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Soil gypsum

Soil gypsum is a method of chemical reclamation of saline soils with a large proportion of sodium in the soil absorbing complex (SAC) and alkaline reaction using gypsum (CaSO4⋅2H2O). Saline soils are characterized by unfavorable physical, chemical, physical-chemical and biological properties and low fertility.

The gypsum method is scientifically grounded and developed by domestic scientists. The main merit in study of solonetz soils belongs to academician K.K. Gedroyts. 

Properties of alkaline (sodic) soils

Sodic soils (solonets and solonchaks) occupy more than 30 million ha (26 million ha) in Russia, of which about 11 million ha are arable. Overall, sodic soils occupy about 115 million ha in CIS countries, of which 23.9 million ha are arable.

Solonetz are widespread in the southern regions of the Volga region, Western and Eastern Siberia, the Southern Urals and Northern Caucasus, the steppe regions of Kazakhstan. There are separate areas of solonetz in the form of inclusions in the main land massifs.

These soils are characterized by high cohesion, poor physical and chemical properties. In a damp condition they disperse under the influence of high concentrations of sodium, turning into a smeary mass. Processing of such soils leads to high clumpiness. In dry condition their processing is impossible. Yields in this case are low and of poor quality. Microbiological activity is weakened because of high alkalinity and unstable water regime.

Improvement of sodic soils by changing reaction and composition of cations is achieved by application of gypsum. 

K.K. Gedroyts suggested method of gypsumification of solonetz soils, which consists in displacement of Na+ cations from soil absorbing complex and replacing them with Ca2+ with simultaneous application of organic fertilizers.

At the same time, organic and mineral colloids peptized with sodium are washed out of the upper soil layers into the lower ones, forming a dense solonetz horizon. Sodium absorbed by SAC is displaced by carbon dioxide solution (carbonic acid), forming carbonates and hydrocarbonates – hydrolytically alkaline sodium salts, the alkalinity (pH) of the soil solution thus becomes more than 9:

[SAC]Na2 + H2O + CO2 → [SAC](Na, H) + NaHCO3;

[SAC](Na, H) + NaHCO3 → [SAC]H2 + Na2CO3.

Alkaline soil reaction is unfavorable for most crops and soil microorganisms. It reduces the availability of phosphorus, iron, manganese, and boron to plants.

Often solonetz are located in spots of different size (from several meters to hundreds of meters across) among the prevailing zonal soils, such as chestnut, brown, chernozem soils of forest-steppe, steppe and semi-desert zones.

Radical improvement of solonetz is achieved by displacement of sodium from soil absorbing complex (SAC) and its replacement with calcium cations, removal of formed sodium salts by leaching and destruction of solonetz horizon. For reclamation of solonetz soils, carbonate sodium salts are eliminated by replacing it with calcium, and the formed Na2SO4 – by leaching.

Negative effect of sodium on physical properties of soil depends on its content in SAC. Significant effect of sodium on soil properties and crop yields is noted at the content of exchangeable sodium more than 10% of the cation exchange capacity. Therefore, gypsum is carried out, if the share of exchangeable sodium in the SAC is more than 10%.

Along with sodium in the composition of exchangeable cations can contain up to 15-35% magnesium.

Importance of soil gypsum

Gypsum application improves the water regime of soils, physical and chemical properties of salts, increases their fertility, reduces alkalinity and content of exchangeable sodium in SAC, increases the degree of its saturation with calcium.

According to field experiments, the application of gypsum as a fertilizer on heavy loamy sod-podzolic soils at a dose of 300-500 kg/ha for clover yields an average increase in hay yield 1.62 t/ha, on medium and light loamy soils – 1.11 t/ha, on sandy loam – 0.72 t/ha, on gray forest and leached chernozem soils – 0.65 t/ha. The effect of gypsum on crop yield on acidic soils is due to increased calcium and sulfur nutrition of plants, as well as increased availability of potassium, which is more displaced from SAC.

