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Use of Gypsum Stones to Lower the Sodium Adsorption Ratio of Irrigation Water¹

ABSTRACT

Many sources of irrigation water have sufficiently high Na⁺/Ca²⁺ ratios to be harmful to soils and crops. When the salt contents of the waters are reasonably low, the primary method of lowering this ratio is to apply extra Ca²⁺ on the fields or in the water. Gypsum is generally the most reasonably priced source of Ca²⁺. A major portion of its current cost is associated with grinding the stones to a powder and application of this powder to the fields.

This study shows that gypsum stones 5 to 20 kg in size are a useable source of the Ca²⁺ needed to bring down the sodium adsorption ratio (SAR) in sodic irrigation waters. The dissolution rate of the gypsum stones is proportional to the square root of the velocity of flow through the bed. Since the time of residence of solution in the bed is inversely proportional to its velocity through the bed, the increase in concentration of Ca²⁺ in solution as it passes through the whole bed is inversely proportional to the square root of the velocity of flow. Solution concentration is also inversely proportional to the square root of the size of the stones in the bed.

Equations were developed to facilitate design of channels with the proper cross section and length to supply the needed amounts of Ca²⁺ to the water when the average size of the stones, the flow rate of the water, and the initial ionic concentration of the water are known.

¹ Research report from the Mona Reclamation Experimental Project, WAPDA, and the Colorado State Univ. Water Management Res. Team. Portions of this work were supported by USAID/Pakistan Agreement no. 204-75 and by USAID/Washington Res. Contract no. AID/ta-C-1411.

² Technical Officer, MREP; CSU Chief of Party, MREP Director of Research and JRO in charge of analysis at MREP respectively. W. D. Kemper is located at the Engineering Res. Center, Colorado State Univ., Ft. Collins, CO 80523.

Received for publication December 4, 1978. Accepted for publication April 16, 1979.