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

Measuring the specific surface area (SSA) of soils that contain much organi c matter (OM) is problematic. The adsorption of p-nitrophenol (pNP) from xy lene at room temperature yielded realistic values for the SSA of a wide ran ge of clays, oxides and subsoils. Here we have extended the same measuremen t to some topsoils with varied OM content, texture and clay mineral composi tion. Specifically, we have compared the surface areas measured by adsorpti on of N-2, and, applying the BET equation, with the values obtained by adso rption of pNP, before and after treatment of the samples with hydrogen pero xide. In all instances, the removal by H2O2 of organic matter - albeit in p art only - led to a marked increase in the SSAs measured by nitrogen becaus e of the exposure of micropores previously blocked or covered by OM. The su rface areas measured by pNP were appreciably larger than those obtained by the standard BET equation, and showed little change after removal of organi c matter. However, the surface area of two smectite-rich samples measured b y pNP increased substantially after peroxidation, presumably because smecti te crystals decomposed during treatment with H2O2. The results suggest that, under the experimental conditions used, pNP could diffuse without hindrance into and through organic matter, enabling it to adsorb on to micropore surfaces within clay aggregates (domains). In keepin g with this suggestion, the relation between the surface areas measured by pNP 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 strong er relation was observed between the measured and calculated values for cat ion exchange capacity.