Brief description of technology
Phosphogypsum is a byproduct of the phosphate fertilizer industry and emanates from the production of phosphoric acid from rock phosphate. It contains the components like calcium oxide and such rare earth elements as silicon, iron, titanium, magnesium, aluminum and manganese. Cheap and affordable phosphogypsum can be used to improve the productivity of soils with high magnesium content.
Long-term studies demonstrated the beneficial effects of phosphogypsum application in irrigated agriculture for land improvement purposes in the Aral Sea Basin, Central Asia. The evidences are provided by the scientists of the Kazakh water management research institute who conducted field experiments in magnesium–affected soils of southern Kazakhstan (100 ha, the Arys Turkestan area) with the aim of increasing the cotton and winter wheat yields. The application of 4-8 tons ha-1 had resulted in cotton yields increase from 10 centers up to 20-35 centners ha-1. The effect of phosphogypsum continued during 5-6 years after its application.
The technology had been tested together with the local farmers within the framework of ADB and ICARDA supported research projects.
Phosphogypsum improves soil quality and soil fertility, increases the calcium and phosphorus content, enhances crop growth and development, increases infiltration rates and irrigation water saving by 25-35 %. Application of PG raises soil moisture uniformity; reduces technological losses of water for evaporation and runoff from the irrigated soils up to 2 times, provides uniform plant development. Phosphogypsum is recommended for takyr and alkali soils for improving soil quality and productivity.
Large quantities of phosphogypsum are available in Kazakhstan that can be used to increase the productivity of high magnesium soils by farmers at an affordable cost in Kazakhstan and elsewhere in Central Asia.
Efficiency of phosphogypsum
In Kazakhstan and CAC countries the traditional methods to combat soil salinity and water logging had led to a reduced content of organic matter and gypsum in the soils, as well as deterioration of soil physical and chemical properties due to soil sodicity. The significant part of irrigated soils has “takyr” properties meaning their high compactness and low permeability. When irrigated such soils are swollen, and under dry conditions the deep cracks are formed. It negatively impacts the productivity of agricultural crops. Nowadays, about 30 % of irrigated soils in southern Kazakhstan are compact with low soil permeability.
The magnesium–affected soils are found in the Aral Sea Basin, Central Asia, whilst more than 30% of the irrigated lands in southern Kazakhstan consist of soils that have exchangeable magnesium percentage (EMP) in the range of 25-45 %, and in some cases, as high as 60%. A typical example of high magnesium soils and irrigation waters is found in the Arys Turkestan area, southern Kazakhstan (Fig.1), where land degradation and gradual decline in cotton and winter wheat yields affect the livelihoods of crop farmers. About half of the irrigation land fund requires reclamation measures.
Photo 1. Map of southern Kazakhstan indicating Arys Turkestan canal command area with high-magnesium soils
The problem becomes more complex when the magnesium concentrations are higher than calcium in irrigation water. The Arys Turkestan canal command area is a typical example where excess levels of magnesium are present in both soil and irrigation water. Cotton is a major crop cultivated in approx. 95% of crop area. Consequently, there has been a gradual decrease in cotton (Gossypium hirsutum L.) yields. Despite its low productivity the farmers rely heavily on this crop for their livelihoods and farm income (Fig.2). Other important crop is winter wheat (Triticum aestivum L.), which is also affected by soil degradation.
Photo 2. Patchy growth of cotton on a high-magnesium soil without additional calcium (gypsum or phosphogypsum)
When magnesium dominant soils are plowed they typically form massive clods that impede the flow of water down the furrows and across irrigated fields resulting in poor water distribution (Figure 3). The problem becomes more complex when the magnesium concentrations are higher than calcium in irrigation water. Consequently, there has been a gradual decrease in cotton (Gossypium hirsutum L.) and winter wheat (Triticum aestivum L.) yields, whilst the farmers rely heavily on these crops for their livelihoods despite low productivity.
