The easiest and most widely used system for classifying soil organisms is by using body size and dividing them into three main groups: macrobiota, mesobiota, and microbiota (Swift et al., 1979). The ranges that determine each size group are not exact for all members of each group.

1. Microorganisms

These are the smallest organisms (<0.1 mm in diameter) and are extremely abundant and diverse. They include algae, bacteria, cyanobacteria, fungi, yeasts, myxomycetes, and actinomycetes that are able to decompose almost any existing natural material. Microorganisms transform organic matter into plant nutrients that are assimilated by plants.

Two main groups are normally found in agricultural soils: bacteria and mycorrhizal fungi.

1a) Bacteria

Bacteria are very small, one-celled organisms that can only be seen with a powerful light (1000×) or electron microscope. They constitute the highest biomass of soil organisms.

They are adjacent and more abundant near roots, one of their food resources. There are many types of bacteria but the focus here is on those that are important for agriculture, e.g. Rhizobium and actinomycetes.

Bacteria are important in agricultural soils because they contribute to the carbon cycle by fixation (photosynthesis) and decomposition.

Some bacteria are important decomposers and others such as actinomycetes are particularly effective at breaking down tough substances such as cellulose (which makes up the cell walls of plants) and chitin (which makes up the cell walls of fungi).

Land management has an influence on the structure of bacterial communities as it affects nutrient levels and hence can shift the dominance of decomposers from bacterial to fungal.

One group of bacteria is particularly important in nitrogen cycling. Free-living bacteria fix atmospheric N, adding it to the soil nitrogen pool; this is called biological nitrogen fixation and it is a natural process highly beneficial in agriculture.

Actinomycetes are a broad group of bacteria that form thread-like filaments in the soil. The distinctive scent of freshly exposed, moist soil is attributed to these organisms, especially to the nutrients they release as a result of their metabolic processes.

Actinomycetes form associations with some non-leguminous plants and fix N, which is then available to both the host and other plants in the near vicinity.

1b) Fungi

These organisms are responsible for the important process of decomposition in terrestrial ecosystems as they degrade and assimilate cellulose, the component of plant cell walls.

Fungi are constituted by microscopic cells that usually grow as long threads or strands called hyphae of only a few micrometers in diameter but with the ability to span a length from a few cells to many meters.

Soil fungi can be grouped into three general functional groups based on how they source their energy:

Decomposers – they are also known as saprophytic fungi – convert dead organic material into fungal biomass, CO2, and small molecules such as organic acids. These fungi generally use complex substrates, such as cellulose and lignin, in wood.

They are essential for decomposing the carbon ring structures in some pollutants. Like bacteria, these fungi are important for immobilizing or retaining nutrients in the soil.

Mutualists – they are also known as mycorrhizal fungi – colonize plant roots through a symbiotic relationship.

Mycorrhizae increase the surface area associated with the plant root, which allows the plant to reach nutrients and water that otherwise might not be available.

Mycorrhizae essentially extend plant reach to water and nutrients, allowing plants to utilize more of the resources available in the soil.

Mycorrhizae source their carbohydrates (energy) from the plant root they are living in/on and they usually help the plants by transferring phosphorus (P) from the soil into the root.

Two major groups are identified:

Ectomycorrhizae: grow on the surface layers of the roots and are commonly associated with trees.
Endo mycorrhizae: such as arbuscular mycorrhizal fungi and vesicular mycorrhizal fungi, grow within the root cells and are commonly associated with grasses, row crops, vegetables, and shrubs.

Arbuscular mycorrhizal fungi can also benefit the physical characteristics of the soil because their hyphae form a mesh to help stabilize soil aggregates. Vesicular-arbuscular mycorrhizae are the most widespread mycorrhizal fungi.

Mycorrhizae are particularly important for phosphate uptake because P does not move towards plant roots easily.

These organisms do not harm the plant, and in return, the plant provides energy to the fungus in the form of sugars.

