Comparison of Specific Surface Area Measurement Approaches for Soils with a Range of Properties.

Specific surface area (SSA) plays an important role in many soil physical properties and processes; such as water retention, shrink swell potential, and contaminant fate and transport. The ethylene glycol monoethyl ether (EGME) and N₂ gas adsorption methods have been used to measure the SSA. The WP4 Dewpoint potentiometer was originally developed to measure the dry end of the water retention curve, but recently its use has been expanded to determine SSA using a mathematical equation that contain an empirical constant. The SSA measured by WP4 Dewpoint potentiometer is believed to mimic that determined by the EGME method.  We have compared WP4 Dewpoint potentiometer and EGME methods by measuring the SSA of 40 different soil samples from North Carolina, representing 10 textural categories and various diagnostic horizons.  The results show that the SSA measured by WP4 is 2 to 6 times greater than the corresponding surface area determined by the EGME method.  Overall, the relationship between the SSAs determined by EGME and WP4 is not linear, and the differences are more pronounced for coarse textured than fine textured soils.  Assuming the EGME method provides an estimate of the specific surface area of soils, the WP4 could either provide a better estimate of the internal soil particle surface area, potentially contributed by a mineralogical component that the EGME cannot measure, or the empirical constant reported in literature does not universally work for all soils. To explore potential causes for the discrepancies between the two methods, the soils used in the measurements are grouped by textural class, organic carbon content, pH, and electrical conductivity.