BACTERIA AND OTHER MICROORGANISMS OF THE SOIL - The Soil - page 130

The reaction of a solution or a medium, that is to say whether it is acid, neutral
or alkaline, is determined by the concentration of hydrogen ions (see chapter 1 ) and it
is an important factor influencing microbial growth and the nature of microbial pop-
ulations. As a rule, fungi seem to require more acid conditions (i.e. a lower pH value)
than bacteria or actinomycetes which generally prefer neutral or slightly alkaline re-
actions (i.e. a pH range of, say, 6 to 8), but there are always exceptions ( Fig. 44 ). Iron-
and sulphur-oxidizing bacteria proliferate under quite acid conditions, where the pH
value is around 1 or 2, as in acid mine waters or in the colliery wastes described in
chapter 10. Differences in the reaction of soils are often reflected in their microbial
populations.
A CTIVITIES OF SOIL MICROBES
Probably because of their minute size, simple unicellular structure and very varied
metabolic activities, microorganisms can affect their surroundings in many ways;
such effects in soils are of particular interest to us. An important example of these
activities is the contribution they make, in association with the small soil fauna, to the
breaking down of the mass of organic material which is constantly reaching the soil
as plant litter and animal debris. The decomposition of organic matter in this way is
an essential stage in the Carbon Cycle in nature; it is vital to the maintenance of life
on this planet. Plants and certain microorganisms produce organic substances from
carbon dioxide with exploitation of solar energy in photosynthesis, or by chemosyn-
thesis. Atmospheric carbon dioxide (the content of CO 2 in the atmosphere is about
0.03% by volume) is the carbon source for plants to synthesize their organic material.
As much as 100 tonnes of organic dry matter per hectare may accumulate annually
in tropical forests. Breakdown of this material and its eventual mineralization to CO 2
is a major process in the Carbon Cycle. Organic substances are the food for animals
or microorganisms and, after their death, the organic matter in their bodies is broken
down again, largely in the soil, with release of CO 2 .
En route , some carbon compounds may be re-cycled by assimilation into the pro-
toplasm of microorganisms and other organisms participating in the degradation of
organic matter, but subsequently these organisms also die and are degraded. Thus the
CO 2 of the atmosphere is derived partly from geological sources as in the combustion
of coal or oil, and from biological sources, including the respiration of all living or-
ganisms. A simplified outline of the Carbon Cycle is presented in Figure 45 .
Most water-soluble substances present in the soft, fleshy tissues of plants are
decomposed fairly quickly, because many microorganisms can readily metabolize
simple organic compounds, such as sugars. The more stable or polymerized plant
structural components, for example, hemicelluloses, cellulose and partially lignified