Agriculture Reference
In-Depth Information
ditions result in increased solubility of iron and other metals along with
reduced forms of carbon and sulfur.
Reduced forms are generally more soluble and thus more easily extracted
than are oxidized forms. High levels of reduced forms, including some plant
nutrients, for instance, iron, may result in levels toxic to plants or other soil
organisms. Thus, not only the species but also the extractability and thus the
apparent level of soil constituents in the rooting zone will be affected by
whether the soil is under oxidizing or reducing conditions at the time of sam-
pling and analysis. Storing soil samples from an aerobic soil under reducing
conditions will drastically alter analytical results, as will storing a soil sample
from an anaerobic soil under aerobic conditions [6-10].
3.3.
MICROORGANISMS
The most diverse group of organisms growing in one location, in soil, are
microorganisms. Here we define microorganisms and any organism that is
visible only under a microscope. Using this definition, algae, fungi, actino-
mycetes, bacteria, and even some worms, arthropods, ciliates, and other organ-
isms, will be included in this group. This represents a truly diverse group of
organisms capable of carrying out an immense diversity of physical, biologi-
cal, and chemical changes in their environment.
1
Aerobic, anaerobic het-
erotrophic, autotrophic, thermophilic, mesophilic, and cryophilic are only some
of the different types of microorganisms found in soil. Table 3.1 gives the char-
acteristics of these different types of organisms. An indication of their power
and importance is seen when it is noted that some of these organisms can
create a whole new cell out of only carbon dioxide, light, and a mix of inorganic
compounds, including nitrogen and ions!
Figure 3.5 is a drawing of soil microorganisms; Figure 3.6 is a photomicro-
graph of common soil microorganisms, which are differentiated by their shape
and size. While the actinomycetes and fungi are both filamentous in growth
pattern, actimomycetes are smaller and less branched while fungi have larger,
more highly branched mycelia. Cocci and bacteria can be found in virtually all
the environments, hot or boiling water, high acidity (pH 1), cold (0°C), basic
(pH 12), and high salt concentrations. Fungi, which are aerobic, are somewhat
more restricted in the environments that they inhabit; however, some species
can live in high-osmotic-potential environments, such as sugar syrups, as well
as in or on hydrocarbon mixes, such as diesel fuels.
Although all microorganisms in soil are important, most attention is
focused on bacteria (often in discussions there is little or no distinction
between bacteria and cocci; both are usually lumped together and simply
referred to as “bacteria”). This is because they are extremely numerous and
1
The three references to Web sites given in the Bibliography provide additional information and
links to other relevant sites concerning microbial activity in soil.
