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This article in SSSAJ

  1. Vol. 43 No. 4, p. 698-702
    Received: Dec 4, 1978
    Accepted: Apr 16, 1979

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

  1. Bashir Ahmad,
  2. W. D. Kemper,
  3. Ghulam Haider and
  4. M. A. Niazi2



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.

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