mixing potassium chloride with crude potassium salts, most often with crushed sylvinite, less often with kainite. Its appearance is that of small mottled crystals. According to technical specifications it must contain not less than 40% of K2O.

The 30% potassium salt is a mixture of sylvinite and kainite, suitable for magnesium-demanding crops on sandy and sandy loam soils poor in magnesium.

Mixed potassium salts are the most suitable fertilizer for beets, cruciferous vegetable crops, carrots and others that respond favorably to sodium and magnesium on light soils.

Potassium sulfate

Potassium sulfate is a concentrated chlorine-free potash fertilizer containing 45-52% K2O. It is a fine crystalline powder of white color with yellow or gray tint, moisture content 1.2%. It is not caking and may be transported in bags or in bulk. It is received by treatment of polymineral potash ores, for example langbeinite, shenite, or by exchange reaction with potassium chloride:

2KCl + MgSO4 = K2SO4 + MgCl2.

In saturated solution, due to low solubility, potassium sulphate precipitates first, which is filtered and dried.

Potassium sulfate is produced in the Western Ukraine by the processing of langbeinite salt. Fertilizer has good physical properties, non-hygroscopic, does not caked.

The advantage of potassium sulfate is that it does not contain chlorine. Compared with chlorine-containing fertilizers, potassium sulfate provides an increase in yields of grapes, buckwheat, tobacco and other chlorophobic crops. It is widely used in vegetable growing, especially in protected ground. Sulfur also has a positive effect on cruciferous crops, legumes and some other crops.

However, in cost terms, potassium sulfate is one of the most expensive potassium fertilizers.

Calimagnesia, potassium-magnesium sulfate

Calimagnesia, potassium-magnesium sulfate (K2SO4⋅MgSO4) contains 26-29% K2O and 9% MgO. It is obtained from cainite-langbeinite rock. It is a dehydrated mineral of schoenite, that is why it is sometimes called so. It is a white, strongly pulverized powder with a grayish or pinkish hue, or grayish-pink irregularly shaped granules. It does not cake and is transported in bags or in bulk.

Primarily used for chlorine-sensitive crops or on light soils.

Kalimag

Kalimag contains 16-20% K2O and 8-9% MgO. It is produced from langbeinite (K2SO4⋅2MgSO4) after grinding and leaching of sodium chloride. Approximate composition: K2SO4 – 39%, MgSO4 – 55%, NaCl – 1%, insoluble residue – 5%. Available in the form of gray granules. It does not cake and can be transported in bulk. Its efficiency is close to that of potassium permanganate.

Potassium-chloride electrolyte

Potassium-chloride electrolyte is potassium chloride with admixtures of sodium and magnesium chlorides. It is a by-product of magnesium production from carnallite. It contains 34-42% K2O, 5% MgO, 5% Na2O and up to 50% Сl. Highly dusty fine crystalline powder with yellow tint. It does not cake and is transported in paper bags or in bulk. The effect is similar to potassium chloride, and is more effective than potassium chloride on magnesium-poor soils. It is produced in Solikamsk.

Potassium-containing cement dust

Potassium-containing cement dust contains 14-35% K2O and is a waste product of cement production. Includes carbonate (K2CO3), hydrocarbonate (KHCO3) and potassium sulfate (K2SO4). It also contains CaCO3, MgO (3-4%), silicic acid, semi-oxides, and some trace elements. It has an alkaline reaction does not contain chlorine, so it can be used for potatoes, buckwheat, grapes, tobacco, citrus.

In Netherlands, Norway, Finland, potassium-containing cement dust is used as potash and lime fertilizer. It is well soluble in water and accessible to plants. The calcium carbonate it contains makes it hygroscopic. Cement dust can be used to produce potassium phosphate and granulated.

Furnace ash

Furnace ash is used as a local potassium-phosphate-lime fertilizer. It is effective for all crops and on all types of soils. Potassium is contained in the form of potassium carbonate (K2CO3, potash). Phosphorus ash is assimilated by plants in the same way as precipitate and tomas slag, unlike superphosphate, it does not bind in hard-soluble phosphorus compounds. Lime eliminates the negative effect of potash on soil structure.

