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

The Exchangeable Sodium Percentage (ESP) is the most widely used index of sodicity for the investigation of soil structural stability and fo r soil classification, However, sensitivity to levels of ESP varies ac ross soil types, An alternative index is the Exchangeable Sodium Conte nt (ESC), in which levels of sodium are expressed on an oven-dried soi l basis rather than relative to the cation exchange capacity, Early wo rkers suggested that the ESC may prove a better index of sodicity acro ss soil types than the ESP, This contention has not been tested in the literature. We compared the abilities of ESP and ESC to explain varia tion in soil structural stability across soil types using data from 15 published data sets. The logarithm of soil structural stability was c orrelated significantly with the logarithm of ESP in only seven data s ets, 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 ES P, In three of four combined data sets where common methodologies were used, ESC explained variation better than ESP, and there was no diffe rence in the fourth, Therefore, we conclude that ESC is generally a be tter index of soil sodium levels than ESP and should be used more wide ly.