Enhancing the productivity of high‐magnesium soil and water resources in Central Asia through the application of phosphogypsum

Abstract

Recent evidences from some irrigated areas worldwide, such as Central Asia, suggest that water used for irrigation contains magnesium (Mg 2þ ) at levels higher than calcium (Ca 2þ ). Excess levels of Mg 2þ in irrigation water and/or in soil, in combination with sodium (Na þ ) or alone, result in soil degradation because of Mg 2þ effects on the soil's physical properties. More than 30 per cent of irrigated lands in Southern Kazakhstan having excess levels of Mg 2þ are characterized by low infiltration rates and hydraulic conductivities. The consequence has been a gradual decline in the yield of cotton (Gossypium hirsutum L.), which is commonly grown in the region. These soils require adequate quantities of Ca 2þ to mitigate the effects of excess Mg 2þ . As a source of Ca 2þ , phosphogypsum-a byproduct of the phosphorous fertilizer industry-is available in some parts of Central Asia. In participation with the local farming community, we carried out a 4-year field experiment in Southern Kazakhstan to evaluate the effects of soil application of phosphogypsum-0, 4Á5, and 8Á0 metric ton per hectare (t ha À1 )-on chemical changes in a soil containing excess levels of Mg 2þ , and on cotton yield and economics. The canal water had Mg 2þ to Ca 2þ ratio ranging from 1Á30 to 1Á66 during irrigation period. The application of phosphogypsum increased Ca 2þ concentration in the soil and triggered the replacement of excess Mg 2þ from the cation exchange complex. After harvesting the first crop, there was 18 per cent decrease in exchangeable magnesium percentage (EMP) of the surface 0Á2 m soil over the pre-experiment EMP level in the plots where phosphogypsum was applied at 4Á5 t ha À1 , and a 31 per cent decrease in EMP in plots treated with phosphogypsum at 8 t ha À1. Additional beneficial effect of the amendment was an increase in the soil phosphorus content. The 4-year average cotton yields were 2Á6 t ha À1 with 8 t ha À1 phosphogypsum, 2Á4 t ha À1 with 4Á5 t ha À1 phosphogypsum, and 1Á4 t ha À1 with the control. Since the amendment was applied once at the beginning, exchangeable Mg 2þ levels tended to increase 4 years after its application, particularly in the treatment with 4Á5 t ha À1 phosphogypsum. Thus, there would be a need for phosphogypsum application to such soils after every 4-5 years to optimize the ionic balance and sustain higher levels of cotton production. The economic benefits from the phosphogypsum treatments were almost twice those from the control.

Potential business opportunities from saline water and salt-affected land resources

“…Recent evidence from different parts of the world has helped in characterizing another type of salt-affected soil, i.e. soil that is affected by magnesium (Vyshpolsky et al, 2008). With high levels of magnesium, when plowed, these soils form large clods that impede water flow resulting in poor water distribution and plant growth.…”

“…For example, a moderately saline soil would require less investment to bring it back to a productive state than a highly saline soil under similar conditions, although both soils fall under the 'saline soil' category (Qadir et al, 2014). Similarly, the quantity and related cost of a calcium source, such as gypsum, increases with the level of sodium in sodic soils or magnesium in magnesium-affected soils (Garcia-Ocampo, 2003;Vyshpolsky et al, 2008). In addition, if there is a compacted layer in the subsoil -which needs to be broken to leach the salts from the root zone -the cost of land restoration is affected.…”

“…Magnesium-affected soils contain an exchangeable magnesium percentage (EMP) in the range of 25 to 45% (Garcia-Ocampo, 2003), and in some cases, this is as high as 60% (Vyshpolsky et al, 2008).…”

“…For example in the Kazakhstan part of the Aral Sea Basin in Central Asia, more than 140,000 ha have these degraded soils (Vyshpolsky et al, 2008), i.e. more than 30% of the irrigated area (Vyshpolsky et al, 2010).…”

“…With low infiltration rate and hydraulic conductivity, these soils form large dense clods during the postirrigation drying phase, which impact the water flow rate. The consequence of using magnesium-affected soils for agricultural production without suitable management practices has been a gradual decline in crop productivity and yield (Vyshpolsky et al, 2008;Karimov et al, 2009). …”