Photo 3. Typical takyr high-magnesium soils. Clods formed after irrigation, impacting hydraulic properties and water flow rate
High-magnesium soils can be brought back to a highly productive state by increasing the levels of calcium to counteract the deleterious effects of magnesium. This is accomplished through the application of a source of calcium to the soil in sufficient amounts.
Gypsum or phosphogypsum are usually used as a calcium source. Gypsum is a mineral, which is found in sedimentary environments. Phosphogypsum is the main waste of phosphoric acid factories, which use phosphate rock as a raw material.
Phosphogypsum contains 1,3.2,9% phosphate, including 0,2.0,9% soluble forms that acidify soil and accelerate the exchange reactions. In addition, the phosphogypsum has a positive effect by providing the mobile phosphorus. Acids in its content increase the solubility and provide a significant improvement of agronomic properties of fused soil, so crop yields increased up to 2 times in the first year of its application.
In 2001, the scientists of the Kazakh Research Institute of Water Resources led by Dr.Vyshpolsky have tested the technology of phosphogypsum application in the Arys-Turkestan area. The results of their long-term studies show that:
Phosphogypsum for cotton
The PG application after autumn plowing before snow fall provides the maximal increase in cotton yield – 0.5-1.5 tons ha-1. Differences in productivity were predetermined by a degree of soil sodicity and application rates for 0.4-0.6-m soil layer, climatic conditions and water availability.
In the first year (2001), when 4.5 and 8.0 tons ha-1 PG were applied after autumn plowing and snowfall, with farmer Z.Duzbaev, the cotton productivity raised by 1.3 and 1.6 tons ha-1and provided profit at the rate of US$52/ha on the first treatment and small losses US$7 /ha on the second treatment (Fig 4 and 5). However, the next 5 years, at application of 4.5 and 8.0 tons ha-1PG the mid-annual profit in the farm has made US$516/ha at the first treatment and US$643/ha at the second treatment. From the economic point of view the second treatment has appeared the most perspective as provided growth of annual profit more than US$120/ha.
Photo 4. Cotton field without PG application
Photo 5. Cotton field under PG application at the rate of 8 t/ha
Technology of PG application under spring plowing by norm 3-4т/га has led to decrease in its efficiency; therefore growth of productivity of cotton did not exceed 0.4-0.6 t/ha (farmers Kasymov and Mirhaidarov). In this case expenses for carrying out of chemical land improvement conjoint soils paid off within two years. The next years the profit changed within the limits of $80-100 US/ha.
Similar results have been received on the soils of farmer Abdraimov where efficiency of PG application technology was studied in the winter and the spring seasons after autumn plowing. At the rates of 3.3 and 8.0t/ha which were established for amelioration of 0.3 and 0.6 m soil layer, expenses for purchase and transportation PG have made US$105 and US$255/ha, and growth of productivity of a cotton (concerning the control) has occurred accordingly by 0.5 t/ha under application of 8.0 t/ha PG in winter and by 0.3 t/ha under its spring application. In the first case the farmer has received profit of US$50 US/ha at the application rate of 3.3 t/ha PG and losses within the limits of US$30/ha at application rate of 8.0 t/ha PG. In the second case losses were US$50/ha at the rate of 3.3 t/ha and US$155/ha at the rate of 8.0t/ha. Hence, the fall-winter period is the best time for PG application.
High efficiency of use PG for land improvement of conjoint soils proves under its application under winter wheat. Farmer Muborakov has applied PG at the rate of 2.5 t/ha after appearance of shoots of wheat. The area of PG application was 10 ha, while that under control is 2 ha. Productivity of wheat has increased, in comparison with control by 1.2 t/ha and has made 3 t/ha (a photo 7, 8). At realization of additional production the farmer has received US$96/ha, and has spent US$95/ha for purchasing and application of PG. In this case ameliorative actions have paid back in the first year of their application.
Table 1. Cotton yields, tons ha-1
application rate, t/ha
Phosphogypsum for winter wheat
Phosphogypsum application proves to be efficient in trials of its application under winter wheat crop. Farmer N.Muborakov has applied 2.5 t/ha Phosphogypsum in the field under germinated wheat. The area of Phosphogypsum application was equal to 10 hectares. Control treatment area was 2 hectares. Wheat yield has increased in compare with control up to 1.2 tons ha-1and made up 3 tons ha-1.
