In this unit you are going to enter the
world of soil organisms and learn what they do for us by living in and on the
soil. It has been said that; "you have to be awfully small to want to live
in the soil, and the smaller you are, the happier you will be; and if you live
in the ocean, you can be as big as you like". In studying soil organisms,
we are going to find that many of the most significant are too small to be seen
without a microscope.
Healthy soil teems with an immense community of living organisms. In fact, a hectare supports about 20,000 kilograms of soil organisms, equivalent to the weight of 40 horses. Although they make up only about 5% of soil organic matter, these organisms are vital to many soil processes. Soil organisms may also just make you feel better? See happy_bacteria
Through their roles in the decomposition cycle, they regulate the flow of
energy through the soil, the cycling of nutrients, and the productivity of
agroecosystems. Soil organisms span a wide range in size, from microscopic
forms, such as bacteria, fungi, and protozoa, to large animals, such as
insects, worms, and burrowing mammals.
The larger organisms assist in decomposition by ingesting plant residues, breaking them into finer particles, and mixing them as waste throughout the moist soil environment. These wastes become food for the microorganisms, which digest the organic matter, releasing plant nutrients and gases, and producing glues that stick the soil mineral particles together to form aggregates. See the soil food web at soil food web
Macro soil organisms influence soils mainly by being mixers of soil materials. Ground squirrels, badgers, gophers (not the Golden variety Golden Spp) and crawfish are some animals that mix soil horizons with their digging.
Earthworms were described by Aristotle as 'the intestines of the earth'. Numerous soil scientists have been equally fascinated by the amount of work done by them, Charles Darwin said of them "It may be doubted whether there many other animals which have played so important a part in the history of the world as these lowly organized creatures".
Earthworms are miniature topsoil factories. The surface soil will eventually pass through an earthworm. Earth worms are amazingly strong, and can easily shift stones 60 times there own weight.
Worms may deposit 10 to 15 tons of castings per acre on the surface of the
soil during a year. Research on the beneficial effects of worms generally does
not show they will increase yields. However, we do know that the casts they
leave behind are high in bacteria, organic matter, and plant nutrients.
Castings have an NPK (nitrogen, phosphorus, potassium) ratio of 0.5-0.5-0.3 and
are 50% organic matter and 11 trace minerals. Worm castings work like
time-released fertilizer.
Worms prefer a moist non-acid environment in which to live. They also need
organic matter, which they use as a food source, and high amounts of available
calcium. Worms also leave numerous channels in the soil which may result in
pesticides and nutrients entering the subsoils at faster rates. These channels
allow preferential flow of water, rather than the water moving only
through the soil pores.
To look at how worms are researched at the Rosemont Experiment Station go to UM Worm Study
A study published in the July-August 1998 Minnesota Volunteer reported on
the changes that are taking place in northern Minnesota because of the
"exotic" earthworm. All earthworms in Minnesota are exotics because
the last period of glaciation wiped out all the natives at that time (if there
were any). The earthworm appears to be rapidly altering the character of the
sugar maple and basswood forests by consuming the leaf litter. The large deep
burrowing night crawler seems to be responsible for most of the changes. Note
the lack of the Oi and Oe layers in the soil with the abundant night
crawlers. 
The nonnative species have been introduced via bait containers and horticultural activities. Earthworms prefer the basic pH of the maple-basswood forest and are able to successfully exploit the food resources contained in the duff layers (O horizons). Normally a tree leaf may take three to five years to decompose and be incorporated into the soil humus. In forests infested with night crawlers, this process can take as little as four weeks. By accelerating the breakdown of plant material, earthworms change the way nutrients are recycled back to the plants. They may also be changing the ecology of the soil microorganism community be reducing the food for fungi and bacteria which rely on the duff layer as a main food source. Additional work since this original study has confirmed this process by the night crawlers.
Grasses and
sedges tend to succeed on forest floors after the invasion of earth worms
and some of the more common woodland plants tend to disappear like wild ginger
and yellow violet. Further study on this problem will help us to better
understand the problem caused by setting free all those night crawlers that did
not catch fish.
Worms should not be put into the soils of northern Minnesota to protect the ecology of the forest floor.
Roots absorb the water and nutrients that are needed by the plants for
photosynthesis and respiration.
Roots in the soil play an important role in the activity of organisms. Roots
are often a little leaky, and the material that they leak is referred to as
root exudates.
The area
immediately around the root is known as the rhizosphere. The rhizosphere
environment has a lower pH, and the soil atmosphere has lower O2 and higher CO2
concentrations. The rhizosphere is higher in soil organism activity due to
increased food supply leaked by the root for the organisms to use. This
includes: amino acids, organic acids, carbohydrates, nucleic acids, growth
factors, enzymes, and soughed-off tissue. Benefits for the plant of having a
rhizosphere include enhanced N mineralization, enhanced N2 fixation, and nutrient solubilization.
The rhizosphere is defined as an intense zone of
stimulated microbial activity around the root. Within the rhizosphere
microbial numbers are much greater than in the bulk soil.
