The ratio of optical densities or absorbances of dilute, aqueous humic and fulvic acid solutions at 465 and 665 nm (E4/E6) is widely used by soil scientists for the characterization of these materials. While it has been suggested that the E4/E6 ratio is related to the degree of condensation of the aromatic carbon network, carbon content, and molecular weight of humic substances, little rigorous experimental evidence is available in the literature to confirm these hypotheses.The results of this investigation show that the E4/E6 ratio of humic and fulvic acid is: (i) mainly governed by the particle size (or particle or molecular weight); (ii) affected by pH; (iii) correlated with the free radical concentration, contents of O, C, CO2H and total acidity in as far as these parameters are also functions of the particle size or particle or molecular weight; (iv) apparently not directly related to the relative concentration of condensed aromatic rings; (v) independent of humic acid and fulvic acid concentrations, at least in the 100–500 ppm range. Our data show, in agreement with M. M. Kononova (1966), that E4/E6 ratios for humic and fulvic acids should be determined between pH 7 and 8. This can best be done by dissolving the humic material in 0.05N NaHCO3 solution at concentrations of 200–400 ppm.

1. From the data presented herein it is possible to deduce the following distribution of total N in humic substances and soils: proteinaceous materials (proteins, peptides, and amino acids) -ca. 40%; amino sugars -5-6%; heterocyclic N compounds (including purines and pyrimidines) -ca. 35%; NH 3 -19%; approximately 1/4 of the NH 3 is fixed NH 4 + . Thus, proteinaceous materials and heterocyclics appear to be major soil N components.

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