Arthropods       Bacteria       Earthworms       Fungi       Nematodes       Protozoa

An incredible diversity of organisms make up the soil food web. They range in size from the tiniest one-celled bacteria, algae, fungi, and protozoa, to the more complex nematodes and micro-arthropods, to the visible earthworms, insects, small vertebrates, and plants. Growing and reproducing are the primary activities of all living organisms. As individual plants and soil organisms work to survive, they depend on interactions with each other for sustenance, both directly and indirectly. By-products from growing roots and plant residue feed soil organisms. In turn, soil organisms support plant health as they decompose organic matter, cycle nutrients, enhance soil structure, and control the populations of soil organisms including crop pests in a delicate game of check and balance. Other organisms enter into a symbiotic relationship with plants resulting in a direct correlation between soil organism and plant process. As these organisms eat, grow, and move through the soil, they make it possible to have clean water, clean air, healthy plants and soil, and moderated water flow.

Arthropods

Many bugs, known as arthropods, make their home in the soil. They get their name from their jointed (arthros) legs (podos). Arthropods are invertebrates, that is, they have no backbone, and rely instead on an external covering called an exoskeleton. Arthropods range in size from microscopic to several inches in length. They include insects, such as springtails, beetles, and ants; crustaceans such as sowbugs; arachnids such as spiders and mites; myriapods, such as centipedes and millipedes; and scorpions. Nearly every soil is home to many different arthropod species. Certain row-crop soils contain

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Bacteria

Bacteria are tiny, one-celled organisms - generally 4/100,000 of an inch wide (1 µm) and somewhat longer in length. What bacteria lack in size, they make up for in numbers. A teaspoon of productive soil generally contains between 100 million and 1 billion bacteria. That is as much mass as two cows per acre. Bacteria put the tang in yogurt and the sour in sourdough bread. They are vital to the food web in

Bacteria fall into four functional groups. Most are "decomposers" that consume simple carbon compounds, such as root exudates and fresh plant litter. By this process, bacteria convert energy in soil organic matter into forms useful to the rest of the organisms in the soil food web. A number of decomposers can break down pesticides and pollutants in soil. Decomposers are especially important in immobilizing, or retaining, nutrients in their cells, thus preventing the loss of nutrients, such as nitrogen, from the rooting zone. A second group of bacteria are the "mutualists" that form partnerships with plants. The most well known of this group are the nitrogen-fixing bacteria called legumes. The third group of bacteria is

Legumes Legumes: Legumes are plants that can fix nitrogen from the air to make nitrates. Nitrate is nitrogen in a form available to plants. Legumes, through pinkish colored nodules on their roots, form a mutually beneficial relationship with soilborne bacteria. It the bacteria who are able to perform the chemistry necessary for nitrogen fixation; the plant pulls the nitrogen from the air through stomata in its leaves and transfers it to the bacteria via its phloem. In return, the legume and the plants nearby are supplied with the nitrates. However, if legumes are fed nitrogen (in the form of fertilizer or manure), they will cease to produce their own. Legumes are heavy feeders of phosphorus, potassium, magnesium, and calcium; so they (or the crops that follow) may need feeding if the soil is deficient in these nutrients. Legumes are used as green manures. Common examples are clover, vetch, soybeans, peas, and alfalfa. are plants that convert atmospheric nitrogen into a usable form. Each legume species requires a specific species of rhizobia to form nodules and fix nitrogen. This relationship occurs in specialized root tissue called nodules. Some legumes, such as alfalfa, can produce enough ammonia to supply all their nitrogen needs. The relationship between the legume and rhizobia is symbiotic Symbiotic relationship : A relationship between two entities in which both benefit from the collaboration. An example being mycchoriza and plants. The antithesis of this is a parasitic relationship in which the host is fed upon by the parasitic entity. . The bacteria invade plant root hairs and multiply in the outer root tissue, similar to the relationshipbetween plants and mycchorhizae fungi. The plant forms tissue that acts as a

The Bacillus strain of bacteria can also be utilized in the garden. Bacillus thuringiensis (commonly known as 'Bt') is an insecticidal bacterium mainly used to control caterpillars and fungus gnat larvae by way of introducing plague in their populations. Caterpillars must ingest it to be effective so proper coverage of crop or soil should be ensured.Bacillus subtilis is another effective bacterium that controls most plant diseases, such as bacterial spot, powdery mildew, rust, gray mold, leaf blight, scab, and many more.

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Earthworms

While not vital to all healthy soil systems, worms contribute to the overall health of the soil in multiple ways: Stimulate microbial activity: Although earthworms derive their nutrition from microorganisms, many more microorganisms are present in their feces or castings than in the organic matter that they consume. As organic matter passes through their intestines, it is fragmented and inoculated with microorganisms.

Mix and aggregate soil: As they consume organic matter and mineral particles, earthworms tunnel through the soil effectively mixing and redistributing materials in the soil zone. Charles Darwin calculated that earthworms could move large amounts of soil from the lower strata to the surface and also carry organic matter down into deeper soil layers. A large proportion of soil passes through the guts of earthworms, and they can turn over the top six inches (15 cm) of soil in ten to twenty years.

