by Mrs. Eilisha
The soil is a
complex system of organic and inorganic matter. It consists of various
layers of this material, each varying in the amount of solid, gases,
liquids, and organic matter. A general analysis of soil shows that
about 40% are rocks and minerals, 25% gases, 25% liquid, and 10%
organic matter. This combination creates the soil environment, which
has several characteristics that make it worthwhile to study. It
provides a place for plants to secure themselves. Soil acts as a filter
that helps purify the water, and it is the place where many nutrients
are recycled. Finally the topic of this paper, soil is the habitat for
thousands of different organisms around the world.
are generally grouped into two categories: micro fauna/microorganisms
and macrofauna. The main soil microorganisms include bacteria,
fungi, and protozoa. The macro fauna include oligochaeta, arthropods,
mollusks, and nematods. There are many other organisms that spend some
time in the soil, but usually just for reproduction or feeding, and are
not included in this paper. The organisms mentioned above play an
essential role in soil formation and the soil environment. The larger
organisms tend to depend more on the microorganisms that are part of
the lower trophic levels. Therefore microorganisms will be
discussed first to provide background information.
most important microorganisms in the soil ecosystem are bacteria. They
single-celled organisms that are responsible for many important
processes. One thing they do is decompose plant material as well as
other organisms' waste. These waste materials contain nutrients, and
the bacteria's cells are able to convert them from an unusable form
into a form that can be used. One essential nutrient that is converted
by bacteria is nitrogen. Nitrogen-fixing bacteria are able to use the
nitrogen that exists in the atmosphere (N2) and change it through cellular
processes into ammonia (NH3). Nitrifying bacteria mix ammonia (NH3) with water (H2O) to create ammonium ion
(NH4+) and nitrate (NO3-), which can be used by other
organisms. Protozoa are also present in the
soil environment and contribute to distributing ammonium into the soil
for use by plants and other organisms. They feed on bacteria but do not
need as much nitrogen as the bacteria provide, and therefore release
ammonium. Plants need this to grow, so much so, that humans began
putting more nitrogen into fertilizers through an industrial process
created by Fritz Haber and Carl Bosch. Denitrifying bacteria are
responsible for putting the
nitrogen back into the atmosphere as N2.
Bacteria are also responsible for decomposing other
compounds as well. One phylum of bacteria called actinobacteria or actinomycetes are responsible for decomposing
chitin, cellulose, and other difficult materials in dead plants,
animals and fungi.
cannot disintegrate these materials because of their tough structure.
The bacteria are able to produce an enzyme allowing them to reduce the
chitin, for example, from a polymer into simple sugars and ammonia that other organisms can then use.
Actinomycetes are a type of
bacteria that used to be considered as fungi. They were thought to be
so because they grow hyphae like fungi, and they can live in harsh
environments in which other bacteria usually do not survive, like
higher pH levels and dryer conditions. They since were reclassified and
renamed actinobacteria. These bacteria give the soil its earthy
scent, which actually
indicates a healthy environment. Bacteria such as this help to maintain healthy
soil environments for plants, allowing them to get the nitrogen they
and decomposing organic waste.
Fungi serve in
a similar capacity in the soil environment. They tend to be more
abundant in places where the bacteria cannot thrive, such as
environments that are acidic, have low amounts of nitrogen, have low
moisture, and high complex carbohydrates (bacteria cannot break them
down). Fungi fill the niche of decomposing tough organic matter such as
chitin, cellulose, keratin and lignins.
Again, most organisms cannot digest these materials, but fungi, like actinobacteria, can produce enzymes that allow them
to degrade those strong compounds into sugars that can be
digested. Not only are they decomposers, but fungi also are
responsible for supplying nutrients to other organisms, in this case
form a mutualistic
relationship with plants
called mycorriza, which allows both parties to
benefit. The fungi grow
on or within (endopyhtic) the roots of the plant. Because of the hyphae's high surface
area to volume ratio,
fungi are able to better access essential nutrients and deliver them to
the plants. The plants, in turn, give the fungi food in the form of
importance of these microorganisms is clear, but in addition to them
converting nutrients into usable forms, they also are the food source
for many of the macro fauna in the soil. The macro fauna depend on the
nutrients that the microbes produce.
beneficial, and probably most recognizable macro fauna in soil is the
earthworm. Oligochaeta, a subclass of annelids, or segmented worms,
contribute to the texture of soil as we know it.
The earthworms burrow throughout the soil creating tunnels and shafts.
These openings loosen the soil structure providing spaces where other
organisms can travel and gain access to other areas. Water also fills
these spaces, adding to the water absorption of the soil, increasing
moisture, which many organisms need, including earthworms. When
water gets absorbed into the ground there is less runoff on the
surface. While the worms burrow, they intake soil and dead plant
material. They digest these nutrient-rich materials and produce waste
called castings. The castings contain all of the nutrients that were
present in the organic material: nitrogen, potassium, calcium,
phosphorus, and magnesium, and contain microorganisms, such as bacteria
and fungi. The protozoa and nematodes feed on the microorganisms
within the castings and release the nutrients within them. The worm
castings are considered to be fertilizers because of their positive
effect on the soil: they help to balance the pH levels, retain
moisture, improve drainage, and control pathogens. Plant nurseries
harvest and sell worm castings for this very reason.
fauna include nematodes, mollusks, and arthropods. These include a wide
variety of animals that help to maintain a balance in the food web.
Some of these animals consume live or dead plant material, and some are
predators of the microorganisms mentioned
above. The larger bugs gain and release some of
the nutrients from bacteria when they feed on them. In addition
to this, macro fauna help maintain natural population sizes of their
prey. One very interesting relationship is between macro fauna and
bacteria. As the bacteria
the more their population grows. This is a negative feedback. This is clearly supported in the
relationship between the arthropods and fungi in the following
situation. Leaf cutter ants harvest basidiomycete fungi. They grow the
fungi, feeding it leaves, providing it with a
desirable living condition and protection from other organisms. In
tern, the ants eat the fungi. The obvious question begs to be asked,
wouldn't the fungi die off if it is constantly being eaten? In
actuality, when it is eaten, it is more stimulated to grow, and it does
just that. As long as it is fed (the leaves provided from the ants) it
will continue to grow, and be satisfied living in this situation. This
is the same relationship with bacteria and their predators.
are not always beneficial. organisms that pose a threat to plants and
microorganisms that aid the plants are viewed as pests. Fungi and
bacteria have been
known for posing many problems to soil organisms. They can form
parasitic relationships with plants,
attacking young or damaged roots and inhibiting their growth. The
enzymes that enable these microorganisms to digest tough plant
are the same enzymes, such as cellulase, that allow the bacteria and
fungi to penetrate a plant's roots in a pathogenic relationship. Macro
fauna are not faultless. Nematodes can also be parasites, feeding on
plants and animals, including humans. They spread diseases and viruses
amongst soil organisms, posing many problems for farmers and gardeners.
Larger nematodes are often introduced as predators to smaller nematodes
to control their population. An arthropod called symphyla, or wire
worms, are pests in greenhouses and gardens. They also feed on plants
and microorganisms. Mites pose similar concerns.
described above, the organisms in the soil contribute greatly to the
wellness of the plants that grow from the soil. They are dependent on
each other and each has a niche to fill to ensure that the environment
stays well-balanced. Organisms do not always behave in a helpful
manner, but pesty organisms occur naturally in an ecosystem.
Each type of organism in the soil plays a role
in keeping the soil an ecosystem that is diverse and worthy of our
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