Role of gypsum in amelioration of saline-sodic and sodic soil

Gypsum is the most commonly used amendment in Pakistan. It is relatively inexpensive and easily available. Large deposits of gypsum are known to exist in Mianwali, Khushab, Jehlum, D.G. Khan, Sibbi, Kohat and D.I. Khan. The deposits are estimated at well over 3.5 billion tons. In order to establish the usefulness of gypsum as an amendment for the reclamation of saline-sodic and sodic soils, various experiments have been conducted in the country. Besides, experiments have also been carried out to see the possibility of using gypsum as a preventive measure to sodicity if the soil is being irrigated with hazardous tubewell water. For this purpose, experiments have been carried out in various parts of the country by various agencies/departments. In Pakistan, soil salinity and sodicity have been recognized as one of the major constraints for agriculture production. Reclamation of saline-sodic and sodic soils, however, cannot be achieved by simple leaching. Reclamation of these soils is difficult, time consuming and more expensive than that of saline soils due to replacement of exchangeable sodium with calcium. Hence, it requires the addition of chemical amendments along with leaching. The results of studies have revealed that by the application of gypsum to saline-sodic and sodic soils, adsorbed sodium on the soil complex is being replaced by the calcium. The action of Ca salts is mainly through exchange reactions with sodium. Na + CaSO4 Ca Soil + Na2 SO4 Soil Na Na2CO3 (if present) + Ca SO4 Ca CO3 + Na2 SO4 Precipitates Leached The effectiveness of gypsum depends upon (i) the degree of fineness (ii) the way in which it is incorporated in the soil and (iii) the efficiency of the drainage system. Regarding use of gypsum as a preventive measure against sodicity due to the use of hazardous tubewell waters, it has been concluded that potentially hazardous tube well water can be used to grow crops successfully without adversely affecting the soils, provided gypsum is applied to the soil. This paper gives an over view of the reclamation experiments carried out so far pertaining to the reclamation of sodic soils in various parts of the Pakistan. It is clear from the results that the use of gypsum increase infiltration rate of the soil. An application of gypsum remains effective for a period of about three years. Application of gypsum @ 50% requirement of the soils appears to be more economical than 100% application. It is recommended that reasonable facilities should be provided to farmers in Pakistan for using gypsum as amendment for the reclamation of their problematic soils to enable them to use this amendment on a large scale which will be highly profitable to both the Government and the farmers community through improvement of sodic soils and increase in over all agriculture production.