The average efficiency of gypsum application on the chernozem soils is 0.3-0.6 t/ha of grain and on chestnut soils – 0.2-0.3 t/ha. Gypsum preparation gives an increase in grain yield by 0.3-0.6 t/ha, clover hay – by 0.6-1 t/ha, sugar beet yield increases.

The efficiency of gypsumification of forest-steppe zone solonchaks is proved in experimental and production conditions. Increase in grain yields of cereal crops annually for 7-8 years after a single application of gypsum at a dose of 10 t/ha amounts to 0.5 t/ha. In the steppe zone the efficiency of gypsum application decreases: on meadow-steppe solonchaks the annual increase in grain yield for 8-10 years is on average 0.3-0.4 t/ha.

Classification of sodic soils

Depending on the content of sodium in the soil absorption complex soils are divided into:

  • non-solonetzic with a share of sodium up to 3-5% of the absorptive capacity;
  • slightly solonetzic – 5-10%;
  • solonetzic – 10-20%;
  • solonets – more than 20%.

Solonetses are also subdivided into shallow, or corky, with deep occurrence of solonet horizon up to 7 cm, medium – with a depth of occurrence of 7-15 cm, and deep-columnar – with occurrence at a depth of over 15 cm.

Saline soils (solonchaks) are widespread apart from solonets. According to the degree of salinization, i.e. according to the amount of salts and depth of occurrence of saline horizons, saline soils are subdivided into:

  • slightly solonchaky with a share of salts more than 0.25% at a depth of 80-150 cm;
  • solonchak with salts over 0.25% at a depth of 30-80 cm
  • solonchak with saline horizon at a depth of 5-30 cm;
  • solonchaks with no less than 1% of salts in the upper layer, sometimes reaching more than 10%.

According to the salt composition, solonchaks are divided into:

  • sulfate with a predominant content of Na2SO4,
  • soda – Na2CO3 and NaHCO3,
  • chloride – NaCl and MgCl2,
  • mixed.

Processes occurring during the application of gypsum to the soil

When gypsum gets into an alkaline environment, it reacts:

Na2CO3 + CaSO4 → CaCO3 + Na2SO4,

calcium gradually displaces sodium from the soil absorption complex (SAC):

[SAC]Na2 + CaSO4 → [SAC]Ca + Na2SO4.

The resulting sodium sulfate is a hydrolytic neutral salt, in small quantities is not harmful to plants, but when gypsum salts, where sodium content is more than 20% of the cationic absorption capacity, it is removed by leaching from the root-containing layer.

The removal of sodium carbonate from the soil solution and replacement of sodium with calcium in the SAC, if not eliminates, then reduces the alkalinity of the environment. At the same time there is a coagulation of soil colloids, improving physical, physico-chemical and biological properties of solonetz soils (improved processing conditions, aeration and water permeability).

Gypsum is simultaneously a source of calcium and sulfur for plant nutrition.

Efficiency of soil gypsum

Soil gypsuming efficiency is influenced by:

  • irrigation;
  • deep plowing;
  • snow retention;
  • application of local and industrial fertilizers;
  • when applying manure, yield increases from gypsum and manure are summed up.

Optimal forms of fertilizers on solonetz soils are ammonium sulfate and simple superphosphate.

The changes caused by gypsum are retained for many years.

In the forest-steppe zone gypsuming of solonetz soils is more effective than in the steppe zone, mainly solonetz spots with participation of up to 30% in meadow-steppe complexes with depth of groundwater occurrence more than 1.5-2.0 m are subjected to gypsuming. Solonetz soils of the steppe zone are subjected to self-melioration instead of gypsumization, i.e. they are cultivated with three-tier ploughs or reclamation ploughs to plow CaCO3 or CaSO4 underlying the solonetz horizon.

Some solonetz undergo complex reclamation, including reclamation treatment with surface application of starter doses of gypsum to eliminate soil crust, as well as phytomelioration to activate self-melioration at the expense of intra-soil calcium reserves.

