Scanning Electron Microscopy of Humic Matter as Influenced by Methods of Preparation
1985
Tan, K. H.
Scanning electron microscopy (SEM) has recently been used to study the macromolecular structure of fulvic (FA) and humic acids (HA). Differences in soils and sample preparations are believed to produce different results, but little is known to confirm the latter. This investigation was initiated to study the effect of different sample preparations on SEM of humic matter extracted from Omega, Mascotte, Onaway, and Cecil soil series. Using air dry specimens, SEM showed massive sheets of FA and HA with granular surfaces observable at high magnification, similar to those obtained by transmission electron microscopy (TEM). The individual spheres of FA and HA reported in earlier TEM studies appeared in the present SEM analysis only after the samples had been frozen at 0°C in a freezer. Use of a modified liquid-N procedure, to provide a simpler and faster method, yielded similar tissue-like structures for FA and HA as with the freon-liquid-N method on mica strips. Fulvic acid (pH 3.0) was composed of fiber-like tissue, whereas HA (pH 7.0) had the appearance of shredded sheets. In both FA and HA, the sheets became prevalent at higher pH values. No effect was noticed by differences in concentration, but differences in soils had a definite influence on the SEM of the humic fractions. The fibers of FA from the Omega soil were coarse and thick, compared to those of the Mascotte soil where they were very fine. In the Onaway series, the FA fibers were medium in size. Fulvic acid from the Cecil soil, on the other hand, had the appearance of shredded or perforated sheets. Six types of structures were distinguished, e.g., fibrous, spheroids connected by fibers, sheets interwoven by fibers, perforated sheets, bladed, and shredded sheets. They were believed to be the result of the method of freezing.
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