Soil Food Web Inhabitants

Bacteria

There are trillions of these one celled organism, that are about 1/25,000 of an inch wide, in an acre, weighing about one ton. They are critical for making nutrients available for plant growth and in providing structure to the soil, creating a balance between air movement and water holding capacities. They also protect plants by chemical and biological means as well as by their production of antibiotics.

Types Of Bacteria

Decomposers, such as Actinomycetes that if present in healthy numbers gives soil its earthy smell, are typical of most bacteria and break down the simpler carbon compounds. By incorporating these digested carbons and nutrients they hold them in the root zone. Some bacteria can break down chemical toxins. 

Mutualists, such as nitrogen fixing bacteria, form a symbiotic relationship with certain plant roots. Nodules are created on root hairs "infected"by these bacteria converting gaseous nitrogen into a form usable by the plant. As the plant in turn decomposes nitrogen levels increase in the soil. 

Lithotrophs or chemoautotrophs obtain their energy from compounds of, nitrogen, surfer, iron or hydrogen instead of from carbon compounds. An important example being nitrobacter that convert nitrite into nitrate.

Soil Fungi

Are both single celled, such as yeast,and multi celled. They usually grow as long strands called hyphae that grow between soil particles adding to its structural properties.

Fungi cells are usually only several thousands of an inch in diameter, but a hyphae can be many feet long. Fungi are capable of functioning in soils that are to dry for bacterial activity.

Types of Fungi

Decomposers, saprophytic fungi, feed on the same simple carbons that bacteria specialize in but most break down the complex carbons such as cellulose and lignin. Like bacteria fungi are important for immobilizing or retaining nutrients in the soil. Dr Elaine Ingham, the leading expert in the soil food web states that many of the secondary metabolites excreted by fungi are organic acids that help accumulate the build up of humic acid rich organic matter.

Mutualists, the mycorrhizal fungi that form an association with plant roots greatly extending their capacity for certain nutrient absorption whilst protecting them from pathogens. Ectomycorrhizae grow on the surface layers of roots, whilst endomycorrhizae grow within the root cells.           

Protozoa

These single celled organisms can be as large as 1/50 of an inch in diameter but most are considerably smaller. They primarily feed on bacteria but also on fungi, other protozoa and organic matter. In the course of their feeding they release nutrients, primarily ammonium from the nitrogen present in bacteria, that feeds other members of the food web the remainder being available for plants to utilize. The three groups of protazoa that vary by shape are, ciliates ,amoebae and flagellates.

Nematodes

Nematodes are non segmented round worms that are about 1 mm long, a cup of healthy soil will contain thousands of specimens. They are classified into groups based on their feeding patterns. Bacterivores eat bacteria, fungivores eat fungi, predatores feed on other nematodes and on protozoa. Omnivores have a mixed diet and root feeding nematodes feed on the outer layer of root cells or enter the plant itself. Large populations may develop in degraded soil effecting the well being of plants.                                                                           

Like protozoa, nematodes release ammonium through their consumption of bacteria and fungi. As they move through the rhizosphere they help disperse bacteria and fungi.

In a healthy food web nematodes play an important role in balancing bacteria and fungi numbers, if the balance of the web is lost they can over graze these microbes effecting plant health. Some researches use a count of the different types of nematodes to give an indication of the health of the soil due to their diversity ans sensitivity to environmental changes.

Arthropods

Commonly called bugs these invertebrates include insects such as beetles, springtails and ants, crustaceans like sow bugs, arachnids including spiders and mites and myriapods such as centipedes and millipedes.

They are grouped according to the functions they perform.

Shredders - Chew up dead plant matter digesting the fungi and bacteria that are present on it, they include sow bugs and millipedes, by shredding organic matter into small particles they provide a medium for other bacteria and fungi to thrive.  

Herbivores - Feed on live organic matter, these root feeding insects such as symphylans can severely damage plants if their numbers are not controlled by other members of the soil food web.

Fungal Feeders -  Springtails, mites and silverfish feed on fungi by grazing the surface of plant roots.  

Predators - Include centipedes, ground beetles, ants, mites and spiders. They feed on other arthropods and some mites feed on nematodes. Predatory arthropods are important in keeping the grazers that feed on bacteria and fungi in check, thus protecting the base of the food web.

Arthropods carry microbes on their exoskeleton and in their digestive system distributing them across the soils surface, adding to the diversity and proper functioning of the soils ecosystem. Their excretions provide plant available nutrients. Passing through an arthropods digestive system organic matter ans minerals are aggregated improving soil structure.

Burrowing arthropods have the same effect as worms in aerating and cultivating the soil, whilst predatory arthropods control other arthropods that attack plants, To perform this critical function the soil food web in its entirety must be in good health to support the top of the food chain. Careless use of chemicals and compacting the ground will lead to a decline in the soil food web, cultivating the web will lead to sustainable plant health.

Earthworms

Earthworms are also invertebrates, having no back bone, there are more than 7000 species of earthworms around the world ranging in size from an inch to over 6 feet in length. They derive their nutrition from eating  decaying organic matter and from consuming bacteria, fungi, protozoa and nematodes. They can consume up to a third of their body weight in a day.

Benefits Of Earthworms

Although they consume soil microbes worm castings ,feces, contain far more microbes than the total eaten. The increase in overall microbial levels benefits the soil and therefore the plants. Consuming organic matter along with mineral particles helps aggregate the soil, improving its structure for water , air and nutrient retention. 

The tunnels made by deep burrowing worms provide easy passage for plant roots with the added benefit that they are lined with nutrients from the worms casts. Earthworms also pull organic matter into their tunnels incorporating into the working soil food web.

Earthworms have been divided into three groups dependent on their feeding and burrowing habits.

Surface soil and litter species live in or near surface plant litter, dead plant material, they are the type of worms found in compost piles and can not survive in a low organic matter environment

Upper soil species  live and feed in the top soil strata ingesting soil and organic matter 

Deep burrowing species live in burrows that may be ten feet deep, they come to the surface and pull plant litter into their burrows to eat.

Soil food web referenced from USDA Natural Resources Conservation Service.

Excellent link visually explaining  the Soil Ecosystem.