Soil Ecology Background Information
Read Plaster (1997), chapter 7
Important terms for this lab:
- nitrogen fixation
- Nitrogen cycle
"We live on the rooftops of a hidden world. Beneath
the soil surface lies a land of fascination, and also of mysteries,
for much of man's wonder about life itself has been connected with
the soil. It is populated by strange creatures who have found ways
to survive in a world without sunlight, an empire whose boundaries
are fixed by earthen walls."
-- Peter Farb. 1959. Living Earth. Harper and
Brothers Publishers, New York.
Soil is not a sterile homogeneous material. Soil in its natural
state is a living environment, teeming with biological activity
functioning in a complex ecosystem. The types of life found in the
soil are many, and the numbers of organisms are enormous. Plaster
(1992) estimates that one-fourth teaspoon of a fertile soil (about
one ml) contains:
A handful of such soil may also contain several earthworms and
many plant roots, insects, mites and spiders, and perhaps an indignant
gopher or prairie dog.
Organisms serve a variety of functions in the soil ecosystem. Some
of them are:
- Recycling of energy and carbon - The decomposition of organic
matter releases stored energy and produces carbon dioxide, which
can then be reused by plants through photosynthesis.
- Recycling of plant nutrients - The decomposition of organic
matter transforms plant nutrients into available forms which can
be taken up by plants.
- Disease transmission and control - Some organisms cause or transmit
diseases while others, such as Penicillium, produce substances
which control disease organisms.
- Nutrient/water absorption - Organisms such as fungi and actinomycetes
form associations with plant roots and aid the plant in exploring
the soil environment for water and nutrients.
- Nitrogen fixation - Some organisms take nitrogen from the atmosphere
and transform it into organic forms which are then utilized by
other organisms in the ecosystem. Such organisms include bacteria
in symbiosis with legume plants, blue-green algae, and some actinomycetes.
- Soil mixing - Organisms that burrow through the soil mix it
through their digging. Subsoil is brought up and mixed with topsoil,
recycling the clays and minerals which had eluviated down out
of the rooting zone.
- Decomposition of chemical materials - Pesticides, petroleum
products, and other organic chemicals are degraded by soil microbes;
first into other organic materials, ultimately into carbon dioxide
and water. Materials which resist microbial action may accumulate
and lead to environmental problems.
- Microorganisms and organic matter produced by soil organisms
are important in the formation of soil structure. Humus and exudates
of roots and microorganisms act as a glue, holding soil particles
together. Fungal hyphae bind particles together. The surfaces
of bacteria carry a net-negative charge, similar to clays, and
have a similar effect on soil structure.
Soil organisms can be classified according to the method in which
they obtain energy and carbon. Organisms are either autotrophs
that obtain energy and carbon from inorganic sources, or heterotrophs
that obtain energy and carbon from organic sources. Autotrophs
get energy and carbon from either photosynthesis (photoautotrophs)
or from chemical reactions (chemoautotrophs). Heterotrophs
get energy and carbon from living plants or animals (consumers)
or from dead organic matter (decomposers).
- Autotrophs (energy and C from inorganic sources)
- Photoautotrophs (energy and C from photosynthesis)
- Chemoautotrophs (energy and C from chemical reactions)
- Heterotrophs (energy and C from organic matter)
- Consumers (energy and C from living organisms)
- Decomposers (energy and C from dead organic material)
Soil organisms are often grouped into five classes:
Animals include earthworms, arthropods (insects,
spiders, centipedes, etc.), and rodents (moles, gophers, prairie
dogs, etc.). Earthworms have an enormous effect on the soil through
their mixing of soil material and by ingesting organic matter. The
organic matter remaining n their casts is partially digested, so
the nutrients in it are more available to plants. Termites digest
large amounts of organic matter, some of which is later available
to plants, and also affect soil physical properties. Other arthropods
are predators, feeding on other organisms. Nematodes are often predators
in the soil, feeding on other microorganisms. Many of them have
a detrimental effect on plant growth. Microscopic protozoa graze
on bacteria, moving within thin films of soil water. When water
or food are limiting they encyst, waiting until conditions improve.