See this diagram for rhizosphere populations Diagram
Microbes in the rhizosphere can be arbitrarily subdivided into the following groups: A. Pathogenic (invades and kills plants) B. Beneficial (often symbiotic with plants) C. Harmful (normally non-pathogenic opportunists on plants) D. Saprophytic (live on dead plants) E. Neutral (no effect on plants)
The microbes listed above are all competing for the some resources (space, nutrients and carbon) in the rhizosphere. The rhizosphere is a battlefield between pathogenic and non-pathogenic microorganisms. While we are mostly concerned with deleterious and pathogenic bacteria in the rhizosphere, there are some micro-organisms present in the rhizosphere which are good for roots. An example of this is the actinomycete Streptomyces which secretes antibiotics and toxins into the soil which then inhibits the growth of other rhizosphere microorganisms. e.g. It prevents the spread of the pathogenic "damping-off" fungus.
Another worm-like organism is the nematode. Nematodes are microscopic worms that feed on organic matter and other soil animals or infect plant roots. Under a 10x hand lens, nematodes appear as transparent, thread-like worms. Parasitic nematodes are the most important from an agricultural standpoint. Many plants are affected, such as tomatoes, carrots, potatoes, peas, alfalfa, turfgrass, and fruit trees. Nematodes can parasitize virtually all crops and ornamental plants and can cause significant economic damage by reducing both yield and quality. Properly taken samples from small field units can reduce production costs by allowing the grower to eliminate nematodes
Lance nematodes,
Hoplolaimus spp., are large nematodes which are highly resistant to effects of
temperature extremes and dry soil conditions. One species, H. columbus,
causes severe damage to soybeans and cotton. Another more widely distributed
species, H. galeatus, is primarily a pathogen on grasses. Lance nematodes feed
externally along root surfaces but may also feed with at least part of the body
embedded in the root. Larvae look similar to adults except that they are
smaller. This group of nematodes is easily detected with soil sampling. The
life cycle of this nematode takes about 30 days under ideal conditions, and
females lay eggs singly rather than in a mass. Though a female may lay as many
as 100 eggs in a lifetime, that lifetime may last an entire growing season.
Root-knot
nematodes, Meloidogyne spp., are one of the important plant-parasitic
nematodes because of their wide host range and widespread distribution.
Root-knot larvae enter roots of host plants near root tips and remain inside
the root at one location throughout their life. As larvae feed, the root cells
divide rapidly near the nematode's head. This rapid cell division and
enlargement cause the swelling or knots on roots. .
Fungi are not plants. Fungi were listed in the Plant Kingdom for many years.
Then scientists learned that fungi show a closer relation to animals, but are
unique and separate life forms. Now, Fungi are placed in their own Kingdom.
The part of the
fungus that we see is only the "fruit" of the organism. The living
body of the fungus is a mycelium made out of a web of tiny filaments called
hyphae. The mycelium is usually hidden in the soil, in wood, or another
food source. A mycelium may fill a single ant, or cover many acres. The
branching hyphae can add over a half mile (1 km) of total length to the
mycelium each day. These webs live unseen until they develop mushrooms,
puffballs, truffles, brackets, cups, "birds nests,"
"corals" or other fruiting bodies. If the mycelium produces
microscopic fruiting bodies, people may never notice the fungus.
The most active decomposers of organic materials in a forested soil are the soil fungi. This is mainly because they are tolerant of acid soil conditions. All of us have seen fungi. Their size varies from single-cell yeasts to molds and mushrooms. The woody residue of the forest floor provides an abundance of food for certain fungi that are effective decomposers of lignin.
Most fungi build their cell walls
out of chitin. This is the same material as the hard outer shells of insects
and other arthropods. Plants do not make chitin. Fungi feed by absorbing
nutrients from the organic material in which they live. Fungi do not have
stomachs. They must digest their food before it can pass through the cell wall
into the hyphae. Hyphae secrete acids and enzymes that break the surrounding
organic material down into simple molecules they can easily absorb.
Fungi have evolved to use a lot of different items for food. Some are decomposers living on dead organic material like leaves. Some fungi cause diseases by using living organisms for food. These fungi infect plants, animals and even other fungi.
Athlete's foot and ringworm are two fungal diseases in humans. The mycorrhizal fungi live as partners with plants. They provide mineral nutrients to the plant in exchange for carbohydrates or other chemicals fungi cannot manufacture. You probably use fungal products every day without being aware of it. People eat mushrooms of all shapes, sizes and colors. Yeasts are used in making bread, wine, beer and solvents. Drugs made from fungi cure diseases and stop the rejection of transplanted hearts and other organs.
Mycorrhizae are fungi associated with the fine roots of most plants. The term itself means "fungus root". There are hundreds species of fungi which function as mycorrhiza; most are basidiomycetes, the class of fungi which form mushrooms. An individual plant may have several different mycorrhizae associated with its roots, and some mycorrhizae may be limited to only a few species of plant. These fungi can benefit plants by enhancing the nutrient absorbing ability of roots.
Mycorrhizae are especially important in facilitating uptake of phosphorous.
