An improved method for determining the specific surface areas of topsoils with varied organic matter content, texture and clay mineral composition

Abstract

Summary Measuring the specific surface area (SSA) of soils that contain much organic matter (OM) is problematic. The adsorption of p ‐nitrophenol ( p NP) from xylene at room temperature yielded realistic values for the SSA of a wide range of clays, oxides and subsoils. Here we have extended the same measurement to some topsoils with varied OM content, texture and clay mineral composition. Specifically, we have compared the surface areas measured by adsorption of N 2 , and, applying the BET equation, with the values obtained by adsorption of p NP, before and after treatment of the samples with hydrogen peroxide. In all instances, the removal by H 2 O 2 of organic matter – albeit in part only – led to a marked increase in the SSAs measured by nitrogen because of the exposure of micropores previously blocked or covered by OM. The surface areas measured by p NP were appreciably larger than those obtained by the standard BET equation, and showed little change after removal of organic matter. However, the surface area of two smectite‐rich samples measured by p NP increased substantially after peroxidation, presumably because smectite crystals decomposed during treatment with H 2 O 2 . The results suggest that, under the experimental conditions used, p NP could diffuse without hindrance into and through organic matter, enabling it to adsorb on to micropore surfaces within clay aggregates (domains). In keeping with this suggestion, the relation between the surface areas measured by p NP and the corresponding values calculated from the clay and OM contents, and clay mineral composition, of the soils was close to 1:1. An even stronger relation was observed between the measured and calculated values for cation exchange capacity.