CHARACTERISTICS OF SOIL HUMIC-ACID FRACTIONS SEPARATED BY ULTRAFILTRATION

Different molecular weight fractions obtained by ultrafiltration of a humic acid (HA) extracted from a Mollisol were analyzed by chemical, i nfrared, electron microscopy, and isotachophoresis methods. The result s showed that the different HA ii-actions did not differ substantially in characteristics from each other, although each fraction was compos ed of subfractions exhibiting different electromobilities. It appeared that fractionation of HA would yield molecular weight fractions as im posed by any exclusion limits used in the procedure. However, the elem ental composition, infrared spectra, and electron micrographs showed t hat the fractions, separated by molecular weight limits, contained sim ilar compounds. These results do not support the concept of humic acid s as a heterogeneous mixture of compounds, but show instead that humic acids may possess a composition more homogeneous in nature than previ ously postulated.