Along with plastering, the system of agrotechnical reclamation measures includes sowing perennial grasses, application of organic and mineral fertilizers. On slightly saline soils, increased doses of manure, compost and other organic fertilizers, applied under deep plowing, contribute to their improvement.

Soils of arid-steppe and semi-desert zones, as a rule, are characterized by high content of absorbed calcium. With development of solonetzation the share of absorbed calcium decreases and absorbed sodium and magnesium increases. The process of desalinization should be accompanied by replacement of exchangeable sodium and part of magnesium by calcium. Radical transformation of solonets is possible at:

  • replacement of sodium by calcium, with a sodium content of more than 10% of the sum of cations;
  • displacement of a part of absorbed magnesium, which is more than 30% of the sum of cations;
  • saturation with calcium of absorbing complex up to 70% of the sum of cations.

Skewness in structure of sown areas of meliorative crop rotations, for example, predominance of perennial grasses in rainfed conditions of steppe zone leads to decrease of desalinization intensity or predominance of grain and fallow links in crop rotation leads to deficit of organic matter in soil. 

In rainfed (non-irrigated) conditions due to slow interaction of meliorants with soil positive effect lasts for a long time, the full effect is manifested in 4-5 years. To increase efficiency of gypsum formation it is necessary to improve moisture availability of rainfed soils, for which snow retention and deep embedding of meliorants are used. Under irrigation there is a risk of secondary salinization of solonetz soils.

Efficiency of gypsum preparation increases in combination with organic and mineral fertilizers. Among mineral fertilizers the greatest effect is achieved by physiologically and hydrolytically acidic forms.

Changes in agrochemical and physical properties of saline soils occur slowly, remain for a long time, so repeated reclamation if necessary is carried out not earlier than in 10 years or more.

Doses, timing and methods of application of gypsum

The degree of saturation of the soil absorbing complex with calcium is the basis for calculating the need for chemical reclamation of solonetz soils.The need for chemical reclamation of solonetz soils increases with transition from slightly solonetzic soils to solonetzic soils and solonets, i.e. with increase of sodium share in cation exchange capacity (CEC) from 5-10 to 20% and more.

From the equation of chemical reaction of interaction of gypsum and sodium carbonate of soil it follows that 0.086 g of CaSO4⋅2H2O is needed to replace 1 g of sodium by equivalent mass, then 0.086⋅(Na – K⋅T)/100 g (CaSO4⋅2H2O) is needed to replace excess sodium in 1 g of soil to safe content (K). For a 1-cm-thick soil layer in a 1 ha area (108 cm2), the dose of gypsum D (t/ha) would be:

where Na – sodium content, mmol per 100 g of soil; K – coefficient of safe sodium content, usually equal to 0.1 (10%); T – absorption capacity, mmol per 100 g of soil.

For the whole reclaimed layer (H, cm) of soil at volume weight d (g/cm3) the equivalent dose of gypsum D (t/ha) will be:

D = 0.086⋅(Na – KT)⋅Hd,

where 0.086 – mg-eq CaSO4⋅2H2O, g; Na – sodium content, mmol per 100 g of soil; K – coefficient of safe sodium content, usually equal to 0.1 (10%); T – absorption capacity, mg-eq/100 g of soil; H – thickness of reclaimed layer, cm; d – volume mass of reclaimed layer, g/cm3.

Example. A mass of southern chernozem solonets is characterized by absorption capacity T = 25 mmol/100 g of soil; sodium content = 6 mg-eq/100 g of soil; thickness of reclaimed layer H = 25 cm; volume weight of reclaimed layer d = 1.6 g/cm3. The dose of gypsum CaSO4⋅2H2O (D) in this case will be:

D = 0,086 (5 – 0,1 ⋅ 20) ⋅ 20 ⋅ 1,7 = 12,0 t/ha.

