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

Recent evidences from some irrigated areas worldwide, such as Central Asia, suggest that water used for irrigation contains magnesium (Mg2?) at levels higher than calcium (Ca2?). Excess levels of Mg2? in irrigation water and/or in soil, in combination with sodium (Na2?) or alone, result in soil degradation because of Mg2? effects on the soil's physical properties. More than 30 per cent of irrigated lands in Southern Kazakhstan having excess levels of Mg2? 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 Ca2? to mitigate the effects of excess Mg2?. As a source of Ca2?, 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, 4_5, and 8_0 metric ton per hectare (t ha_1)?on chemical changes in a soil containing excess levels of Mg2?, and on cotton yield and economics. The canal water had Mg2? to Ca2? ratio ranging from 1_30 to 1_66 during irrigation period. The application of phosphogypsum increased Ca2? concentration in the soil and triggered the replacement of excess Mg2? from the cation exchange complex. After harvesting the first crop, there was 18 per cent decrease in exchangeable magnesium percentage (EMP) of the surface 0_2m soil over the pre- experiment EMP level in the plots where phosphogypsum was applied at 4_5 t ha_1, and a 31 per cent decrease in EMP in plots treated with phosphogypsum at 8 t ha_1. Additional beneficial effect of the amendment was an increase in the soil phosphorus content. The 4-year average cotton yields were 2_6 t ha_1 with 8 t ha_1 phosphogypsum, 2_4 t ha_1 with 4_5 t ha_1 phosphogypsum, and 1_4 t ha_1 with the control. Since the amendment was applied once at the beginning, exchangeable Mg2? levels tended to increase 4 years after its application, particularly in the treatment with 4_5 tha_1 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

Optimizing the rate and timing of phosphogypsum application to magnesium-affected soils for crop yield and water productivity enhancement

The levels of magnesium (Mg2+) in irrigation waters and soils are increasing in several irrigation schemes worldwide. Excess levels of Mg2+ in irrigation waters and/or in soils negatively affect soil physical properties (infiltration rate and hydraulic conductivity) and ultimately crop growth and yield. Although few studies have been undertaken on productivity enhancement of magnesium-affected soils by adding a source of calcium (Ca2+) to mitigate the effects of excess Mg2+, there is no information available on optimizing the rate and time of the Ca2+-amendments. A 2-year field study was undertaken in southern Kazakhstan by applying phosphogypsum (PG), a source of Ca2+ and a byproduct of the phosphorous fertilizer industry, to a magnesium-affected soil. There were five treatments with four replications: (1) control (without PG application); (2) PG application in January (before snowfall) equivalent to PG requirement for 0.3m soil depth (3.3tha-1); (3) PG application in January equivalent to PG requirement for 0.6m soil depth (8.0tha-1); (4) PG application in April (after snowmelt) at 3.3tha-1; and (5) PG application in April (after snowmelt) at 8.0tha-1. All treatment plots were grown with cotton (Gossypium hirsutum L.), which is the most important summer crop in the region. The PG treatments performed significantly better than the control in terms of (1) improved soil quality with a reduction in exchangeable magnesium percentage (EMP) levels; (2) enhanced water movement into and through the soil vis-à-vis increased moisture storage in the root zone for use by the plant roots; (3) increased irrigation efficiency; (4) increased cotton yield and water productivity; and (5) greater financial benefits. In terms of the best rate and time of application, PG applied before the snowfall improved the soil properties to a greater extent than its application in spring after snowmelt. The economic benefits from the amendment application at 3.3tha-1 were double those from the treatments where it was applied at 8.0tha-1, suggesting that the lower rate was economically optimal. In addition to improving crop productivity, the study demonstrated the beneficial use of an industrial waste material in agriculture.Magnesium to calcium ratio Exchangeable magnesium percentage Salt-affected soils Phosphogypsum Water quality Central Asia Kazakhstan Cotton