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Predicting Exchangeable Sodium Ratios in Irrigated Tropical Vertisols¹

ABSTRACT

Batch equilibrium studies were used to investigate Nadivalent exchange and to derive Gapon constants for three Hawaii Vertisols. Obtained regression coefficient values indicated that a linear relationship exists between exchangeable sodium ratios (ESR) and sodium adsorption ratios (SAR) for tropical Vertisols. ESR values obtained for two of the soils were slightly lower than the values predicted by Bower's equation (3) and higher than those predicted by the U.S. Salinity Laboratory's equation (11). ESR values obtained for the third soil were lower than those predicted by either equation. Generally, the higher preferences of surface than of subsurface horizons for Na was confirmed. Prevailing electrolyte concentration levels were found more important than any other experimental variable in affecting the precision of ESR estimates for a given SAR. Although ESR values were generally higher at 0.1N than at 0.01N, the valence-dilution effect was reversed for the surface horizon of one Vertisol during Mg-Na exchange.

While little difference was noted between Ca-Na and Mg-Na exchange in one of the soils, in the other two significantly smaller ESR values were obtained for the first than for the second reaction. An unexpected type of hysteresis effect was noted, whereby divalent-soils exhibited greater preference for Na than did Na-soils. Possible reasons for this effect are discussed.

¹ Contribution no. 532 of the Hawaii Inst. of Geophysics. Journal Series no. 1659 of the Hawaii Agr. Exp. Sta. Work supported in part by Western Regional Project W-107 and AID 211-d funds. Paper based on a dissertation by the senior author in partial fulfillment of her Ph.D. requirements.

² Former Graduate Assistant and Associate Professor of Soil Science, respectively. Senior author is now with the Land Development Department, Bangkok, Thailand.

Received for publication April 15, 1974. Accepted for publication June 5, 1974.