K2O content depends on the fuel burned. For example, hardwood ash contains 10-14% K2O, 7% P2O5, 36% CaO, and softwood ash contains 3-7% K2O, 2.0-2.5% P2O5, and 25-30% CaO. From young trees, more ash is produced and the content of nutrients is higher.

The ash contains micronutrients. The dose of ash for plowing or cultivation is 5-6 cwt/ha. Peat ash and ash to neutralize excessive acidity is applied in an amount of 1.5-3 t/ha, better under plowing.

Interaction of potassium fertilizers with soil

Potassium fertilizers are well soluble in water. When applied to the soil are dissolved in the soil solution and enter into an exchange (physical and chemical) interaction with the soil absorbing complex (SAC), partially in non-exchange interaction.

The exchange absorption of potassium cations is a small part of the whole absorption capacity. The exchange reaction is reversible:

[SAC]Ca2 + 2KCl ⇔ [SAC](K2, Ca) + CaCl2;

[SAC](Al, H) + 4KCl ⇔ [SAC]K4 + AlCl3 + HCl.

In the exchanged-absorbed state, potassium loses mobility, which prevents leaching outside the arable layer, except for light soils with low absorption capacity. The exchange-absorbed potassium remains available to plants.

Secondary processes of interaction between soil solution and soil absorbing complex gradually displace potassium cations from it. The root system of plants takes an active part in this exchange at the expense of root excretions.

Potassium cations displace an equivalent amount of calcium, magnesium, ammonium, hydrogen, aluminum cations from the SAC. On slightly acidic and neutral soils with high absorption capacity and buffering, exchange processes almost do not affect the reaction of the soil solution. On acid and highly acidic soils, especially on light granulometric composition, with exchangeable hydrogen and aluminum in the SAC, the application of potash fertilizers leads to acidification of the soil solution. Therefore, on such soils the efficiency of potash fertilizers decreases.

Additional acidification of the soil solution occurs as a result of physiological acidity of potassium salts, but it is much less than that of ammonium salts, and appears only with long-term application for potassium-loving crops.

Non-exchange (fixed) potassium is much less mobile than exchange-absorbed, practically not available to plants. Non-exchangeable absorption (fixation) of cations with ion radius of 0.130-0.165 nm (K+, NH4+, Rb+, Cs+) is typical for clay minerals of montmorillonite group and hydromica group with three-layer swelling crystal lattice. Therefore, the value of non-exchange potassium uptake depends on mineralogical composition: the more minerals of montmorillonite group and hydromica, the more pronounced is the fixation of potassium.

Fixation occurs due to penetration of cations into the interstitial spaces of minerals in the state of swelling, occupy hexagonal voids in the grid of oxygen atoms of the tetrahedral layers, pull down both negatively charged oxygen layers, as a result of which they appear in a closed space. Variable wetting and drying of the soil intensify the process of fixation. Potassium fixation also occurs in damp soil, but to a lesser extent.

The share of fertilizer potassium fixation in different soils depending on mineralogical composition and fertilizer dosage is from 14 to 82% of the applied amount.

According to the results of experiments conducted at the All-Russian Institute of Fertilizers and Agrochemistry, the form of potassium fertilizer does not affect potassium fixation by the soil. This process is influenced by the size of fertilizer particles: when applying coarse-crystalline or granular fertilizers, the fixation is reduced by 20-30% due to less contact between the fertilizer and the soil.

The size of non-exchangeable absorption also depends on the fertilizer dose: the absolute amount of fixed potassium increases with increasing dose, in percentage terms – slightly decreases. The potential capacity of the soil to fix potassium is high. In V.U. Pchelkin’s laboratory experiment at a potassium dose of 1000mg/100g the slightly leached black earth fixed 147.3 mg/100g, which is equivalent to 4420 kg/ha of soil.

