N2-BET SPECIFIC SURFACE-AREAS OF SOME LOW ACTIVITY CLAY SOILS AND THEIR RELATIONSHIPS WITH SECONDARY CONSTITUENTS AND ORGANIC-MATTER CONTENTS

Size fractions (2000-20-mu-m, 20-2-mu-m, 2-0.2-mu-m, and 0.2-0-mu-m) were obtained after Na-resin treatment of five top horizons from low-activity clay soils. Their specific surface areas (SSA) were measured by the N2-BET method. The same method was applied after organic matter removal by H2O2 treatment to two of the bulk soils and their size fractions. The weighted sum of the SSA of the fractions fitted well to the SSA of the bulk soils, indicating that the association of particles, destroyed by the fractionation procedure had no effect on the SSA. The SSA of the coarsest fractions were greater than theoretical values from a sphere model, because of the presence of microaggregates. A good relationship was found between the SSA of the soils and subfractions and their total contents in R2O3 (R2O3 = Al2O3 + Fe2O3), i.e., an estimation of the importance of the secondary minerals present. Organic matter removal from two of the soil samples and their subfractions resulted in systematical but weak increase in SSA (less-than-or-equal-to 8 m2.g-1). This increase was not correlated to the amount of C removed from the samples. It was mainly due to changes in SSA occurring in the clay fractions where amorphous organic matter was assumed to act as a glue binding some clay plates together.