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
surface are brought deeper into the soil.
normal functions of getting energy for growth from
We classify soil organisms in several different ways.
organic molecules, they “respire”—using oxygen and
Each can be discussed separately or all organisms
releasing carbon dioxide to the atmosphere. (Of course,
that do the same types of things can be discussed as a
as we take our essential breaths of air, we do the same.)
group. We also can look at soil organisms according
An entire field can be viewed as breathing as if it is one
to their role in the decomposition of organic materi-
large organism. The soil is like a living being in another
als. For example, organisms that use fresh residues as
way, too—it may get “sick” in the sense that it becomes
their source of food are called primary (1°), or first-
incapable of supporting healthy plants.
level, consumers of organic materials (see figure 4.1).
The organisms living in the soil, both large and
Many of these primary consumers break down large
small, play a significant role in maintaining a healthy
pieces of residues into smaller fragments. Secondary
soil system and healthy plants. One of the main reasons
(2°) consumers are organisms that feed on the primary
we are interested in these organisms is because of their
consumers themselves or their waste products. Tertiary
role in breaking down organic residues and incorporat-
(3°) consumers then feed on the secondary consumers.
ing them into the soil. Soil organisms influence every
Another way to treat organisms is by general size, such
aspect of decomposition and nutrient availability. As
as very small, small, medium, large, and very large. This
organic materials are decomposed, nutrients become
is how we will discuss soil organisms in this chapter.
available to plants, humus is produced, soil aggregates
There is constant interaction among the organ-
are formed, channels are created for water infiltration
isms living in the soil. Some organisms help others, as
and better aeration, and those residues originally on the
when bacteria that live inside the earthworm’s digestive
Photo by Jerry DeWitt
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chAPter 4 the living soil
energy flows in direction of arrows
1° = first-level consumers
(25 mm = 1 in)
2° = second-level consumers
3° = third-level consumers
ground
beetle
centipede
8–20mm
50 mm
2o–3o
2o
ant
pseudo-
5–10 mm
scorpion
8–20 mm
flatworm
predatory
springtail
mite .5–1 mm
rove beetle
70–150 mm
10 mm
.5–3 mm
nematode
2o
2o
2o
protozoa
.01–.5 mm
feather-
winged
mold mite
beetle
beetle mite
1–2 mm
fly 1–2 mm
1 mm
rotifera
.1–.5 mm
sowbug
nematode
1o
10 mm
1 mm
bacteria
actinomycetes
1o
fungi
earthworm millipede
1o
20–80 mm
1o
beetle
50–150 mm
mite
1 mm
white
land slug
worm
& snail 2–25 mm
organic residues
10–25 mm
Figure 4.1. Soil organisms and their role in decomposing residues. Modified from D.L. Dindal (1972).
system help decompose organic matter. Although there
systems should be to create conditions that enhance
are many examples of such mutually beneficial, or
the growth of beneficial organisms, which are the vast
symbiotic, relationships, an intense competition occurs
majority, while decreasing populations of those few that
among most of the diverse organisms in healthy soils.
are potentially harmful.
Organisms may directly compete with each other for the
same food. Some organisms naturally feed on others—
SOIl MIcROORgANISMS
nematodes may feed on fungi, bacteria, or other nema-
Microorganisms are very small forms of life that can
todes, and some fungi trap and kill nematodes. There
sometimes live as single cells, although many also form
are also fungi and bacteria that parasitize nematodes
colonies of cells. A microscope is usually needed to see
and completely digest their content.
individual cells of these organisms. Many more microor-
Some soil organisms can harm plants, either by caus-
ganisms exist in topsoil, where food sources are plenti-
ing disease or by being parasites. In other words, there
ful, than in subsoil. They are especially abundant in the
are “good” as well as “bad” bacteria, fungi, nematodes,
area immediately next to plant roots (called the rhizo-
and insects. One of the goals of agricultural production
sphere), where sloughed-off cells and chemicals released
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chAPter 4 the living soil
by roots provide ready food sources. These organisms
can be deficient in nitrogen when we are surrounded
are primary decomposers of organic matter, but they do
by it—78% of the air we breathe is composed of nitro-
other things, such as provide nitrogen through fixation
gen gas. Yet plants as well as animals face a dilemma
to help growing plants, detoxify harmful chemicals (tox-
similar to that of the Ancient Mariner, who was adrift
ins), suppress disease organisms, and produce products
at sea without fresh water: “Water, water, everywhere
that might stimulate plant growth. Soil microorganisms
nor any drop to drink.” Unfortunately, neither animals
have had another direct importance for humans—they
nor plants can use nitrogen gas (N2) for their nutrition.
are the source of most of the antibiotic medicines we use
However, some types of bacteria are able to take nitro-
to fight diseases.
gen gas from the atmosphere and convert it into a form
that plants can use to make amino acids and proteins.