With a systematic application of potassium fertilizers and a positive balance of potassium in the soil increases the content of mobile forms (water-soluble and exchangeable) and fixed forms.

With a negative balance of potassium the opposite process takes place. As the available water-soluble and exchangeable forms of potassium are consumed, there is a gradual transition of fixed potassium and part of the crystal lattice potassium to more mobile forms. Thus, in the experiment on a loamy soil (England) for 101 years, plants took out with crops 3-4 times more potassium than it was contained in the soil in the exchangeable form. In the experiments of Kobzarenko (1998), plants on control variants took out from sod-podzolic light loamy soil (Moscow region) 583 kg/ha of potassium for 17 years that in 2,9 times more than the initial content of exchangeable potassium in the soil. At the same time, there were no significant changes in the content of exchangeable potassium during the reference period. These studies confirm the possibility of gradual replenishment of exchangeable potassium by other forms.

Experiments also confirm the weak migration of fertilizer potassium along the soil profile, except for sandy and sandy loam. In lysimetric experiments annual leaching of potassium outside the root-containing layer was 0.4-7.0 kg/ha in the Non-Black Earth zone on loamy soils, and up to 12 kg/ha on sandy loamy soils.

Effectiveness of potassium fertilizers

Potassium (2.5-4.5:1:3.5-6) predominates in the ratio N:P:K for potassium-loving crops, and nitrogen (2-3:1:1:1.5-3.5) for grain crops.

Average removal of potassium with crops per 1 ton of marketable products and the corresponding amount of by-products is 25-30 kg for cereals, potatoes – 7-10 kg, sugar beets – 6,7-7,5 kg, vegetable crops – 4-5 kg, perennial grass in hay – 20-24 kg.

Crop sufficiency with potassium can be judged by its content in the soil in the exchangeable form. Methods of determination differ for types of soils:

  • for sod-podzolic soils – Kirsanov method (0.2 n. HCl), Peyvet method (1 n. NaCl), Maslova method (1 n. CH3COONH4);
  • for gray forest soils and chernozems (except for carbonate ones) – Chirikov’s method (0,5 n. CH3COONH4);
  • for carbonate chernozem, chestnut soils and gray soils – Machigin’s method (1%-m (NH4)2CO3).

All methods of determining potassium in the soil available to plants are based on the extraction of the exchange form, adsorptionally held by colloidal particles. This amount also includes water-soluble potassium.

The effectiveness of potassium fertilizers depends on:

  • soil type and granulometric composition;
  • availability of available potassium in the soil;
  • the needs of the crops in the crop rotation;
  • the amount of precipitation;
  • temperature;
  • organic matter content in the soil;
  • nitrogen and phosphorus fertilizers;
  • the method of incorporation;
  • forms of potash fertilizers.

Potassium fertilizers are highly effective on sod-podzolic soils, red soils, gray forest soils, and northern chernozems. Especially poor in mobile (exchangeable) potassium are sod-podzolic sandy and sandy loam soils, dried peatlands and peat-bog soils.

Potassium fertilizers have a positive effect when the content of mobile potassium in the soil at the level of 1st-3rd classes. With a higher supply, the effectiveness of potassium fertilizers decreases and is determined primarily by crop rotation, doses of nitrogen and phosphorus fertilizers and agronomic practices.

The efficiency of potassium fertilizers, as well as phosphorus and nitrogen fertilizers, is higher on slightly acidic and neutral soils than on strongly acidic ones. Therefore, liming of acidic soils is a condition for increasing efficiency. However, due to the antagonism of potassium and calcium ions in limed soils, the doses of potassium fertilizers are increased.

Table. Effectiveness of potash fertilizers depending on the acidity of sod-podzolic soils (by Mineev)

рНKCl
Yield increase from 1 kg K2O, t/ha
barley
winter rye
potato
< 4,5
0,29
0,38
2,01
4,6-5,0
0,46
0,30
2,67
5,1-5,5
0,50
0,63
2,99
5,6-6,0
0,56
0,67
3,76

The application of manure, which itself is a good source of potassium, usually reduces the effect of mineral potash fertilizers.

