Geoscience Reference
In-Depth Information
1985-86 Hellriegel and Willfarth recognized the connection between nitrogen
fixation and the nodules on legumes.
1888 Beijerinck grew nodule bacteria in pure culture.
1890-91 Winogradsky isolated autotrophic nitrification bacteria.
Beijerinck isolated the free-living, nitrogen fixing organism, Azotobac-
ter.
1901
Hiltner introduced the term 'rhizosphere' for the space with close inter-
actions between plant roots and microorganisms.
1904
1918
Conn devised a technique for direct soil microscopy.
Winogradsky, on the basis of direct microscopic observation of soils,
advanced the concept of autochthonous and zymogenous soil bacteria.
1924
Lochhead suggested grouping soil microorganisms according to their
nutritional requirements.
1940
1943
Waksman reported the discovery of streptomycin
In its top 15 cm layer, a typical arable soil may contain approximately two to
three tonnes (wet weight) of bacterial biomass; equivalent to the weight of some 40
to 60 sheep. If we suppose that the soil contains, say, three per cent of organic mat-
ter, then about one per cent of the latter is made up of microorganisms. Although one
gram of soil may contain hundreds of millions of bacteria, because of their minute
size, these constitute only a fraction of the total weight of the soil. By reason of their
manifold metabolic activities, however, microorganisms represent a significant pro-
portion of the biomass of the soil. The total biomass of soil, of course, is the weight
of all the living organisms to be found therein.
The detailed scientific investigation of microorganisms is for the specialist, but
some indication of what microorganisms are like, and how they are isolated and iden-
tified, is essential to a full appreciation of the living soil. Bacteria are too small to
be visible to the naked eye and, even with the help of the microscope, special tech-
niques are required for their observation. Under the ordinary transmitted light micro-
scope (i.e. bright field), bacteria cannot be seen unless first stained by suitable dyes,
or rendered opaque by treatment with certain chemicals. By staining, the organisms
are killed. Unstained, living microorganisms, however, may be observed directly by
the use of either a dark background microscope or a phase-contrast microscope. In a
dark, dusty room, tiny dust particles become visible if a beam of light is shone across
the room and, as the dust particles reflect the light, they are revealed as tiny specks of
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