According to the degree of calcium saturation of solonets they are subdivided into:

  • unsaturated with calcium – when its content in the absorbing complex is less than 70%. Such solonets are found in semi-desert zone;
  • calcium-saturated – with its content in the absorbing complex of about 70% of the sum of cations. Gypsum dosage for reclamation of such solonets can be determined by the above formula.

Unsaturated solonetz may be high-sodium, or typical, and low-sodium, i.e. with absorbed sodium content less than 10% of the sum of cations. The rate of gypsum application for high-sodium solonets should correspond to the sum of substituted absorbed sodium up to 10% and the part of magnesium that exceeds 30% of the sum of cations:

T = 0.086(Na – 0.1T) + (Mg – 0.3T)Hd.

For low-sodium solonets, gypsum doses are determined by the content of absorbed magnesium:

T = 0.086(Mg – 0.3T)Hd.

For solonetz soils containing sodium carbonate (soda), the rate of gypsum application is increased to neutralize the negative effect of soda on plants. The saturation of the soil absorbing complex with calcium of soils of arid-steppe and semi-desert zones up to 65-70% contributes to suppression of the dispersing role of sodium and magnesium.

In conditions of rainfed steppe (rainfed) agriculture gypsum application is effective under condition of average annual precipitation of more than 400 mm. In arid steppe zone with average annual precipitation of less than 300-350 mm chemical melioration is effective only with irrigation.

Large doses of gypsum can be applied in stages over 2-3 years. The best place in the crop rotation for applying gypsum are bare fallows and row crops. It is usually applied under autumn plowing. It is allowed to apply gypsum under spring wheat and annual grasses. On crust solonets, gypsum is applied after plowing with incorporation by cultivators. On medium and deep columnar solonetz with thickness of humus layer more than 20 cm gypsum is brought in by ploughs with skimmers. On solonets with lower thickness of humus layer gypsum is applied in two ways: before plowing and under cultivation after plowing. And the ratio of the first and second parts of the dose depends on the amount of turned out by plows on the surface of the solonetz horizon: the more it is, the greater part of the dose is applied after plowing.

Solonets and saline soils, as a rule, are found in spots among zonal soils. If they occupy up to 30% of the field area, gypsum is applied to solonetz spots, if more than 30%, gypsum is applied to the whole area.

To take into account the content of the active substance in the materials used for gypsum application, the application rate is adjusted by the formula:

where Df is the actual rate of application of material for soil gypsum, t/ha, Da.s. – application rate of pure gypsum, t/ha, %a.s.– content of gypsum in the material intended for soil gypsuming, %.

Zonal recommendations can be used to determine approximate rates of gypsum application:

  • in the zone of chernozems:
    • on crusted sodic solonets the rate is 8-10 t/ha,
    • at weak alkalinity – 3-4 t/ha,
    • on medium and deep columnar solonets – 3-4 t/ha,
    • in the presence of soda 5-10 t/ha;
  • in the zones of chestnut and brown soils:
    • on solonetzic soils – 1-3 t/ha,
    • on medium and deep columnar solonets – 3-5 t/ha,
    • on crust chloride-sulfate solonets – 5-8 t/ha.

Table. Approximate rates of gypsum, t/ha

For chernozems
For chestnut soils
Crust solonets
а) soda
if the gypsum is slightly alkaline
б) chloride-sulfate medium and deep columnar solonets
in the presence of soda
Solonetzic soils

Ameliorating effect of gypsum materials depends on its solubility, determined by soil moisture, granulometric composition of ameliorant and degree of mixing with saline layer. Therefore, in irrigated conditions gypsum doses can be reduced by 25-30%, in rainfed conditions it is better to apply it under bare fallow, in their absence – in the main tillage under annual grasses, row crops or spring crops.

