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A Study of the Sodium Adsorption Ratio and Residual Sodium Carbonate Concepts of Irrigation Waters as They Affect Exchangeable Sodium of Soil Under Semiarid Conditions¹

ABSTRACT

The object of this study was to investigate the significance of "residual Na₂CO₃," as engendered by high HCO₃^- content in irrigation water, using synthetic and natural waters in greenhouse experiments, and a natural water in a field experiment. The significance is evaluated in terms of exchangeable Na⁺ changes in soils on which the waters were applied.

Synthetic waters and unadulterated natural waters of high HCO₃^- content produced exchangeable-sodiumpercentages (ESP) in greenhouse cultures 4 to 6% above that expected from the sodium-adsorption-ratio (SAR) value of the waters used. The use of gypsum with natural waters held the ESP to approximately 2 to 4% above that predictable from the SAR-ESP nomogram.

A field experiment of 5 years' duration with natural and gypsum-treated reservoir water under conditions of about 15 inches annual rainfall resulted in exchangeable Na⁺ levels much lower than those found in greenhouse experiments. This occurred, even though more than twice as much water was used. The highest ESP level found was 2% below that expected from the SAR value of the water.

The apparent discrepancy between field and greenhouse results can be ascribed in part to Ca return to the soil surface by alfalfa and possibly to slow weathering of noncarbonate minerals. There was a decrease in exchangeable H⁺ during the 5-year field experiment.

¹ Journal Paper Number 409 South Dakota Agr. Exp. Sta., Brookings. Contribution of the South Dakota Agr. Exp. Sta. and the Soil and Water Conservation Research Division, ARS, USDA. The cooperation of the United States Department of Interior, Bureau of Reclamation, on field phases of this work is gratefully acknowledged. This paper was presented to the Western Soil Science Society June 17, 1958, at Logan, Utah.

² Agronomist, S. D. Exp. Sta., formerly Soil Scientist, ARS; Extension Soils Specialist, formerly Grad. Res. Ass't., S. D. State College; Agr. Eng., U. of Arizona, formerly Agr. Eng., SWC, ARS, and Ass't. Agronomist, S. D. Exp. Sta.; and Agr. Eng., SWC, ARS, respectively.

Received for publication October 21, 1958. Accepted for publication December 23, 1958.