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Scanning electron microscopy was used to evidence the aggregated structure of a forest soil as well as the presence of fungal hyphae external to soil aggregates. The supernatant of soil suspension in water mainly contained isolated bacteria, while ultrathin sections of aggregates frequently revealed groups of bacteria surrounded by a sheath of mucilage with adhering clay minerals on the outside. These results confirm the existence of two particular biotopes in the soil studied: one is located inside aggregates, and the other, in the inter-aggregate spaces.

Soil dilutions were studied by different techniques: They were retained on cellulose filters and observed either directly by scanning electron microscopy or after inclusion and ultra-thin sectioning by transmission electron microscopy. Morphology and locomotor organs of bacteria were observed after negative staining. They were plated on various culture media in order to isolate and identify most of the nonexacting microorganisms. Results are compared and the use of electron microscopy in soil studies is discussed.

Variations between seasonal samplings of a forest soil were studied during a one year cycle. The total number of microorganisms was maximal in march, while qualitative variations of the bacterial and fungal flora were unimportant. Electron microscope studies showed physiological differences and enabled to evaluate the seasonal variations of the type of associations of soil microbes.