Geoscience Reference
In-Depth Information
growth factors that are found in soil extracts or yeast extracts. Trace amounts of vari-
ous minerals are also frequently needed.
Although not very precise, these nutritional classifications have been used by
some investigators seeking to count groups of organisms in soil by culturing them on
media with differing nutritional contents. Microbial populations in soils are generally
determined by the varying supplies of nutrients.
Winogradsky classified soil bacteria into two general groups. Those constantly
present, not necessarily in large numbers, but which he considered to be the indi-
genous inhabitants, he termed 'autochthonous' organisms. His second group included
microorganisms that may proliferate rapidly in response to recent additions of any
readily utilized nutrients; these organisms, he termed 'zymogenous'. As a rule, zymo-
genous microbes are among the initial decomposers of fresh organic material, such as
plant debris. Consequently, populations of these organisms can develop quickly, and
under favourable conditions may flourish for a time but decline when the nutrients
are exhausted. On the other hand, autochthonous microbes represent the more stable,
indigenous populations. They are less subject to marked changes in numbers or in
activities. Winogradsky's concept may also apply to the populations of fungi in soil
and possibly to some of the soil fauna. Often, fungi are obligate parasites of plants
or animals and therefore can be either temporary soil invaders or normal inhabitants;
in a sense, they are somewhat analagous to the zymogenous and autochthonous soil
bacteria, respectively.
As with other complicated biological systems, there are often mutual interactions
in soil between different microorganisms as well as between microorganisms and
plants or animals. In addition, there are the phenomena of parasitism and predation.
The various microbiological populations in soil are rarely to be found in stable equi-
libria: changes are constantly in progress. It seems certain that there is much compet-
ition for available nutrient supplies and as these become used up, some organisms die
off. The precise counting of bacteria is soil is well-nigh impossible although counts
of different groups of microorganisms can be of comparative value if determinations
are made repeatedly on samples of the same plot of soil on successive occasions.
Such counts frequently fluctuate from day to day or even hour to hour. The causes of
these fluctuations are not easily ascertained. Factors like temperature, moisture con-
tent, aeration or nutritional conditions seldom remain constant, they vary from day
time to night. As nutrients are used up, some sections of the population may decline
and others will become dominant. Even where there may be continued additions of
fresh litter, nutrient supplies in soil are usually at a low level and only a slow turnover
of microbial populations is possible.
