IS EXCHANGEABLE SODIUM CONTENT A BETTER INDEX OF SOIL SODICITY THAN EXCHANGEABLE SODIUM PERCENTAGE?: A REASSESSMENT OF PUBLISHED DATA

Abstract

The Exchangeable Sodium Percentage (ESP) is the most widely used index of sodicity for the investigation of soil structural stability and for soil classification. However, sensitivity to levels of ESP varies across soil types. An alternative index is the Exchangeable Sodium Content (ESC), in which levels of sodium are expressed on an oven-dried soil basis rather than relative to the cation exchange capacity. Early workers suggested that the ESC may prove a better index of sodicity across soil types than the ESP. This contention has not been tested in the literature. We compared the abilities of ESP and ESC to explain variation in soil structural stability across soil types using data from 15 published data sets. The logarithm of soil structural stability was correlated significantly with the logarithm of ESP in only seven data sets, compared with 14 data sets that had significant correlations with the logarithm of ESC. In 10 of the data sets, between 15 and 60% more of the variation in structural stability was explained by ESC than ESP. In three of four combined data sets where common methodologies were used, ESC explained variation better than ESP, and there was no difference in the fourth. Therefore, we conclude that ESC is generally a better index of soil sodium levels than ESP and should be used more widely.