The fungus is actually a network of filaments that grows in and around the plant root cells, forming a mass that extends considerably beyond the root system of the plant.

Pathogens or Parasites: cause reduced production or death when they colonize roots and other organisms. Root-pathogenic fungi, such as Verticillium, Pythium, and Rhizoctonia, cause major economic losses in agriculture each year.

Many fungi help control diseases, e.g. nematode-trapping fungi that parasitize disease-causing nematodes, and fungi that feed on insects may be useful as biocontrol agents.

2. Micro-Fauna

The micro-fauna (<0.1mm in diameter) includes inter alia small collembola and mites, nematodes, and protozoa that generally live in the soil water films and feed on microflora, plant roots, other micro-fauna and sometimes, larger organisms (e.g. entomopathogenic nematodes feed on insects and other larger invertebrates). They are important to release nutrients immobilized by soil microorganisms.

2a) Nematodes

Nematodes are tiny filiform roundworms that are common in soils everywhere. They may be free-living in soil water films; beneficial for agriculture or phytoparasitic (Figure 2), and live at the surface or within the living roots (parasites).

Free-living nematodes graze on bacteria and fungi, thus they control the populations of harmful microorganisms. These nematodes are 0.15-5 mm long and 2-100 mm wide; an exception is Mermithidae nematodes, which may be 20 cm long and are very common in tropical soils, being parasites of some arthropods such as locusts.

Nematodes can only move through the soil where a film of moisture surrounds the soil particles. They live in the water (they are hydrobionts) that fills spaces between soil particles and covers roots. In hot and dry conditions, they enter into a dormant stage, and as soon as water becomes available, they spring back to activity.

Nematodes are recognized as a major consumer group in soils and are generally grouped into four to five trophic categories based on the nature of their food, the structure of the stoma (mouth) and esophagus, and the method of feeding (Yeates and Coleman, 1982): bacterial feeders, fungal feeders, predatory feeders, omnivores, and plant feeders.

The bacterial feeders prey on bacteria (bacterivores) and may ingest up to 5 000 cells/minute, or 6.5 times their own weight daily. This helps disperse both the organic matter and the decomposers in the soil.

Bacterial- and fungal-feeding nematodes release a large percentage of N when feeding on their prey groups and are thus responsible for much of the plant available N in the majority of soils (Ingham et al., 1985).

The annual overall consumption may be as much as 800 kg of bacteria per hectare and the amount of N turned over in the range of 20-130 kg (Coleman et al., 1984).

Phytophages or plant-feeding nematodes damage plant roots, with important economic consequences for farmers.

They possess styles with a wide diversity of size and structure, and they are the most extensively studied group of soil nematodes because of their ability to cause plant disease and reduce crop yield.

3. Meso-Fauna

Meso-fauna (0.1-2mm in diameter) includes mainly micro-arthropods, such as pseudoscorpions, springtails, mites, and worm-like enchytraeids. Meso-fauna have limited burrowing ability and generally live within soil pores, feeding on organic materials, micro-flora, micro-fauna, and other invertebrates.

3a) Collembola

Collembola or “springtails” are microarthropods that live in the litter or in the pore space of the upper 10-15 cm of soil. They are saprophagous and feed mainly on fungi, bacteria, and algae growing on decomposing plant litter (Ponge, 1991).

They are important as epigeic decomposers. Unlike most insects, they have no wings at any stage. They measure a few millimeters in length and elongate with a characteristic salutatory organ, a forked “tail” which enables them to spring when in danger.

Springtails are probably the most abundant group of insects on Earth.

3b) Pseudoscorpions

Pseudoscorpions are tiny arachnids rarely longer than 8 mm. They live in litter, decaying vegetation, and the soil.

Pseudoscorpions superficially resemble true scorpions, bearing relatively large chelae on the pedipalps, but they do not have a telson or stinger. Pseudoscorpions feed on very small arthropods such as springtails and mites.