The greatest efficiency of potash fertilizers is achieved in an optimal ratio with nitrogen and phosphorus. One-way application of potash fertilizers is carried out on drained peatlands and peat-bog soils, with sufficient content of other nutrients.

Timing, methods and forms of potassium fertilizer application

On soils of medium and heavy granulometric composition chloride-containing potash fertilizers in full dose, except for row application in small doses for some crops, it is advisable to make under autumn autumn autumn autumn tillage. This allows you to place fertilizers in a wetter layer of soil, where the bulk of the roots develop, and the chlorine is washed from the arable layer during the fall and spring. Only on light, peat-bog and floodplain soils, potash fertilizer is made in spring. Under row crops and vegetable crops, a portion of the total dose of potassium is appropriate to give as a top-up.

In the rotation of potassium fertilizers made in the first place for potassium-loving crops, which give a significant increase in yield.

Flax and hemp consume relatively little potassium, but because of the weak root system, which under normal conditions can not provide plants with sufficient potassium, for these crops make increased doses of potash fertilizer.

For chlorophobic crops, it is advisable to apply fertilizers with minimal chlorine content. When using chlorinated potassium fertilizers for potatoes, the amount of starch is reduced by 7-15% compared with chloride-free fertilizers.

Application of potassium fertilizers on different soils

In Russia, more than 1/3 of the arable land areas are characterized by low and medium levels of exchangeable potassium and need to apply potash fertilizers. Their use is most effective on sandy, sandy loam sod-podzolic, peat-bog, floodplain soils and red soils. They also have a positive effect in the zone of sufficient moisture on loamy sod-podzolic, gray forest soils, podzolized and leached chernozems with low and medium potassium supply.

Sod-podzolic soils

Sod-podzolic soils have relatively small reserves of available potassium. Non-exchangeable potassium is part of the prevailing in these soils of secondary clay minerals – kaolinite and montmorillonite, which can not provide restoration of stocks of exchangeable potassium. This explains the positive effect of potash fertilizers on sod-podzolic soils.

According to data from many years of experience, the annual introduction of potassium fertilizers in an amount of 30-90 kg K2O per 1 ha increases the yields of root crops, particularly potatoes, and cereal crops. The largest yield increases of up to 30-50% are obtained on light loamy and sandy soils, where potassium reserves are extremely low. In shorter experiments with nitrogen-phosphorus background, potash fertilizers also provide an increase in yields of all crops in the first rotation of the rotation: the increase in grain yields averaged 10-20%, root crops – over 30%.

With the depletion of soil reserves of potassium and the improvement of nitrogen-phosphorus nutrition of plants the need for potash fertilizers increases, increasing their effectiveness, which is especially manifested in heavy soils with a granulometric composition. Because of the small stocks of mobile potassium sod-podzolic soils moderate doses of up to 90 kg/ha K2O does not provide a positive balance. However, the content of exchangeable potassium in the soil due to dynamic equilibrium between forms of potassium in the control is maintained at the initial level, which is associated with the mobilization of natural soil potassium caused by physiological acidity of fertilizers, biological accumulation by plants due to their better development on fertilized variants as well as by inclusion of potassium of subsoil and underlying soil layers. Mobilization of unexchangeable potassium under the influence of fertilizers provides an increase in mobile forms of potassium and is accompanied by a decrease in its reserves.

Gray forest soils

Gray forest soils are characterized by a low content of exchangeable potassium in the arable layer. However, in comparison with sod-podzolic soils, the effect of potassium fertilizers on the yield of various crops is weaker. It is connected with the fact that unexchangeable potassium of silt fraction is a part of hydromica of loess-like loam – the main soil-forming rock of gray forest soils. Hydromica have a high fixing ability in relation to monovalent cations and the ability to easily release unexchangeable absorbed potassium, which either passes into the exchange state or is directly used by plants.