Bacteria
This conversion process is known as nitrogen fixation.
Bacteria live in almost any habitat. They are found
Some nitrogen-fixing bacteria form mutually benefi-
inside the digestive system of animals, in the ocean and
cial associations with plants. One such symbiotic rela-
fresh water, in compost piles (even at temperatures over
tionship that is very important to agriculture involves
130°F), and in soils. Although some kinds of bacteria
the nitrogen-fixing rhizobia group of bacteria that live
live in flooded soils without oxygen, most require well-
inside nodules formed on the roots of legumes. These
aerated soils. In general, bacteria tend to do better in
bacteria provide nitrogen in a form that leguminous
neutral pH soils than in acid soils.
plants can use, while the legume provides the bacteria
In addition to being among the first organisms to
with sugars for energy.
begin decomposing residues in the soil, bacteria benefit
People eat some legumes or their products, such
plants by increasing nutrient availability. For example,
as peas, dry beans, and tofu made from soybeans.
many bacteria dissolve phosphorus, making it more
Soybeans, alfalfa, and clover are used for animal feed.
available for plants to use.
Clovers and hairy vetch are grown as cover crops to
Bacteria are also very helpful in providing nitrogen
enrich the soil with organic matter, as well as nitrogen,
to plants, which they need in large amounts but is often
for the following crop. In an alfalfa field, the bacteria
deficient in agricultural soils. You may wonder how soils
may fix hundreds of pounds of nitrogen per acre each
reLAtIve AMounts of bActerIA And funGI
All soils contain both bacteria and fungi, but they may have different relative amounts depending on soil conditions. The general ways in which you manage your soil—the amount of disturbance, the degree of acidity permitted, and the types of residues added—will determine the relative abundance of these two major groups of soil organisms. Soils that are disturbed regularly by intensive tillage tend to have higher levels of bacteria than fungi. So do flooded rice soils, because fungi can’t live without oxygen, while many species of bacteria can. Soils that are not tilled tend to have more of their fresh organic matter at the surface and to have higher levels of fungi than bacteria. Because fungi are less sensitive to acidity, higher levels of fungi than bacteria may occur in very acid soils. Despite many claims, little is known about the agricultural significance of bacteria- versus fungal-dominated soil microbial communities, except that bacteria-prevalent soils are more characteristic of more intensively tilled soils that tend to also have high nutrient availability and enhanced nutrient levels as a result of more rapid organic matter decomposition.
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chAPter 4 the living soil
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, espe-
cially 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 those of fungi, but they
are sometimes grouped by themselves and given equal
billing with bacteria and fungi.
Fungi
Figure 4.2. Root heavily infected with mycorrhizal fungi (note round
spores at the end of some hyphae). Photo by Sara Wright.
Fungi are another type of soil microorganism. Yeast is a
fungus used in baking and in the production of alcohol.
and apple scab. Fungi also initiate the decomposition
Other fungi produce a number of antibiotics. We have
of fresh organic residues. They help get things going by
all probably let a loaf of bread sit around too long only to
softening organic debris and making it easier for other
find fungus growing on it. We have seen or eaten mush-
organisms to join in the decomposition process. Fungi
rooms, the fruiting structures of some fungi. Farmers
are also the main decomposers of lignin and are less
know that fungi cause many plant diseases, such as
sensitive to acid soil conditions than bacteria. None are
downy mildew, damping-off, various types of root rot,
able to function without oxygen. Low soil disturbance
resulting from reduced tillage systems tends to promote
organic residue accumulation at and near the surface.
