In the Direct Method, Vinogradskii added fresh “natural soil to
various artificial milieu” containing a variety of plant and animal
matter chosen for their nutritive abilities. These experiments produced
a range of what he called “auxiliary cultures,” which he
examined by microscope directly, to study the “released processes,”
i.e., the range of biochemical activities the soil microbes were
capable of. He called the changes brought about by addition of nutrients
the soil’s “biological reaction.” These changes could be estimated
by comparing the state of the altered soil at different points in
time with an unaltered control sample of the same soil. Growing
these soils on gelatin plates and photographing the bacterial
colonies that grew on the plates allowed quantitative comparisons
of the proportions of different species. Here Vinogradskii drew on
some staining methods on soil water and dairy samples developed
by American microbiologists Robert Breed and H. J. Conn