Humic substances in soils are commonly believed to have an "aromatic" backbone. A dominant hypothesis for the origin of soil humic substances holds that phenolic acids derived from biodegradation of lignin or synthesized by microorganisms are oxidized to quinones and that the quinones spontaneously or enzymatically copolymerize with other degradation products of biopolymers (e.g., amino acids, fatty acids, nucleic acids, sugars, peptides, oligosaccharides, etc.). We tested this theory by using intensive physical fraction to isolate the coarse (0.2-2 um), medium (0.02-0.2 um), and fine (<0.02 um) clay fractions from a typical Midwestern agricultural soil (Ap horizon of a Webster soil (fine-loamy, mixed, superactive Typic Endoaquoll)). X-ray diffraction analysis showed that the mineralogy of the fine clay fraction was smectitic while the coarse clay fraction contained quartz, kaolinite, illite and several primary minerals but little or no smectite. Organic C levels associated with the fractions were similar (6.38, 7.38, and 5.64% for the coarse, medium, and fine clay fractions, respectively), however, C:N ratios (23.6, 13.4, and 13.1 for the coarse, medium, and fine clay fractions, respectively) indicated substantial differences in the nature of the humic materials associate with the various fractions. Spectroscopic (13C-MAS NMR) and scanning electron microscopic analyses revealed that the coarse clay fraction was overwhelmingly dominated by discrete particles of condensed aromatic C that appear to be pyrogenic. The fine clay fraction contained diffuse filamentous material dominated by alkyl, o-alkyl, and carboxylic C with only small amounts of aromatic C. The organic matter associated with the fine clay fraction appears to be biogenic. The results suggest the existence of at least two independent pathways for the formation of soil humic substances (biogenic and pyrogenic) and indicate that humic substances in the studied soil do not have an aromatic backbone.