MYcorrhIzAL funGI
This tends to promote fungal growth, as happens in
Mycorrhizal fungi help plants take up water and
many natural undisturbed ecosystems.
nutrients, improve nitrogen fixation by legumes, and
Many plants develop a beneficial relationship with
help to form and stabilize soil aggregates. Crop rotations
fungi that increases the contact of roots with the soil.
select for more types of and better performing fungi
Fungi infect the roots and send out rootlike structures
than does mono cropping. Some studies indicate that
called hyphae (see figure 4.2). The hyphae of these
using cover crops, especial y legumes, between main
mycorrhizal fungi take up water and nutrients that
crops helps maintain high levels of spores and promotes
can then feed the plant. The hyphae are very thin,
good mycorrhizal development in the next crop. Roots
about 1/60 the diameter of a plant root, and are able to
that have lots of mycorrhizae are better able to resist
exploit the water and nutrients in small spaces in the
fungal diseases, parasitic nematodes, drought, salinity,
soil that might be inaccessible to roots. This is espe-
and aluminum toxicity. Mycorrhizal associations have
cially important for phosphorus nutrition of plants in
been shown to stimulate the free-living nitrogen-fixing
low-phosphorus soils. The hyphae help the plant absorb
bacteria azotobacter, which in turn also produce plant
water and nutrients, and in return the fungi receive
growth–stimulating chemicals.
energy in the form of sugars, which the plant produces
in its leaves and sends down to the roots. This symbiotic
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chAPter 4 the living soil
interdependency between fungi and roots is called a
on the root, sometimes cause greater disease severity
mycorrhizal relationship. All things considered, it’s a
and more damage than the nematode itself. A number
pretty good deal for both the plant and the fungus. The
of plant-parasitic nematodes vector important and
hyphae of these fungi help develop and stabilize larger
damaging plant viruses of various crops. However, there
soil aggregates by secreting a sticky gel that glues min-
are many beneficial nematodes that help in the break-
eral and organic particles together.
down of organic residues and feed on fungi, bacteria,
and protozoa as secondary consumers. In fact, as with
Algae
the protozoa, nematodes feeding on fungi and bacteria
Algae, like crop plants, convert sunlight into complex
help convert nitrogen into forms for plants to use. As
molecules like sugars, which they can use for energy and
much as 50% or more of mineralized nitrogen comes
to help build other molecules they need. Algae are found
from nematode feeding. A number of nematodes alone
in abundance in the flooded soils of swamps and rice
or with special bacteria parasitize and kill insects such
paddies, and they can be found on the surface of poorly
as the larvae of the cabbage looper and the grubs of the
drained soils and in wet depressions. Algae may also
Japanese beetle. Finally, several nematodes infect ani-
occur in relatively dry soils, and they form mutually ben-
mals and humans, causing serious diseases such as river
eficial relationships with other organisms. Lichens found
blindness and heartworm.
on rocks are an association between a fungus and an alga.
Earthworms
Protozoa
Earthworms are every bit as important as Charles
Protozoa are single-celled animals that use a variety
Darwin believed they were more than a century ago.
of means to move about in the soil. Like bacteria and
They are keepers and restorers of soil fertility. Different
many fungi, they can be seen only with the help of a
types of earthworms, including the night crawler, field
microscope. They are mainly secondary consumers of
(garden) worm, and manure (red) worm, have different
organic materials, feeding on bacteria, fungi, other
feeding habits. Some feed on plant residues that remain
protozoa, and organic molecules dissolved in the soil
on the soil surface, while other types tend to feed on
water. Protozoa—through their grazing on nitrogen-
organic matter that is already mixed with the soil.
rich organisms and excreting wastes—are believed to
The surface-feeding night crawlers fragment and mix
be responsible for mineralizing (releasing nutrients
fresh residues with soil mineral particles, bacteria, and
from organic molecules) much of the nitrogen in
enzymes in their digestive system. The resulting mate-
agricultural soils.
rial is given off as worm casts. Worm casts are generally
higher in available plant nutrients, such as nitrogen,
SMAll AND MEDIUM-SIZE SOIl ANIMAlS
calcium, magnesium, and phosphorus, than the sur-
Nematodes
rounding soil and, therefore, contribute to the nutrient
Nematodes are simple multicellular soil animals that
needs of plants. They also bring food down into their
resemble tiny worms but are nonsegmented. They
burrows, thereby mixing organic matter deep into the
tend to live in the water films around soil aggregates.
soil. Earthworms feeding on debris that is already below
Some types of nematodes feed on plant roots and are
the surface continue to decompose organic materials
well-known plant pests. Fungi such as Pythium and
and mix them with the soil minerals.