This enhancement of nutrient uptake is a result of the extensive system of
hyphae and mycelia (thread-like filaments of the mycorrhizal fungus) that
pervade soils. They function like root hairs but are much more far
reaching.
The relationship of this fungus with plants is a mutually beneficial one, with the fungi receiving energy in the form of carbohydrates from the host plant. There are two types of mycorrhizae. Filaments of the first type, called ectomycorrhizae, penetrate between cells of roots, but not into root cells, and also form a thick cylindrical sheath around young lateral roots. The affected roots become short and thickened, and are deficient in root hairs. It is believed that this sheath may protect roots from invasion by plant pathogens. Most trees have this type of mycorrhizae.
A second type, called endomycorrhizae, actually penetrate into root cells and extend out from roots like root hairs to absorb nutrients from the soil solution. They do not form a sheath around roots nor do they alter the structure of roots. Endomycorrhizae are found on a greater range of plants than are ectomycorrhizae. For drawings of mycorrhiza see ectomycorrhizae or VA mycorrhizae tree roots with mycorrhizae
Studies have
repeatedly shown that plant growth is enhanced by the presence of mycorrhizae.
Mycorrhizae are particularly abundant in forest soils but are found in almost
all soils, with the possible exception of grasslands where no trees have
previously grown. Growth enhancement is especially significant for plants
growing on infertile soils and dry soils. Interestingly, mycorrhizae
development decreases following heavy fertilization of soil. The reduced growth
of the pine seedlings in the middle of this photo was because of the lack of
mycorrhizae on the roots. These seedlings were planted in an old limestone rock
roadbed. The soil has a pH greater than 8, which the fungus could not tolerate.
While further studies are needed, it seems that mycorrhizal inoculations may indeed benefit tree and shrub plantings, especially on sites where exposure to stresses are commonplace.
The most abundant organisms in the soil are bacteria. Bacteria are minuscule, one-celled organisms that can only be seen with a powerful light (100X) or electron microscope. They can be so numerous that a pinch of soil can contain millions of organisms. Soils often have between 1,000,000 to 10,000,000 bacteria per gram. Bacteria are tough, they occur everywhere on earth and have even been found over a mile down into the core of the earth.
Bacteria have an extremely varied metabolism. Bacteria can use reduced inorganics, the sun, or organics as an energy source. Some bacteria can live without free molecular oxygen. .
Bacteria are common throughout the soil, but tend to be most abundant in or adjacent to plant roots, an important food source. Actinomycetes are a broad group of bacteria that form thread-like filaments in the soil. They are responsible for the distinctive scent of freshly exposed, moist soil. Actinomycetes are particularly effective at breaking down tough substances like cellulose (which makes up the cell walls of plants) and chitin (which makes up the cell walls of fungi) even under harsh conditions, such as high soil pH.
Free-living bacteria fix atmospheric nitrogen, adding it to the soil nitrogen pool. (see Chapter 2. Other nitrogen-fixing bacteria form associations with the roots of leguminous plants such as lupine, clover, alfalfa, and milk vetch. Actinomycetes form associations with some non-leguminous plants (important species are bitterbrush, mountain mahogany, cliff rose, and ceanothus) and fix nitrogen, which is then available to both the host and other plants in the near vicinity.
Some bacteria exude a sticky substance that helps bind soil particles into small aggregates. So despite their small size, they help improve water infiltration, water- holding capacity, soil stability, and aeration.
Bacteria are becoming increasingly important in bioremediation, meaning that we (people) can use bacteria to help us clean up our messes. Bacteria are capable of filtering and degrading a large variety of human-made pollutants in the soil and groundwater so that they are no longer toxic. The list of materials they can detoxify includes herbicides, heavy metals, and petroleum products.
Bacteria can be divided into 2 large groups based on their carbon source.
Autotrophic bacteria are independent of any carbon in the soil since
they fix atmospheric CO2; and obtain energy from the reactions of
nitrogen and sulfur compounds in the soil. Heterotrophic bacteria
require carbon compounds as a food source. They are extremely important in
decomposing organic matter. In the carbon cycle, CO2; is absorbed by
plants during photosynthesis. As these plants die and are incorporated into the
soil, heterotrophic bacteria and other soil organisms decompose this organic
matter and release CO2; into the soil atmosphere. This movement of
CO2 into the atmosphere completes the carbon cycle. Go to
Carbon Cycle web page for more information on this
important cycle. This diagram also helps explain this cycle.
Carbon cycle diagram
To view some movies of life in the soil including bacteria go to Life in the soil
The living organisms in the soil represent the key to plant health and to human health. Soil microorganisms are the essential link between mineral reserves and plant growth. Ecological soil management aims at assisting all soil organisms not substituting for them with a chemical system.
For an interesting Web site at Michigan State on microorganisms go to the
Microbe Zoo and
another web page about soil biology is,
Soil
Biology in the Key to Healthy Soil and the Direct-Seeding Advantage
Chapter 2 Nitrogen Cycle
Laboratory 9 Chapters
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