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The Living Soil
The plow is one of the most ancient and most
valuable of man's
inventions; but long before he existed the land was in fact regularly
ploughed, and continues to be thus ploughed by earthworms.
Charles Darwin, 1881
When soil organisms
and roots go about their normal functions of getting energy for growth
from organic molecules they "respire" using oxygen and releasing
carbon dioxide to the atmosphere. (Of course, as we take our essential
breaths of air, we do the same.) An entire field can be viewed as breathing
as if it is one large organism. The soil is like an organism in another
way too a field also may get "sick" in the sense that it becomes
incapable of supporting healthy plants.
The organisms
living in the soil, both large and small, play a significant role in
maintaining a healthy soil system and healthy plants. One of the main
reasons we are interested in these organisms is because of their role
in breaking down organic residues and incorporating them into the soil.
Soil organisms influence every aspect of decomposition and nutrient
availability. As organic materials are decomposed, nutrients become
available to plants, humus is produced, soil aggregates are formed,
channels are created for water infiltration and better aeration, and
those residues originally on the surface are brought deeper into the
soil. We classify soil organisms in several different ways. Each organism
can be discussed separately or all organisms that do the same types
of things can be discussed as a group. We also can look at soil organisms
according to their role in the decomposition of organic materials. For
example, organisms that use fresh residues as their source of food are
called primary (1°), or first-level, consumers of organic materials
(see figure 3.1). Many of these primary consumers break down large pieces
of residues into smaller fragments. Secondary (2°) consumers are
organisms that feed on the primary consumers them selves or their waste
products. Tertiary (3°) consumers then feed on the secondary consumers.
Another way to treat organisms is by general size, such as very small,
small, medium, large, and very large. This is how we will discuss soil
organisms in this chapter.
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Figure 3.1 Soil organisms
and their role in decomposing residues. Modified from D.L.
Dindal, 1978. |
There is constant
interaction among the organisms living in the soil. Some organisms help
other organisms, as when bacteria that live inside the earthworm's digestive
system help decompose organic matter. Although there are many examples
of such mutually beneficial symbiotic relationships, an
intense competition occurs among most of the diverse organisms in healthy
soils. Organisms may directly compete with each other for the same food.Some
organisms naturally feed on others nematodes may feed on fungi, bacteria,
or other nematodes, and some fungi trap and kill nematodes.
Some soil organisms
can harm plants either by causing disease or by being parasites. In
other words, there are "good" as well as "bad" bacteria,
fungi, nematodes, and insects. One of the goals of agricultural production
systems should be to create conditions that enhance the growth of beneficial
organisms, which are the vast majority, while decreasing populations
of those few that are potentially harmful.
SOIL
MICROORGANISMS
Microorganisms
are very small forms of life that can sometimes live as single cells,
although many also form colonies of cells. A microscope is usually needed
to see individual cells of these organisms. Many more microorganisms
exist in topsoil, where food sources are plentiful, than in subsoil.
They are especially abundant immediately next to plant roots, where
sloughed off cells and chemicals released by roots provide ready food
sources. These organisms are important primary decomposers of organic
matter, but they do other things, such as providing nitrogen through
fixation to help growing plants. Soil microorganisms have had another
direct importance for humans they are the origin of most of the antibiotic
medicines we use to fight various diseases.
Bacteria
Bacteria live
in almost any habitat. They are found inside the digestive system of
animals, in the ocean and fresh water, in compost piles (even at temperatures
over 130°F), and in soils. They are very plentiful in soils; a
single teaspoon of topsoil may contain more than 50 million bacteria.
Although some kinds of bacteria live in flooded soils without oxygen,
most require well-aerated soils. In general, bacteria tend to do better
in neutral soils than in acid soils.
In addition
to being among the first organisms to begin decomposing residues in
the soil, bacteria benefit plants by increasing nutrient availability.
For example, many bacteria dissolve phosphorus, making it more available
for plants to use.
Bacteria are also very helpful in providing nitrogen to
plants. Although nitrogen is needed in large amounts by plants,
it is often deficient in agricultural soils. You may wonder how
soils can be deficient in nitrogen when we are surrounded by it
- 78 percent of the air we breathe is composed of nitrogen gas.