Photo 6. Winter wheat field with no PG application
Determination of the proper application rates is a very important step. Rating depends on the initial level of magnesium in the soil, which must be reduced to a critical level, in order to bring the soil to its productive state. Applying the phosphogypsum without proper assessment could lead to excessive or inadequate rates. Applying the PG below the actual requirements only partially improves the soil, while the excessive application has economic consequences for farmers. Therefore, rates should be aimed at the soil improvement (physical and chemical properties) to a depth of 0,5-0,6m.
Photo 7. Field visit
The rate of PG application for high magnesium soil can be estimated using the Antipov-Karataev equation:
N=К× (Mg-0.3CЕC) ×h×d:с,
Where N - Rate of PG application, t/ha;
K - Factor for translation of the Gypsum maintenance in amendment, which is corresponding to 1 mg-eq (it is equal to 0.08 for PG producing chemical factories in Taraz)
Mg – Quantity of exchangeable magnesium expressed in mg-eq per 100 g soil;
CЕC –Cation exchange capacity of the soil expressed in mg-eq per 100 g soil;
h – Depth of soil amelioration, cm;
d – Bulk density of the soil, t/m3;
c- Content of gypsum (CaSO4×2H2O) in PG depends on moisture content in PG.
Chemical methods for improvement of low-permeable (sodic, solonetz) soils
The operating experience of irrigation systems shows, that efficiency of the irrigated soils decreases due to their structural degradation, increase in soil bulk density, soil sodicity. The impact of such changes depends on the irrigation technology, farming system, crops pattern, water supply, quality of irrigation water and stability soils towards the anthropogenic pressure. Therefore the system of reclamation measures should foresee the acceleration of exchange reactions during the application, increase of the ploughing depth, as well as the areas under perennial grasses, cultivation of green manure crops, use of water-saving irrigation technologies. Such optimized measures shall lead to a radical soil improvement and raise the economic efficiency of farms and agroholdings.
Moreover, other factors like the local climatic conditions, farm management, an economic solvency (financial resources) and availability of equipment farms also predetermine the conditions for application. Depending on the specific context, the following schemes of land improvement measures are suggested:
For early maturing crops (winter wheat, melon, silage corn)
1. After crop harvesting in autumn the soil plowing is carried out. The recommended plowing depths are 0.25-0.30 m, 0.30-0.35 m and 0.35-0.40 m for low, moderate and high soil sodicity respectively when PG application rate exceeds 8 tons ha-1.
2. In dry and windy autumn, the meliorant should be applied after the soil plowing and harrowed.
3. In damp autumn there is no need for protection against blowing.
4. In case of increased precipitations, after plowing, the amendment should be applied in winter season on the frozen soils or a snow, since the application on damp soils will lead to its compactness.
5. When financial means are limited and the plowing is postponed till spring season and credits are received, the amendment shall be applied in autumn and harrowed (disk harrow).
For moderate-and late-maturing crops (cotton, corn for grain, sunflower)
Use of other application schemes will lead to decrease in their efficiency. For example, in early spring the topsoil is over moistened after winter precipitations, the machinery is used to apply the amendments. The use of machinery leads to the compactness of the arable soil horizon, the delay in exchange reactions, and decrease in leaching of exchange products from the reclaimed depths. The similar phenomena occur during the application on spring plowing.
In view of change of climatic conditions, financial and technical resources of farmers, the suggested technological schemes will provide the greatest possible soil improvement and increased crop yields. The increased yields will provide an economic return to conduct the chemical improvement measures within 2-3 years.
Acting as an information repository and knowledge hub, this website helps to increase the use of innovations developed by the well-established CACILM Project in Central Asia. Its synthesis, compilation, and dissemination of current research provide a secure knowledge base that policymakers and other stakeholders can access and utilize to develop sustainable strategies capable of addressing the region’s severe land degradation.