The study of the potassium regime of gray forest soils in long-term experiments has shown that during one rotation of the crop rotation when making low doses of fertilizers is a negative balance of potassium. But, despite this, there is an increase in the content of available forms of potassium (exchangeable and readily hydrolysable) while maintaining the level of non-exchangeable form in fertilized variants. Under the influence of plants and fertilizers all reserve forms of potassium are mobilized and their transition into the exchangeable state occurs.

With prolonged use of nitrogen-phosphorus fertilizers in the rotation of gray forest soils potassium effect increases from rotation to rotation.

Effect of potash fertilizers on crop rotation productivity
Effect of potassium fertilizers on crop rotation productivity, 100 kg/ha grain units (1967-2000)

Black soil of the forest-steppe and steppe zones

Black earth soils (chernozems) of forest-steppe and steppe zones contain sufficient reserves of potassium available to plants. Soil-forming rocks and clay minerals of chernozems are rich in non-exchangeable potassium, which actively passes into mobile forms, so the effectiveness of potash fertilizers on these soils is small. Even potassium-loving crops (row crops and technical), little response to the application of potassium fertilizers. It is especially noted on soils of heavy granulometric composition.

Over time, the effectiveness of potassium fertilizers increases, which is especially noticeable when growing sugar beets and other potassium-loving crops, as well as when applying potassium on the background of nitrogen-phosphorus fertilizers, which is explained by the depletion of soils not potassium due to the removal of crops.

Systematic fertilization does not lead to a significant increase in the content of mobile forms in chernozems even with a positive balance, which is due to the high saturation of the absorbing complex of chernozems with divalent bases that prevent the absorption of potassium.Favorable conditions for potassium fixation in chernozems: mineralogical composition of the silty fraction – hydromica and highly dispersed minerals of the montmorillonite group, which are characterized by a high ability to fix univalent cations, as well as high saturation of the soil absorbing complex (SAC) with bases, increased soil acidity, high content of organic matter, absence of potassium competitor – absorbed ammonium, irreversible coagulation of colloids with periodic drying of the top layer. These conditions contribute to non-exchangeable absorption of potassium in the arable and subsoil layers. At negative balance on fertilized variants increase non-exchange potassium is explained with mobilization of less mobile forms under the influence of plants and fertilizers, as well as the release of secondary minerals potassium – hydromica.

Potassium has a positive effect under adverse weather conditions. With an abundance of precipitation, it reduces lodging of crops in dry years helps to combat floods caused by drought. Proper use of potassium fertilizers on chernozems, that is, on the background of nitrogen-phosphorus fertilizers in wet and dry years, increases the yields of major crops, especially potassium-loving.

Chestnut soils and gray soils

The content of mobile potassium in chestnut soils of dry steppe and grey soils of Central Asia is high, reaching 40-60 mg K2O/100 g of soil. Potassium reserves are huge, as it is a part of hydrosols and is easily released, so the efficiency of potassium fertilizers is insignificant.

On old-fallow, long used irrigated grey soils with the systematic application of nitrogen and phosphorus fertilizers the content of mobile forms is small, so the yield and product quality of crops, especially cotton, increases.

Soils of steppe and arid-steppe regions

Soils of steppe and dry-steppe regions, most often, are well supplied with potassium. Due to the variability of moisture conditions, on typical, ordinary, southern chernozems, chestnut soils and gray soils the effect of potash fertilizers is small or does not appear at all. Application of potassium fertilizers is justified only for potassium-loving crops – sugar beet, sunflower, vegetable crops, on chestnut soils and sierozem under irrigation.

The process of potassium depletion in soils of arid steppe and desert zones due to large reserves of non-exchangeable potassium, mineralogical composition of soils and soil-forming rocks proceeds slowly. It is important to periodically replenish potassium reserves by adding it with irrigation water during irrigation. Reducing the content of potassium in the soil with prolonged use and systematic use of nitrogen and phosphorus is manifested by signs of potassium starvation of plants and the growth of efficiency of potash fertilizers.

