Gypsum usage in irrigated agriculture: A review

Gypsum is the source of calcium most commonly used to reclaim sodic soils and to improve soil water infiltration when it has been decreased by low electrolyte concentration. The electrolyte concentration of the soil water and exchangeable sodium fraction,E _Na, are the two predominant chemical factors that influence soil hydraulic properties. In sodic soil reclamation with gypsum, the maximum electrolyte concentration increases with increasingE _Na: for soil solutions in equilibrium with both gypsum and anE _Na of 0 and 0.4, the concentrations are 15 and 133 mol m^–3. These concentrations are generally adequate to maintain the existing hydraulic conductivity. Higher concentrations can increase the soil hydraulic conductivity and the rate of reclamation. Thus, combining calcium chloride or sulphuric acid with gypsum, can reduce both the time and amount of water required to achieve reclamation.Infiltration is especially sensitive to low electrolyte concentration. Thus, surface application of gypsum or its direct addition to irrigation water, can potentially increase water infiltration rates of soils susceptible to aggregate dispersion and crusting. This is particularly true in areas where rainfall is interspersed with the application of irrigation water — as in the Midwest and Plains States of the US — or where mediterranean or monsoon climates prevail and wet and dry (irrigation) seasons alternate. As supplemental irrigation and the use of poor quality irrigation waters (sewage effluents, brackish well waters, irrigation drainage waters, etc.) intensify the need for gypsum will increase and phosphogypsum may help meet this need.Recent data indicate phosphogypsum dissolves faster than mined gypsum, and hence it develops a higher electrolyte concentration during an infiltration event. This attribute of phosphogypsum, in conjunction with its acid content, would increase its effectiveness as compared to mined gypsum for both sodic soil reclamation and maintenance of infiltration rates. The acid content of phosphogypsum is of direct benefit for reducing aggregate dispersion through promotion of soil particle flocculation and bonding by calcium, iron and aluminium released by dissolution of calcite and other soil minerals. It would decrease soil pH, thereby increasing the availability of trace metal nutrients and of phosphate which are typically deficient in sodic soils (E _Na > 0.15) because of high pH. Also, the phosphate content of phosphogypsum has value as a phosphate fertilizer.Significant expansion of the agricultural use of gypsum would depend on its application in both irrigated and dryland agriculture to increase water infiltration. An annual production rate of phosphogypsum of 30 × 10⁶ Mg (1 Mg = 1 tonne) is sufficient to treat 73 000 km² (29 000 mi²) at a rate of 4 Mg per ha, or nearly half the total area irrigated in the USA. Although extensive farmland areas with limited rainfall exist along the Gulf of Mexico within the North American continent, market development within this area would require extensive field evaluation by local agricultural research personnel in cooperation with the phosphate fertilizer industry to determine if the economic benefits exceed the cost of phosphogypsum.