[Sidenote: Soil probably original home of micro-organisms.]
[Sidenote: Distribution of micro-organisms in soil.]
The soil is probably the original home of all micro-organisms, from
which they have emigrated into other media. It contains vast numbers,
and, according to some observers, 1 ccm. may hold 100,000 germs. By
far the greater number is found on or near the surface, and in lower
layers the numbers gradually diminish, until at last a depth is
reached, which depends on local conditions, where the soil is
perfectly sterile. The aerobes live near the surface and carry on
their work in this region, whereas the anaerobes are at work lower
down in the soil.
[Sidenote: Cycle of micro-organic activity during the year.]
The picture of the cycle of micro-organic activity in the upper layers
of the soil during the various seasons of the year is probably the
following. In winter, especially during that period when frost and ice
bind the earth, micro-organic life is apparently at its lowest ebb,
and may in some very cold climates come to a standstill altogether,
when micro-organisms may be said to hold their vegetative winter
sleep. With the return of life and the awakening of nature in
spring—especially with the approach of higher temperatures and the
formation of moisture—micro-organic activity once more makes itself
felt all round. During the summer months it is exposed to some
injurious influences such as the heating and drying up of the upper
layers of the soil, but, still gradually increasing, micro-organisms
reach the climax of their activity during the autumnal rains, to
remain in this state until with the advent of the cold season their
activity gradually declines again.
[Sidenote: Micro-organic life in layers from 3 ft. to 6 ft. in depth.]
In the lower layers of the soil, down to 3 feet and 6 feet,
micro-organisms are more protected against the injurious influences of
the atmosphere, sunlight and drying up, but the want of oxygen,
together with the greater difficulty of removing such products as
carbonic acid, has an injurious influence. As the temperature in these
layers is considerably more uniform, it may be inferred that the
micro-organic activity is there of a more uniform kind, less
influenced by sudden changes, probably also less intense, but without
pronounced periods of rest.
[Sidenote: Micro-organisms probably quickly perish in depths greater
than 6 ft.]
In depths greater than 6 feet micro-organisms probably perish very
quickly owing to unfavourable conditions, and if found their presence
must be explained by emigration from higher layers, not by actual
growth at these depths.
On sewage farms the micro-organic activity is without doubt greatly
modified, and proceeds all the year round at a more uniform rate than
on ordinary land, as the sewage always contains the necessary warmth
and moisture so beneficial for it.