Increase infiltration: Earthworms enhance porosity as they move through the soil. Some species make permanent burrows deep into the soil. These burrows can persist long after the inhabitant has died, and can be a major conduit for soil drainage, particularly under heavy rainfall. At the same time, the burrows minimize surface water erosion. The horizontal burrowing of other species in the top several inches of soil increases overall porosity and drainage.

Improve water-holding capacity: By fragmenting organic matter, and increasing soil porosity and aggregation, earthworms can significantly increase the water-holding capacity of soils.

Provide channels for root growth: The channels made by deep-burrowing earthworms are lined with readily available nutrients and make it easier for roots to penetrate deep into the soil.

Bury and shred plant residue: Plant and crop residue are gradually buried by cast material deposited on the surface and as earthworms pull surface residue into their burrows.

Vermicomposters can be purchased in order to intensify the production of worm castings. For more info on worms and vermicomposting go here.

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Fungi

The mycorrhizae Mycorrhiza : "Fungus roots" which are symbiotic with the plant roots on which they occur. This mycorrhiza relationship facilitates the sharing of some of the plant's storehouse of organic compounds (which are essential to fungi, as they are to all living organisms). In addition, water is exchanged along with the organic compounds for assistance from the fungus in the absorption of nutrients like phosphorus and some other minerals. There are endo- and ectomyccorhizae, the endo- aiding in material uptake, the ecto- forming a rhizosphere protecting the roots from potential pathogens or diseases. (my-cor-ry-'zee) group of fungi lives either on (ecti-) or in (endo-) plant roots and act to extend the reach of root hairs into the soil. By inoculating your soil of media, myccorhizae and the plant enter into a symbiotic relationship thereby increasing the plant's uptake of water and nutrients, especially in less fertile soils, while the fungi benefit from plant association by taking nutrients and carbohydrates from the plant roots they live in or on. The superfine, root-like structures of these fungi are more extensive and more effective than plant root hairs at absorbing phosphorus, and other nutrients as well. Phosphorus moves slowly in soils but the fungi can absorb it much faster than the plant alone can. This enhanced root feeding makes it possible to reduce fertilizer rates for plants having a healthy colony of

This relationship developed over millions of years. During extreme periods, such as ice ages, throughout history when plants have experienced a bottle-neck in nutritional availability plants evolved to accept benefits from certain aspects of its soil environment in order to sustain themselves. Specifically, those that developed a relationship with mycorrhizae had a better chance of survival than those that did not. The result is that most plants today have a symbiotic beneficial relationship with its fungal friends. While almost all plants benefit from the presence of mycorrhizae, some do more than others. Some plants including citrus, grapes, avocados, and bananas, are dependent on mycorrhizae fungi. Others that benefit from having them are artichokes, melons, tomatoes, peppers, and squash.

In soils where mychorrhizae have been killed off, an inoculation may be beneficial. In healthy soils where they already exist there will be little or no benefit to adding more. There are dozens of mychorrizae species in nature. Additionally, the species found on plant roots may change as the plant matures. If those that are available are of the correct species, and are handled properly at all stages, they offer interesting potential benefits to farmers in well-managed systems. Generally it is preferred to inoculate with several species rather than a single one.

There are other forms of beneficial fungi, such as Trichoderma, which is a biocontrol that triggers plant defense responses.

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Nematodes

Nematodes are non-segmented worms typically 1/500 of an inch (50 µm) in diameter and 1/20 of an inch (1 mm) in length. Those few species responsible for plant diseases have received a lot of attention, but far less is known about Trophic level : A group of organisms that occupy the same position in a food chain. the majority of the nematode community that plays beneficial roles in soil. An incredible variety of nematodes function at several trophic levels of the soil food web. Some

Free-living nematodes can be divided into five broad groups based on their diet. "Bacterial-feeders" consume bacteria. "Fungal-feeders" feed by puncturing the cell wall of fungi and sucking out the internal contents. "Predatory nematodes" eat all types of nematodes and protozoa. They are also effective against fleas, flies, crickets, beetles, moths, weevils, and many types of worms. They eat smaller organisms whole, or attach themselves to the cuticle of larger nematodes, scraping away until the prey's internal body parts can be extracted. "Omnivores" eat a variety of organisms or may have a different diet at each life stage. "Root-feeders" are plant parasites, and thus are not free-living in the soil.

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Protazoa

Protozoa are single-celled animals that feed primarily on bacteria, but also eat other protozoa, soluble organic matter, and sometimes fungi. They are several times larger than bacteria - ranging from 1/5000 to 1/50 of an inch (5 to 500 µm) in diameter. As they eat bacteria, protozoa release excess nitrogen that can then be used by plants and other members of the food web. Protozoa are classified into three groups based on their shape: "Ciliates" are the largest and

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