Yet plants as well as animals face the dilemma of the Ancient Mariner,
who was adrift at sea without fresh water: "Water, water, everywhere
nor any drop to drink." Unfortunately, neither animals nor
plants can use nitrogen gas (N2) for their nutrition. However, some
types of bacteria are able to take nitrogen gas from the atmosphere
and convert it into a form that plants can use to make amino acids
and proteins. This conversion process is known as nitrogen fixation.
Some nitrogen-fixing bacteria form mutually beneficial associations
with plants. One such symbiotic relationship that is very important
to agriculture is the nitrogen-fixing rhizobia group of bacteria
that live inside nodules formed on the roots of legumes. These bacteria
provide nitrogen in a form that leguminous plants can use, while
the legume provides the bacteria with sugars for energy.
People eat some
legumes or their products, such as peas, dry beans, and tofu made from
soybeans. Soybeans, alfalfa, and clover are used for animal feed. Clovers
and hairy vetch are grown as cover crops to enrich the soil with
organic matter, as well as nitrogen, for the following crop. In an alfalfa
field, the bacteria may fix hundreds of pounds of nitrogen per acre
each year. With peas, the amount of nitrogen fixed is much lower, around
30 to 50 pounds per acre.
The actinomycetes,
another group of bacteria, break large lignin molecules into smaller
sizes. Lignin is a large and complex molecule found in plant tissue,
especially stems, that is difficult for most organisms to break down.
Lignin also frequently protects other molecules like cellulose from
decomposition. Actinomycetes have some characteristics similar to fungi,
but are sometimes grouped by themselves and given equal billing with
bacteria and fungi.
Fungi
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| Figure 3.2 Root heavily infected with mycorrhizal
fungi (note round spores at the end of some hyphae). Photo by
Sara Wright. |
Fungi are another
important type of soil microorganism. Yeast is a fungus used in baking
and in the production of alcohol. A number of antibiotics are produced
by other fungi. We have all probably let a loaf of bread sit around
too long only to find fungus growing on it. We have seen or eaten mushrooms,
the fruiting structure of some fungi. Farmers know that many plant diseases,
such as downy mildew, damping-off, various types of root rot, and apple
scab, are caused by fungi. Fungi are also important for starting the
decomposition of fresh organic residues. They help get things going
by softening organic debris and making it easier for other organisms
to join in the decomposition process. Fungi are also the main decomposers
of lignin. Fungi are less sensitive to acid-soil conditions than are
bacteria. None are able to function without oxygen.
Many plants develop a beneficial relationship with fungi
that increases the contact of roots with the soil. Fungi infect
the roots and send out rootlike structures called hyphae
(see figure 3.2). The hyphae of these mycorrhizal
fungi take up water and nutrients that can then feed the
plant. This is especially important for phosphorus nutrition of
plants in low-phosphorus soils. The hyphae help the plant absorb
water and nutrients and in return the fungi receive energy in the
form of sugars, which the plant produces in its leaves and sends
down to the roots. This symbiotic interdependency between fungi
and roots is called a mycorrhizal relationship. All things considered,
it's a pretty good deal for both the plant and the fungus. The hyphae
of these fungi help develop and stabilize soil aggregates by secreting
a sticky gel that glues mineral and organic particles together.
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Mycorrhizal Fungi
Mycorrhizal fungi help plants take up nutrients,
improve nitrogen fixation by legumes, and help to form and
stabilize soil aggregates. Crop rotations select for more
types and better performing fungi than does mono-cropping.
Some studies indicate that using cover crops, especially legumes,
between main crops helps maintain high levels of spores and
promotes good mycorrhizal development in the next crop. Roots
that have lots of mycorrhizae are better able to resist fungal
diseases, parasitic nematodes, and drought.
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Algae
Algae, like crop plants, convert sunlight into complex molecules
like sugars, which they can use for energy and to help build the other
molecules they need. Algae are found in abundance in the flooded soils
of swamps and rice paddies. They also can be found on the surface of
poorly drained soils or in wet depressions. Algae also occur in relatively
dry soils, and they form mutually beneficial relationships with other
organisms. Lichens found on rocks are an association between a fungus
and an alga.
