In the soil, there
are many different types of creatures that live on and in it. Each of the
creatures has a role to play in or on the soil. These organisms work for the
farmer's benefit if they are simply managed for their survival. Consequently,
we should address them as soil livestock. While there are a great variety of
organisms that contribute to the fertility of the soil, the organisms are earthworms,
arthropods, and the various microorganisms merit particular attentions which
are also present in the soil.
Earthworms
Earthworm burrows
in the soil enhances water infiltration and soil aeration. Earthworm tunneling
can lead to the increasing of the rate of water entry into the ground 4 to 10
times higher than the fields that lack worm tunnels. This reduces water runoff,
recharges groundwater, and helps in the storing of more soil water for dry
spells. Vertical earthworm burrows pipe air deeper into the soil, thereby stimulating
microbial nutrient cycling at those deeper levels. Tillage which is done by
earthworms can replace some expensive tillage work which could be done by
machinery.
Worms eat dead
plant material that are left on top of the soil and redistribute the organic
matter and nutrients throughout the topsoil layer. Nutrient-rich organic
compounds line the tunnels that may remain in place for years if they are not
disturbed. During droughts these tunnels allow for deep plant root to penetration
into the subsoil regions of higher moisture content. In addition to organic
matter, worms also consume soil and soil microbes as they move through the
soil. When the soil clusters, they expel from their digestive tracts which is a
worm cast or casting. Each worm cast is separate from other casts and they
range in size from that of a mustard seed to sorghum seed which is depending on
the size of the worm. The soluble nutrient content of worm casts is
considerably higher than those of the original soil. A good population of
earthworms can process 20,000 pounds of topsoil in a year, with turnover rates
as high as 200 tons per acre.
Earthworms thrive
where there is no-tillage–generally, the less tillage in a field, the better,
and the shallower the tillage, the better. Worm numbers can be reduced by as
much as 90% by the means of deep and frequent tillage. Tillage reduces
earthworm populations by drying the soil, burying the plant residue in which they
feed on, and making the soil easier to freeze. Tillage operation also destroys
their vertical burrows and can kill and cut up the worms themselves. Emergence
times for young worms are spring and fall their most active periods just when
most farmers are interested in tillage. Worms are very dormant in the hot part
of the summer as well as the cold of winter.
Arthropods
In addition to
earthworms, there are many other species of soil organisms which can be seen by
the naked eye. Among these species are sowbugs, millipedes, centipedes, slugs,
snails and springtails. These are the primary decomposers in the soil. Their
role is to eat and shred the large particles of plant and animal residues. Some
bury residue, which then bring it into contact with other soil organisms that
further decompose it. Some members of this group prey on smaller soil
organisms. The springtails of these set are a small insect, which eat mostly
fungi. Their waste is rich in plant nutrients that are released after other
fungi and bacteria decompose it. Also of interest are the dung beetles, which
play a very valuable role in recycling manure and reducing livestock intestinal
parasites and flies from within.
Bacteria
The most numerous
among soil organisms are bacteria, every gram of soil contains at least a
million of these tiny one-celled organisms. In bacteria, there are many different
species, each of this species with its own role in the soil environment. One of
the major benefits bacteria provide for plants is in helping them take up
nutrients. Some of these species release nitrogen, sulfur, phosphorus, and
trace elements from organic matter. Others break down soil minerals and release
potassium, phosphorus, magnesium, calcium and iron. Still other species make,
also release natural plant growth hormones, which stimulate root growth to
plant.
Some species of
bacteria fix nitrogen in the roots of legumes while others fix nitrogen
independently of plant association. Bacteria are responsible for the converting
of nitrogen from ammonium to nitrate and back again, this is depending on
certain soil conditions. Other benefits to plants provided by various species
of bacteria include increasing the solubility of nutrients, improving soil
structure, fighting root diseases, and detoxifying of the soil.
Fungi
Fungi come in many
different species, sizes as well as shapes in the soil. Some species appear as
thread-like colonies, while others are one-celled yeasts. Slime molds and
mushrooms are also known as fungi. Many fungi aid plants by breaking down
organic matter or by releasing nutrients from soil minerals. Fungi are
generally early to colonize larger pieces of organic matter and begin the
decomposition process. Some fungi produce plant hormones, while others produce
antibiotics which include penicillin. There are even species of fungi that trap
harmful plant-parasitic nematodes.
The mycorrhizae
group of fungi lives either on or in plant roots and act to extend the reach of
root hairs into the soil. Mycorrhizae increase the uptake of water and
nutrients most especially in less fertile soils. Roots which are colonized by
mycorrihizae are less likely to be penetrated by root-feeding nematodes since
the pest cannot pierce the thick fungal network.
Actinomycetes
Actinomycetes,
these are thread-like bacteria that look like fungi. They not as numerous as the
bacteria, they also perform vital roles in the soil. Like the bacteria, they
help decompose organic matter into humus, releasing nutrients. They also
produce antibiotics to fight diseases of roots. These same antibiotics are used
for the treatment of human diseases. Actinomycetes are responsible for the
sweet, earthy smell of biologically active soil noticed whenever a field is
tilled.
Algae
Most different
species of algae also live in the upper half-inch of the soil. Unlike most of
the soil organisms, algae actually produce their own food through
photosynthesis. They appear as a greenish film on the soil surface after the
fall of a good rain. Algae improve soil structure by producing slimy substances
that glue soil together into water-stable aggregates. Some species of algae which
are the blue-greens can fix their own nitrogen, some of which is later released
to plant roots.
Protozoa
Protozoa are very
free living microorganisms that crawl or swim in the water between soil
particles. Many soil protozoa are predatory in nature, eating other microbes.
One of the most common is the amoeba that eats bacteria. By eating and digesting
bacteria, protozoa speed up the cycling of nitrogen from the bacteria, making
it more available to plants.
Nematodes
Nematodes are
abundant in most soils, only a few species of nematodes are harmful to plants.
The harmless species eat decaying plant litter, bacteria, fungi, algae,
protozoa and other nematodes. Like other soil predators, nematodes speed the
rate of nutrient cycling.
All these
organisms–from the tiny bacteria up to the large earthworms and
insects–interact with one another in a multitude of ways in a whole soil
ecosystem. Organisms not directly involved in decomposing plant wastes may feed
on each other or each other's waste products or the other substances they
release. Among the other substances released by the various microbes are
vitamins, amino acids, sugars, antibiotics, gums, and waxes.
Roots can also
release various substances into the soil that stimulate soil microbes. These
substances serve as food for select organisms. Some scientists and
practitioners theorize that plants use this means to stimulate the specific
population of microorganisms capable of releasing or otherwise producing the
kind of nutrition needed by the plants.
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