Plants have major effects in the soil environment
by the cycling of water and nutrients, production of organic matter,
formation of pores by root growth, secretion of root exudates which
promote soil aggregation, and production of organic acids which
influence nutrient availability. Bacteria and actinomycetes that
fix atmospheric nitrogen use root exudates for energy; root nodules
provide a home for some of these microorganisms. Blue-green algae
also fix nitrogen; especially in wet environments such as rice paddies
Bacteria have perhaps the greatest influence in
the soil environment of all organisms. Species are either autotrophic
or heterotrophic. They perform many functions in the ecosystem,
most importantly in enzymatic transformations. They are able to
oxidize or reduce many chemical elements in the soil. For example,
nitrogen is tansformed from ammonium to nitrite and nitrate by two
different species. Fixation of nitrogen from atmospheric N2
into organic forms is also performed by bacteria, either in symbiotic
associations with plants or independently. The net-negative electrical
charge on bacteria and organic materials produced by them are important
in the formation of soil aggregates and soil structure.
Actinomycetes are a special type of bacteria.
They are single-celled and small, but they have filamentous structures
(mycelia) like fungi. They have important roles in the decomposition
of organic matter, release of plant nutrients, and in some cases
form symbiotic relationships with plants including nitrogen fixation.
Many of our important antibiotics (all the mycin compounds such
as streptomycin) were first discovered through the study of actinomycetes
in the soil. Streptomyces spp. are responsible for the
typical odor of a plowed field.
Fungi are very important in the soil ecosystem.
The decomposition of organic matter releases plant nutrients into
the ecosystem for other organisms to use. Some fungi form mycorrhizal
associations with plant roots - plants provide the fungi with food
(sugars and other root exudates) and the mycorrhizae provide the
plants with enhanced availability of plant nutrients (P,Zn,Ca,Mg,Mn,Fe,
and Cu). Some fungi are important in other ways: mushrooms are important
food products; many molds, like Penicillium, produce useful
materials such as antibiotics. Fungal hyphae also play a role in
the developement of soil structure.
Lab Study Questions
Study Questions Answers
- The five classes into which soil organisms are often grouped
||earthworms, arthropods, rodents, nematodes...
||grass, legumes, bluegreen algae...
||Rhizobium, Azobacter, Nitrosomonas,
||mushroom, molds, Penicillium
|soil drainage & aeration
- Humus and root and microorganism exudates act as glue - hold
soil particles together. Fungal hyphae bind particles together
and bacteria may add to net negative charge to aid in binding
- Crop rotation would encourage the most active and diverse soil
ecosystem. Factors encouraged by this include good structure,
organic matter content, optimum pH, high fertility, good porosity,
- Earthworms need organic matter, aeration, good moisture (not
excessive, not dry), and absence of toxic chemicals.
- A healthy soil ecosystem would be promoted with a management
plan that included crop rotations, animal manure applications,
low levels of applied chemicals, tillage methods that encouraged
good structure, soil and water conservation, etc.
Exam Questions from Labs
- Which of the following would promote earthworm populations?
a. mulching the soil surface
b. removing all crop residue from the field
c. treating the soil with insecticides
d. keeping the soil acidic
e. all of the above
- Which of the following agricultural practices would be most
detrimental to soil organisms?
a. draining a somewhat poorly drained soil
b. legally acceptable application of sewage sludge to a soil
c. driving over the soil with heavy equipment
d. applying a complete fertilizer to the soil
e. applying barnyard manure to the soil
- Which of the following would you expect to be most numerous
in an acre of soil?
A really neat site with some videos of soil organisms can be found
at: Soil Life
Movies Check it out!
For more information on soil ecology and soil organisms, visit
Microbe zoo , making sure to stop at Dirtland!
Interesting earthworm web sites are
Lowly Earthworm: The Gardener's Friend , Building
your Soil , and The
Earthworms as Fertilizer Factory .
For additional information on the fascinating nematodes, check
in General and Basic
Information about nematodes .
Plaster, E. J. 1997. Soil Science and Management (3rd Ed.). Delmar
Publishers, Inc., Albany.
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