Fusarium, which may enter nematode-feeding wounds
A number of types of earthworms, including the
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chAPter 4 the living soil
surface-feeding night crawler, make burrows that allow
Although the night crawler is certainly beneficial in farm
rainfall to easily infiltrate the soil. These worms usually
fields, this European introduction has caused problems
burrow to 3 feet or more, unless the soil is saturated
in some northern forests. As fishermen have discarded
or very hard. Even those types of worms that don’t
unused worms near forest lakes, night crawlers have
normally produce channels to the surface help loosen
become adapted to the forests. They have in some
the soil, creating channels and cracks below the surface
cases reduced the forest litter layer almost completely,
that help aeration and root growth. The number of
accelerating nutrient cycling and changing species com-
earthworms in the soil ranges from close to zero to over
position of the understory vegetation. So some forest
a million per acre. Just imagine, if you create the proper
managers view this organism, considered so positively
conditions for earthworms, you could have 800,000
by farmers, as a pest!
small channels per acre that conduct water into your soil
during downpours.
Insects and Other Small to Large Soil Animals
Earthworms do some unbelievable work. They move
Insects are another group of animals that inhabit soils.
a lot of soil from below up to the surface—from about 1
Common types of soil insects include termites, spring-
to 100 tons per acre each year. One acre of soil 6 inches
tails, ants, fly larvae, and beetles. Many insects are
deep weighs about 2 million pounds, or 1,000 tons. So 1
secondary and tertiary consumers. Springtails feed on
to 100 tons is the equivalent of about .006 of an inch to
fungi and animal remains, and in turn they themselves
about half an inch of soil. A healthy earthworm popula-
are food for predacious mites. Many beetles, in particu-
tion may function as nature’s plow and help replace the
lar, eat other types of soil animals. Some beetles feed
need for tillage by making channels and bringing up
on weed seeds in the soil. Termites, well-known feeders
subsoil and mixing it with organic residues.
of woody material, also consume decomposed organic
Earthworms do best in well-aerated soils that are
residues in the soil.
supplied with plentiful amounts of organic matter. A
Other medium-size to large soil animals include
study in Georgia showed that soils with higher amounts
millipedes, centipedes, mites, slugs, snails, and spiders.
of organic matter contained higher numbers of earth-
Millipedes are primary consumers of plant residues,
worms. Surface feeders, a type we would especially like
whereas centipedes tend to feed on other organisms.
to encourage, need residues left on the surface. They
Mites may feed on food sources like fungi, other mites,
are harmed by plowing or disking, which disturbs their
and insect eggs, although some feed directly on residues.
burrows and buries their food supplies. Worms are usu-
Spiders feed mainly on insects and keep insect pests
ally more plentiful under no-till practices than under
from developing into large populations.
conventional tillage systems. Although many pesticides
have little effect on worms, some insecticides are very
VERY lARgE SOIl ANIMAlS
harmful to earthworms.
Very large soil animals, such as moles, rabbits, wood-
Diseases or insects that overwinter on leaves of
chucks, snakes, prairie dogs, and badgers, burrow in the
crops can sometimes be partially controlled by high
soil and spend at least some of their lives below ground.
earthworm populations. The apple scab fungus—a major
Moles are secondary consumers, their diet consisting
pest of apples in humid regions—and some leaf miner
mainly of earthworms. Most of the other animals exist
insects can be partly controlled when worms eat the
on vegetation. In many cases, their presence is consid-
leaves and incorporate the residues deeper into the soil.
ered a nuisance for agricultural production or lawns and
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chAPter 4 the living soil
gardens. Nevertheless, their burrows may help conduct
water away from the surface during downpours and thus
a
hyphae of mycorrhizal
decrease erosion. In the southern U.S., the burrowing
fungi
action of crawfish, abundant in many somewhat poorly
root hair
drained soils, can have a large effect on soil structure.
(In Texas and Louisiana, some rice fields are “rotated”
root
interior
with crawfish production.)
mucigel layer
PlANT ROOTS
cell on root surface
soil aggregate
Healthy plant roots are essential for good crop yields.
water film
Roots are clearly influenced by the soil in which they