Properties and distribution of humic substances and inorganic structure-stabilizing components in particle-size fractions of cultivated Mediterranean soils

Aggregate-size fractions of two cultivated calcareous Mediterranean soils (Mevo Horon and Palmahim) were characterized with regard to organic matter and other soil components involved in soil structure stabilizing reactions. The parameters determined in each aggregate-size fraction included: particle size distribution and contents of CaCO₃, organic C and N, fulvic acid fraction (FF), humic acid (HA), total humic substances (HS), and extractable Al and Fe. Except for the organic C and HS in the Mevo Horon (MH) soil, all of the calculated ratios between the contents of each of the components in any size fraction and that calculated for the same component in the <2-μm fraction, exhibited a parabolic type curve, reaching a maximum for the 20- to 50-μm aggregates. It is hypothesized that this ratio is a reflection of the degree of surface coating (or association) of the clay by these structure-stabilizing components. The values of coating observed in each of the aggregate-size fractions were normalized with that observed for the <2-μm fraction (which presumably exhibits the highest degree of association). In practically all combinations of aggregate-size fractions and all soil components (in both soils), the individual stabilizing component is present at higher relative concentration than in the <2-μm fraction. These components, therefore, act first as surface coating materials, with the excess material forming interparticle associations. ¹³C-NMR spectra of both soils (OM not extracted) displayed similar characteristics, with generally high content of both aliphatic (50-60%) and aromatic carbons (22-30%) in both the largest (>250 μm) and the smallest (<2 μm) aggregate-size fractions. Higher polysaccharide content was observed in the larger particle size fraction, reflecting the influence of recent plant material.