On solonets, as a rule, rich in potassium, potash fertilizers are not used, as they increase solonetzation and do not bring the expected effect.

Application of potassium fertilizer and liming

Application of potassium fertilizers on sandy soils that need liming increases the need to neutralize soil acidity, as potassium displaces ions of hydrogen, aluminum, manganese, which reduce pH, from the soil absorbing complex. When liming acidic soils, the need for potassium fertilizers increases. Additions of potassium on the background of lime increases in absolute and relative values. The effect of lime, in addition to improving the physical and chemical properties of soils, is also manifested in the improvement of nitrogen-phosphorus nutrition of plants and a slight decrease in the availability of potassium for plants due to its increased fixation by soil colloids. With increasing yields at liming also increases removal of potassium from the soil, and the transition to available forms goes less intensively than in acidic soils.

Because of the antagonism of potassium and calcium, there is a need to increase the dosage of potash fertilizers in liming and soils with a neutral reaction. Liming of soils in such cases significantly increases the effectiveness of potash fertilizers.

On the other hand, the improvement of the potassium regime increases the benefits of liming. The use of manure reduces the effect of potash fertilizers, as it affects the nutrient regime of soils, while being a good source of potassium.

Increasing the efficiency of potash fertilizers

The main ways to increase the efficiency of potash fertilizers:

  1. Application taking into account natural and economic conditions and the provision of soils with mobile forms of potassium.
  2. Increasing the culture of farming, cultivation of soils, optimal provision of crops of rotation with other fertilizers, i.e. balanced nutrition of crops.
  3. Liming of acidic soils.
  4. Introduction of potassium in the rotation in the first place for crops with high responsiveness to potassium.
  5. Selection of forms of potash fertilizers, taking into account the biological features of crops. For example, potassium sulfate and potassium chloride have the same effect on the yield of most crops. Chloride-free forms contribute to increased yield of buckwheat, millet and some varieties of tobacco, increase the sugar content in berries of certain grape varieties, the starch content in the tubers of late varieties of potatoes, improve the quality of flax fiber.
  6. Proper selection of the timing and methods of application. In most regions of the country potash fertilizer is made in autumn under autumn plowing, except for sandy soils and floodplains. This contributes to the uniform distribution of potassium in the arable layer and leaching of chlorine into the underlying horizons during the fall-winter and spring periods.
  7. Optimization of potassium fertilizer doses taking into account meteorological conditions. Thus, the application of 80 kg/ha of K2O against the background of N60P60 increased winter wheat yields by 240 kg/ha, by 660 kg/ha and by 1190 kg/ha, compared with the background at an average temperature of 16.5°C during May-July, at 15.2°C and at 13°C. This is due to the difficulty in getting potassium into the plants at low temperatures. Potassium fertilizers improve the physical properties of grain, especially with excess precipitation of more than 80 mm in July, during ripening and ripening. For example, in the Non-Black Earth zone during this period, a lack of potassium produces small and puny grain.
  8. Full provision of optimum doses of potassium fertilizers in combination with other nutrients peat (developed and old), peat-bog soils, which are poor in this element (0,02-0,3 of gross content). In these soils, potassium is mobile, not accumulated in the arable horizon and is almost completely used by plants in the first year of application. The effect of fertilizers increases with double regulation (irrigation and drainage) of these soils. The greatest effect is achieved under vegetables and root crops, which pay back the application of potassium by an increase in yield.

Several groups of soils are distinguished by the availability of exchangeable potassium.

