Geoscience Reference
In-Depth Information
BACTERIA AND OTHER MICROORGANISMS OF THE SOIL
All fertile soils harbour a vast range of microscopic organisms which include bacteria,
micro-fungi, actinomycetes, yeasts, algae, myxomycetes, myxobacteria, protozoa and
others, not to mention the large numbers of small soil animals. There is a bewildering
array of species and genera, some of them present in great numbers, often many mil-
lions in a gram of soil. Soil, of course, is a particularly favourable habitat, with a vari-
ety of nutrients, for this world of microorganisms.
Until about the middle of the seventeenth century, the realm of microorganisms
was virtually unexplored. But at that time, in the town of Delft in Holland, Antony
van Leeuwenhoek (1632- 1723) was busily engaged in grinding and polishing tiny
glass lenses which he mounted and assembled into compound microscopes; with these
somewhat primitive instruments, he became an enthusiastic observer of many kinds of
natural microscopic phenomena. It seems likely that he was able to achieve magnific-
ations of several hundred times and so was able to detect tiny, motile living creatures
in, for example, drops of dirty canal water, infusions of pepper grains, scrapings from
his teeth and other such preparations. Today, we realize that the tiny animalcules that
he saw were indeed living motile bacteria. He reported many of his observations to
the Royal Society of London, in a series of letters which are preserved in the Society's
Archives, but he was never willing to allow others access to his remarkable micro-
scopes.
Little more than a century ago, our knowledge of the multitude of microorganisms
in soil or indeed elsewhere was almost negligible. Since then, however, especially dur-
ing the last eighty years or so, impressive advances have been made in scientific micro-
scopy, culminating in the astonishing transmission and scanning electron microscopes
of today. These modern instruments can achieve very high magnifications, up to a hun-
dred thousand times or more, and so make possible detailed observations of the fine
structure of bacteria or even the much smaller virus particles and similar objects.
During the nineteenth century, the majority of microbiologists, for example,
Pasteur, Koch, Cohn, Lister and others, directed their attention to the study of disease-
causing microbes. The isolation and identification of these pathogenic bacteria was a
vital initial step towards the eventual treatment and control of such diseases.
Apart from investigations dealing with yeasts, the operative organisms of bread-
making and alcoholic fermentation, and moulds and fungi, which cause rotting and
spoilage of crops or materials, the existence and activities of microorganisms in soil
