Soil Mesofauna, the springtails, mites and nematodes, relate with the other features of the Soil Food Web, as followsUSDA Soil Food Web
EarthwormsThey do a lot of soil moving, they are the big earth mixers and movers. There are:
surface active non-burrowing worms which consume decaying plant residues on the soil surface,
burrower in the mineral horizons of the soil but feed at the surface usually at night and
deep horizon worms inhabit the deep organo-mineral horizons constructing branching burrows
More on worm biodiversity.
While earthworms do the digging, the mesofauna do all the running around. The worms create the spaces for many other creatures to move through
Fungi and humus probably contribute most to the texture of soil. They mix with mineral elements (sand silt & clay, each different sizes) to produce a 'colloid' that can dry up, expand, and also aggregate, giving soil that crumbly feel when you rub it in your hands. The texture of the sand, gravel and silt, together with the bacteria and fungi explain the soil properties than simply chemical description. More on this with Mycorrhiza. The best soil is 'friable' - when you rub the soil in your hand it 'crumbles'. Also, it can be dry as a bone yet come back to life soon after adding water. Springtails accidentally bring fungal spores to plant roots, that established the mycorrhiza . For more about 'the bees of the soil'.
The recent discovery of glomalin, a 'superglue' produced by fungi, is opening up insights into how the soil feels and works.
The fungus Glomus moves down the growing roots and forms new hyphae. Older hyphae left higher up the roots slough off their protective glycoprotein, called 'glomalin', into the soil. There it attaches to particles of minerals, like sand, silt, and clay, and organic matter, forming clumps. (More on Glomalin). Glomalin gives soil its unique property of being like a lump of rock until water is added when it turns into a friable material
Fungi tend to use more complex compounds, such as fibrous plant residues, wood and soil humus. Fungi and roots can form a very close relationship or mycorrhiza; the relationship is symbiotic, which benefits both by providing food for each, and sometimes essential (eg citrus). There are two main types of fungi: endotrophic, where the fungus invades the hosts’ roots (e.g., orchids), and ectotrophic, in which the fungus forms a mantle around the smaller roots (e.g., pines).
nutrients from trapping and digesting animals like nematodes. First world video of nematode caught by fungi. They use sticky exopolysaccharide - a strong mycoglue produced by a nematophagous fungus to trap and immobolize soil nematodes.
There are more than 200 species belonging to the phyla Ascomycota, Mucoromycotina, and Basidiomycota. They live in the soil and trap nematodes using webs of hyphae, while others attack amoebae or collembola.
Numbers 100um apart
that 'fix' nitrogen from the air, like this Rhizobium
When the plants die or the leaves drop off, the micro and mesofauna work together to break the plant structures down.
First the leaves leach out chemicals, then there is mineralisation, following by the small soil creatures breaking down the mainplant structures. Springtails have a powerful jumping organ. (see in operation) that can also be used to prise the epidermis of the leaf from the underlying parts.
humus and 'active organic matter'. Humus is a chemical entity, consisting of such compounds as humic acid - now available as a commercial product to add carbon to your soil. The 'active organic matter' is not consistent but includes all sorts of bits of debris. This is where the soil animals are at work.
The small soil animals eat the fungi, dead springtails and other debris, remnants from the decomposing process. By transforming bits of plant and animal debris and digesting them into 'poo' chemicals, they provide nutrients for plant roots to absorb and for other creatures to live on. .
Oribatid mites are everywhere in the soil and have been there for 100s of millions of years, breaking down the plant debris into smaller particles, that the fungi and bacteria can get at. Oribatids burrow away, increasing the surface area for bacteria and fungi to digest the plant into chemical compounds.
Nematodes are another common small soil animal (or 'soil mesofauna'). There are an incredible variety of nematodes, which function at several food 'levels' of the soil food web. Some feed on the first level - plants and algae, others graze on the second level - on bacteria and fungi, and some feed on other nematodes - higher food (trophic) levels.
The absorption of the decomposing mass into the soil and mixing with minerals and clay, occurs in both the soil and earthworm guts. There is repeated absorption of the clayey humus and organic matter by earthworms and enchytraeid worms, The burrowing and digging animals loosen up the clayey material into 'crumbs', so that eventually the classic soil state - 'friable and crumbly' is reached.