Materials for soil gypsum

Raw gypsum (CaSO4⋅2H2O) is a gray or white soft powder containing 71-73% (Class B) and 85% (Class A) of CaSO4⋅2H2O. It is obtained by grinding natural gypsum. The remainder on a sieve with a mesh size of 0.25 mm should not be more than 25% of CaSO4. Produced by grinding natural gypsum. It is poorly soluble in water, so the efficiency is affected by the fineness of grinding. Granulometric composition: all particles should have a size < 1 mm, at least 70% of particles < 0.25 mm with a humidity of no more than 8%. Higher humidity leads to caking of gypsum, turning it into blocks.

Phosphogypsum is a waste from the production of double superphosphate, precipitate and wet-process phosphoric acid, is a gray or white powder containing 70-75% (89-93%) CaSO4 and 1,5-3% P2O5 (15-25 kg / t). It surpasses gypsum in efficiency. At high humidity, it freezes, so, as well as crushed gypsum, it is stored in dry warehouses. When using it, take into account the phosphorus it contains.

Crushed gypsum and phosphogypsum can be used as a calcium fertilizer for legume crops at the rate of 300-400 kg/ha.

Clay gypsum is a natural deposits of loose, unmilled rock containing up to 60-90% CaSO4 and 1-11% clay.

Pyritic pellets and technical acids such as sulfuric H2SO4, nitric HNO3 and phosphoric H3PO4 can also be used to acidify alkaline soils. Acidification is a faster and more efficient, but economically more costly way to eliminate alkalinity in the soil solution by replacing sodium with hydrogen in the SAC. Despite this, plant nutrition and availability of nitrogen, phosphorus and other nutrients are improved.

Calcium-containing industrial wastes, such as defecate, can be used to replace sodium in the soil absorption complex, but their use is limited.

Soil self-gypsum

Some chloride-sulfate and sulfate-chloride solonets of chestnut soils may contain layers of gypsum at a depth of 35-45 cm. To improve the state of cultivation of such soils, self-gypsum is used, i.e. a three-tiered plough is plowed to a depth of 45-50 cm, during which the gypsum-bearing layer is mixed with the upper solonetz horizon. In spring, after deep plowing, the field is kept under fallow for up to 1.5 months, then disced and plowed every 30-40 days. In winter, snow retention is used, and the following year spring wheat is sown with a seeding of perennial grasses, such as alfalfa, vetch, melilot, and others.

With high content of calcium carbonate (calcareous rocks) in the subsoil layer, it can also be used for self-reclamation of saline soils, but the effect of CaCO3 is much inferior to gypsum CaSO4⋅2H2O. The formed Na2SO4 is removed by irrigation. For self-reclamation the treatment of steppe and meadow-steppe solonets is carried out with three-tier ploughs or other ameliorative ploughs. With a single tillage in the steppe zone, a stable increase of grain crops yields of 0.4-0.6 t/ha and grass hay yields of 0.7-0.8 t/ha is obtained.

Sometimes method of leveling is used, i.e. non-saline soil is transported to solonetz plots during 3-5 years at the rate of 500 tons per 1 hectare. The method is labor-consuming, requires high economic expenses and cannot be widely spread.

To improve properties of poorly saline soils, as a rule, self-melioration, earth-melioration and phytomelioration are resorted to.

Earth-melioration (earthing) – method of moving by scraper (bulldozer) on solonetzic spots of fertile soil of adjacent basic zonal type of soils, most often chernozems, with a layer of 15-20 cm. Approximately 10 tons of calcium per 1 ha, part of which is involved in reclamation of the underlying solonetz horizon.

Phytoreclamation is effective with any type of land reclamation, provided the proper selection of crops, alternation and optimal cultivation technologies. For different regions of Russia grouping of crops by salt and solonetz resistance, resistance to drought, overwatering and other adverse conditions have been developed. Thus, in the structure of meliorative crop rotation, the ratio of fallow areas and crops should contribute to intensive desalinization and desalinization taking into account the types of melioration used (gypsumification, acidification, self-gypsumification, earthing).

Phytomelioration in combination with other types of melioration should help to provide optimal regime of soil organic matter to improve watertight structure, increase biological activity and activation of meliorant interaction with SAC.


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