4. Macro-Fauna

Members of species classed as macro-fauna are visible to the naked eye (generally> 2 mm in diameter).

Macro-fauna includes vertebrates (snakes, lizards, mice, rabbits, moles, etc.) that primarily dig within the soil for food or shelter, and invertebrates (snails, earthworms, and soil arthropods such as ants, termites, millipedes, centipedes, caterpillars, beetle larvae and adults, fly and wasp larvae, spiders, scorpions, crickets and cockroaches) that live in and feed in or upon the soil, the surface litter, and their components.

In both natural and agricultural systems, soil macrofauna is an important regulator of decomposition, nutrient cycling, soil organic matter dynamics, and pathways of water movement as a consequence of their feeding and burrowing activities.

4a) Earthworms

The effects of earthworms in the soil differ according to the ecological category of the species (Lavelle, 1981) involved:

Epigeic: they live in the litter layers, a very changing environment, subject to drought, high temperatures, and predator presence. These earthworms are generally small and pigmented (green or reddish) with rapid movements.

Endogeic: these are unpigmented (with no color) worms that live and feed in the soil. This group is further divided into three subgroups: oligohumic, mesohumic, and polyphonic, depending on the organic matter content of the soil ingested.

Anecic: these earthworms feed on the surface litter that they generally mix with soil, but they spend most of their time in the soil. They are large, with dark anterodorsal pigmentation, and they dig sub vertical burrows.

As a result of this wide range of adaptations, earthworms have diverse functions in the soil. Epigeic worms can be used as compost makers with no impact on soil structure. Anecics and endogenic do have an impact on soil structure owing to their mixing and burrowing activities and on soil organic matter.

4b) Termites

Termites are important members of soil macrofauna in various regions of the world. They are social insects, living in organized colonies with a number of castes (different individuals) with a set of morphological and physiological specializations.

The main castes are the queen (the termite that founds the colony), worker, and soldier. Neither individual termites nor colonies normally travel long distances as they are constrained to live within their territorial border or within their food materials.

A number of species feed on living plants and some may become serious pests in agricultural systems where dead residues are scarce (Wood, 1996).

Most species feed on dead-plant materials above and below the soil surface. Their food sources include plant-decaying materials, dead foliage, woody materials, roots, seeds, and the faeces of higher animals (Lavelle and Spain, 2001).

There are also soil-wood feeders and soil feeders, which means that they ingest a high proportion of mineral material. Their nutrition derives mainly from well-decayed wood and partly humified soil organic matter.

Another group of termites grows fungi in their nests (fungus-growing termites). Termites may be classified by their feeding habits: grass harvesters, surface litter feeders, wood feeders, soil-wood feeders, and soil feeders (humidors).

4c) Ants

Ants build a large variety of structures in the soil. However, because of their feeding habits, they are of less importance in regulating processes in the soil than termites or earthworms.

4d) Beetles

Beetles (Coleoptera) are diverse taxonomically and differ widely in size and in the ecological role they perform in soil and litter. They are either saprophagous, phytophagous, or predators.

Two groups are of particular relevance in agricultural soils: larvae from the family Scarabaeidae (dung-beetles), crucial to burying cow dung in natural savannahs and grasslands used for cattle grazing; and Melolonthinae beetles, whose larvae may be abundant in grasslands and affect crop production by feeding on living roots (Villalobos and Lavelle, 1990).

5. Biogenic structures

Biogenic structures are those structures created biologically by a living organism. Three main groups of biogenic structures are commonly found in agricultural systems: earthworm casts and burrows, termite mounds, and ant heaps.

The biogenic structures can be deposited on the soil surface and in the soil, and generally, they have different physical and chemical properties from the surrounding soil.

The color, size, shape, and general aspect of the structures produced by large soil organisms can be described for each species that produces it.

The form of the biogenic structure can be likened to simple geometric forms in order to facilitate the evaluation of the volume of soil moved through each type of structure on the soil surface.

Read Also: Components of Soil Organic Matter