Table. Gradations of soil provision with mobile (exchangeable) potassium, mg/kg of soil (Methodological Guidelines for Comprehensive Monitoring of Soil Fertility of Agricultural Lands, 2003)

Level of provision
Kirsanov's method
Chirikov's method
Machigin's method
Maslova Method
Egner-Riem method
Very low
< 40
< 20
< 100
< 50
Low
41-80
21-40
101-200
51-100
< 70
Medium
81-120
41-80
201-300
101-150
71-140
Increased
121-170
81-120
301-400
151-200
> 140
High
171-250
121-180
401-600
201-300
Very high
> 250
> 180
> 600
> 300

Generalization of data from long-term stationary experiments of the geographical network has shown that the content of 10-15 mg K2O/100 g of soil and application of 60-90 kg K2O/ra on sod-podzolic, gray forest soils and leached chernozems ensures the productivity of the crop rotation of 3-5 t/ha of grain units. However, significant reserves of potassium and dynamic equilibrium between its forms make the index of exchangeable potassium content, which characterizes the ability of the soil to provide potassium nutrition to plants, less reliable. In the process of plant nutrition all forms of soil potassium participate, therefore mobile forms (soil solution and exchangeable) and unexchangeable potassium of primary and clay minerals, as well as mobility, ability and recovery rate of exchangeable potassium from reserve forms should be taken into account.

The provision of soils with unexchangeable potassium depends on the type of clay minerals, genetic features and the granulometric composition of soils. The greatest quantity is bound by mica minerals (hydromica, illite, vermiculite), less – by montmorillonite, the least one absorbs non-exchangeable kaolinite.

According to the degree of provision of unexchangeable potassium soils are divided into groups:

  • low 10-20 mg K2O/100 g of soil,
  • medium 20-50 mg K2O/100 g of soil,
  • increased 50-100 mg K2O/100 g soil,
  • high 100-150 mg K2O/100 g soil.

With prolonged use of potassium fertilizers increases the amount of exchangeable potassium, its mobility and share in easily hydrolyzable and non-exchangeable fractions increase. With the increase of these indicators, the effectiveness of “freshly applied” potassium decreases and the effect of “residual” potassium accumulated as a result of fertilizer application increases. However, these data do not allow us to establish common criteria for all soils potassium availability.

There are different methodological approaches to determine the degree of supply of mobile potassium. Thus, it is possible to determine this index by saturation of absorbing complex with exchange potassium, taking 1.8-3.0% as an active level. However, the saturation value of the soil absorbing complex exchangeable potassium must be established for each type of soil, depending on the mineralogical composition of the silt fraction of the soil and the underlying soil-forming rock, biological characteristics of cultures, the conditions of nitrogen-phosphorus nutrition and moisture supply. In each case, the less saturation of the absorbing complex of potassium, the higher the effectiveness of fertilizers.

Optimal concentrations of potassium in the soil are established, although they need to be clarified.

  1. Potassium in the soil is in various interrelated forms. When applying potassium fertilizers, its reserves are replenished, but due to differences in the composition of clay minerals in sod-podzolic soils more increases the content of the exchange form, and on chernozems – non-exchange. In all cases, during plant nutrition available forms of potassium are replenished at the expense of non-exchangeable forms. Therefore, when determining the potential reserves of available potassium is taken into account.
  2. Potassium in the soil is less mobile than nitrogen, but more mobile than phosphorus. Therefore, when trying to create an optimal potassium level by periodic application of high doses on heavy clay soils, potassium is fixed by minerals, and on light sandy and sandy loam soils migrates along the soil profile beyond the root layer.
  3. The application of higher doses of fertilizers and chemical meliorants changes the availability of potassium to plants. For example, when liming acidic soils, even with high potassium content its availability due to the antagonism of calcium and potassium decreases. Therefore, optimal doses of potassium fertilizers on calcareous soils are increased by 1.5-2 times. Antagonism and synergy exist between potassium and other cations and anions.
  4. Cultivated plants respond differently to potassium nutrition. A group of potassium-loving crops has been identified for which the optimal level of exchange potassium should be higher than, for example, cereals, legumes, annual and perennial grasses.
  5. Chlorophobic crops react negatively to excess chlorine in the soil. This negative phenomenon can be eliminated by adjusting the doses, timing and application methods. For example, the early application of chloride-containing potash fertilizers in autumn under autumn plowing.

Doses of potassium fertilizers

Optimal doses of potash fertilizers in crop rotations depend on the location, frequency and sequence of lime and manure applications. With liming of sod-podzolic soils and the corresponding increase in yields, the need for potash fertilizers increases particularly strongly.

