STATISTICAL EVALUATION OF THE ELEMENTAL COMPOSITION OF HUMIC SUBSTANCES

Elemental data (C, H, O, N, S, atomic H/C and O/C ratios) for humic acids (410 samples), fulvic acids (214 samples) and humin (26 samples), isolated from environments all over the world, were compiled from the literature. These data were analyzed statistically using the mean, median, mode, range, standard deviation and t-test. The large data base allows statistically significant differences between various humic substances to be established. Prior to some of the statistical evaluations, the humic substances were grouped according to source-soil, freshwater, marine or peat. The average elemental compositions for humic acid, fulvic acid and humin, in toto, and also when segregated by source, are presented. Standard deviations for carbon contents are remarkably small suggesting that perhaps an optimum composition exists for humic substances in nature. The evaluation shows that humic acids in toto are significantly (P < 0.0005) different from fulvic acids in toto with respect to C, N, and O contents and atomic H/C ratios. Fulvic acid in toto is significantly (P < 0.0005) different from humin in toto in terms of C and O contents. When segregated by source, some significant (P < 0.0005) differences between humic acids isolated from freshwater, marine, and soil environments are evident; similarly, significant differences are found between fulvic acids from freshwater and soil sources. Three van Krevelen diagrams based on: (1) the 650 samples of humic acid, fulvic acid and humin; (2) humic acids segregated by source, and; (3) fulvic acids segregated by source, are discussed.