Protozoa
Protozoa are single-celled animals that use a variety of means to move
about in the soil. Like bacteria and many fungi, they can be seen only
with the help of a microscope. They are mainly secondary consumers of
organic materials, feeding on bacteria, fungi, other protozoa, and organic
molecules dissolved in the soil water. Protozoa through their grazing
on nitrogen-rich organisms and excreting wastes are believed responsible
for mineralizing much of the nitrogen (released from organic molecules)
in agricultural soils.
SMALL AND
MEDIUM-SIZED SOIL ANIMALS
Nematodes
Nematodes are simple soil animals that resemble tiny worms. They tend
to live in the water films around soil aggregates. Some types of nematodes
feed on plant roots and are well known plant pests. Diseases such as
pythium and fusarium, which enter feeding wounds on the root, sometimes
cause more damage than the feeding itself. However, most nematodes help
in the breakdown of organic residues and feed on fungi, bacteria, and
protozoa as secondary consumers. In fact, as with the protozoa, nematodes
feeding on fungi and bacteria also helps convert nitrogen into forms
for plants to use. As much as 50 percent or more of mineralized nitrogen
comes from nematode feeding.
Earthworms
Earthworms are every bit as important as Charles Darwin believed more
than a century ago. They are keepers and restorers of soil fertility.
Different types of earthworms, including the night-crawler, field (garden)
worm, and manure (red) worm, have different feeding habits. Some feed
on plant residues that remain on the soil surface, while other types
tend to feed on organic matter that is already mixed with the soil.
The surface-feeding
nightcrawlers fragment and mix fresh residues with soil mineral particles,
bacteria, and enzymes in their digestive system. The resulting material
is given off as worm casts. Worm casts are generally higher in available
plant nutrients, such as nitrogen, calcium, magnesium, and phosphorus,
than the surrounding soil and, therefore, make an important contribution
to the nutrient needs of plants. They also bring food down into their
burrows, thereby mixing organic matter deep into the soil. Earthworms
feeding on debris already below the surface continue to decompose organic
materials and mix them with the soil minerals.
A number of
types of earthworms, including the surface-feeding nightcrawler, make
burrows that allow rainfall to easily infiltrate into the soil. These
worms usually burrow to three feet or more under dry conditions. Even
those types of worms that don't normally produce channels to the surface
help loosen the soil, creating channels and cracks below the surface
that help aeration and root growth. The number of earthworms in the
soil ranges from close to zero to over a million per acre. Just imagine,
if you create the proper conditions for earthworms, you can have 800,000
small channels per acre that conduct water into your soil during downpours.
Earthworms do
some unbelievable work. They move a lot of soil from below up to the
surface from about 1 to 100 tons per acre each year. One acre of soil
6 inches deep weighs about 2 million pounds, or 1,000 tons. So 1 to
100 tons is the equivalent of about .006 of an inch to about half an
inch of soil. Agricultural soils that use conservation practices may
still erode, though at low annual rates of 1 to 4 tons/acre. A healthy
earthworm population can counteract some of the effects of erosion creating
topsoil by bringing up subsoil and mixing it with organic residues.
Earthworms do
best in well-aerated soils that are supplied with plentiful amounts
of organic matter. A study in Georgia showed that soils with higher
amounts of organic matter contained higher numbers of earthworms. Surface
feeders, a type we would especially like to encourage, need residues
left on the surface. They are harmed by plowing or disking, which disturbs
their burrows and buries their food supplies. Worms are usually more
plentiful under no-till practices than under conventional tillage systems.
Although many pesticides have little effect on worms, others, such as
aldicarb, parathion, and heptachlor, are very harmful to earthworms.
Diseases or
insects that overwinter on leaves of crops can sometimes be partially
controlled by high earthworm populations. The apple scab fungus a major
pest of apples in humid regions and some leaf miner insects can be partly
controlled when worms eat the leaves and incorporate the residues deeper
into the soil.
