Characterization and structural modelling of humic substances in field soil displaying significant differences from previously proposed structures

Humic substances (HS) often are dominant constituents of soil organic matter. In this study 28 fractions of HS were purified and chemically characterized from horizons of three Danish soils differing in texture. A commercial humic acid (HA) was included for comparison. The HS fractions were chemically characterized by CHNS analysis, pH-titration and liquid state ¹³C-NMR experiments. Amino acids and carbohydrates could clearly be differentiated from other O- and N-aliphatic compounds. The data were analysed by principal component analysis. The analysis revealed greater difference between HS fractions than between locations. The characteristics of the HS fractions were compared with several models in the literature and an average structural model for one HA and one FA fraction is proposed.