According to long-term field experiments on sod-podzolic soils with initial pH of 4,2, when potassium fertilizers were made in parallel on the background of lime and without it, showed that after the first rotation (6-8 years) the amount of exchangeable potassium in limed soils with pH 5,0-5,6 compared with unlimed with pH 4,0-4,3 decreased in 2 times and is associated with increasing potassium fixation. At the application of 35-50 kg K2O/ha, this ratio was maintained, although the content of exchangeable potassium in both backgrounds increased by 10%. When increasing the dose of potassium to 70-100 kg/ha, the difference between the backgrounds was 20% and disappeared at doses of more than 100 kg K2O/ha. This explains the fact that higher doses of potassium fertilizer were effective on the limed background, as the same level of exchangeable potassium content (11 mg K2O/100 g) was achieved without liming at a dose of 60 kg K2O/ha, and with liming – 100 kg/ha of potassium.

Thus, the effectiveness of potassium fertilizers on sod-podzolic soils is related to acidity: the lower it is, the higher is the effect of K2O application. On acid sod-podzolic soils increasing of potassium doses more than 60 kg/ha on the average across the crop rotation does not lead to significant growth of productivity with reduced payback. On calcareous soils with the application of nitrogen and phosphorus in doses of 100-120 kg/ha efficiency of potassium fertilizers is significant, not reduced with the increase of doses of potassium to 140 kg/ha and averages 20-25%. Payback of increased doses of potassium is not less than 5 grain units/kg.

Application of organic fertilizer without liming reduces the effectiveness of potassium fertilizer the stronger the higher doses of manure.

On black soils, especially in the steppe areas, the effect of potassium fertilizers is weaker, which is explained by the high content of mobile forms of potassium. However, with the systematic application of high doses of nitrogen-phosphorus fertilizers, the role of potassium increases. Therefore, in determining the effectiveness of potassium fertilizers take into account the intensity of crop rotation, saturation with potassium-loving crops and optimization of plant nutrition with macro- and microelements.

Relationship of crops to potassium nutrition

There are peculiarities of potassium fertilizer application by crops. For example, for sugar beets the need for potassium remains during all periods of growth and development, the lack is especially undesirable in the second half of the growing season with intensive sugar accumulation. Lack of potassium during this period delays protein synthesis and contributes to the accumulation of nitrate nitrogen. Potassium starvation, as well as excess nitrogen, increases “blooming”, which reduces yields and sugar content. Under the sugar beet optimum time of application of potassium fertilizer is considered the main under the plowing. Along with this application is used in the rows and in top dressing.

Potatoes are a typical potassium-loving plant. By the time of harvesting, up to 96% of the potassium contained in the potato crop is concentrated in the tubers. It is sensitive to chlorine: its excessive potassium reduces the starchiness of the tubers. The best potassium fertiliser for potatoes is potassium sulphate. Chlorinated fertilizers cause physiological diseases, the external signs of which are blackening of the stem and leaves. With the autumn application of potassium fertilizers, due to the washout of chlorine, the negative effect is eliminated. Kalimag and kalimagnesia are also applied under potatoes, especially on light soils. Potassium fertilizers additionally increase yield when applied to the fertilized background. On chernozem with the application of manure, the need for potash fertilizer is reduced. Doses of fertilizers for potatoes are determined by the planned yields, fertilized background fertilizer doses are lower.

Optimal provision of cereals with potassium increases the strength of straws, reducing lodging. Potassium combined with phosphorus increases winter hardiness of winter crops. Forms of potash fertilizer for crops are equal. Of legumes, lupine is sensitive to chlorine.

Flax responds to the introduction of potassium, improving fiber quality: the length and number of elementary fibers increases, flexibility and fiber strength increase.

Potassium fertilizers are effective for fruit trees on all soils, especially on light soils: the percentage of flowering branches in apple trees increases, the marketability of fruits increases, the period of their preservation lengthens.

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.