Insects and
Other Small to Medium-Sized Animals
Insects are another group of animals that inhabit soils. Common types
of soil insects include termites, springtails, ants, fly larvae, and
beetles. Many insects are secondary and tertiary consumers. Springtails
feed on fungi and animal remains. Many beetles, in particular, eat other
types of soil animals. Some beetles feed on weed seeds in the soil.
Termites, well-known feeders of woody material, also consume decomposed
organic residues in the soil.
Other medium-
to large-sized soil animals include millipedes, centipedes, mites, slugs,
snails, and spiders. Millipedes are primary consumers of plant residues,
whereas centipedes tend to feed on other organisms. Mites may feed on
food sources like fungi, other mites, and insect eggs, although some
feed directly on residues. Spiders feed mainly on insects and their
role in keeping insect pests from developing large populations can be
important.
VERY LARGE
ANIMALS
Very large animals,
such as moles, rabbits, woodchucks, snakes, prairie dogs, and badgers,
burrow in the soil and spend at least some of their lives below ground.
Moles are secondary consumers, with their diet consisting mainly of
earthworms. Most of the other animals exist on vegetation. In many cases,
their presence is considered a nuisance for agricultural production
or lawns and gardens. Nevertheless, their burrows may help conduct water
away from the surface during downpours and thus decrease erosion. In
the South, the burrowing action of crawfish, abundant in many of the
somewhat poorly drained soils, can have a large effect on soil structure.
(In Texas and Louisiana, some rice fields are "rotated" with
crawfish production.)
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| Figure 3.3 Close-up view of a plant root. A) The
mucigel layer containing some bacteria and clay particles on
the outside of the root. Also shown is a mycorrhizal fungus
sending out its rootlike hyphae into the soil. B) Soil aggregates
surrounded by thin films of water. Plant roots take water and
nutrients from these films. Also shown is a larger aggregate
made up of smaller aggregates pressed together and held in place
by the root and hyphae. |
PLANT ROOTS
Healthy plant
roots are essential for good crop yields. Roots are clearly influenced
by the soil in which they live. If the soil is compact, low in nutrients
or water, or has other problems, plants will not grow well. On the other
hand, plants also influence the soil in which they grow. The physical
pressure of roots growing through soil helps form aggregates by bringing
particles closer together. Small roots also help bind particles together.
In addition, many organic compounds are given off, or exuded, by plant
roots and provide nourishment for soil organisms living on or near the
roots. A sticky layer surrounding roots, called the mucigel, provides
very close contact between microorganisms, soil minerals, and the plant
(figure 3.3).
For plants with
extensive root systems, such as grasses, the amount of living tissue
below-ground may actually weigh more than the amount of leaves and stems
we see above ground.
BIOLOGICAL
DIVERSITY AND BALANCE
A diverse biological
community in soils is essential to maintaining a healthy environment
for plants. There may be over 100,000 different types of organisms living
in soils. Of those, only a small number of bacteria, fungi, insects,
and nematodes might harm plants in any given year. Diverse populations
of soil organisms maintain a system of checks and balances that can
keep disease organisms or parasites from becoming major plant problems.
Some fungi kill nematodes and others kill insects. Still others produce
antibiotics that kill bacteria. Protozoa feed on bacteria. Some bacteria
kill harmful insects. Many protozoa, springtails, and mites feed on
disease-causing fungi and bacteria. Beneficial organisms, such as the
fungus Trichoderma and the bacteria Pseudemonas fluorescens,
colonize plant roots and protect them from attack by harmful organisms.
Some of these organisms, isolated from soils, are now sold commercially
as biological control agents.
Sources
Alexander, M. 1977. Introduction to Soil Microbiology. 2d
ed. John Wiley & Sons. New York, NY.
Hendrix, P.F., M.H. Beare, W.X. Cheng, D.C. Coleman,
D.A. Crossley, Jr., and R.R. Bruce. 1990. Earthworm effects on soil
organic matter dynamics in aggrading and degrading agroecosystems
on the Georgia Piedmont. Agronomy Abstracts, p.250, American
Society of Agronomy, Madison, WI.
Paul, E.A., and F.E. Clark. 1996. Soil Microbiology
and Biochemistry. 2nd ed Academic Press. San Diego, CA.
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