Without life, soil would just be sediment. Life is what makes soil so interesting and important. So what’s in there?
It may seem obvious, but actually plants are the number one most important form of soil life. They are the major source of almost all the carbon – read food – in the soil. This is one of the major reasons that having a constant ground cover is so important. If there are plants in a soil, the soil is being fed. If the ground is bare, the rest of the soil life will slowly starve.
bacteria, archaea and fungi
These guys are the bottom of the food chain. They feed on bits of dead plants and on root exudates from living plants and produce the bulk of the non-plant biomass in the soil. They are often referred to collectively as micro-flora, despite the fact that none of them are at all related to plants (or even act like plants).
And then there is everything else. Protozoa, nematodes, mites, ants, worms and a thousand more besides. So how do we make sense of all the diversity? One approach is to separate them based on size
Soil organisims fall out into three and a bit natural groups: the micros, the mesos and the macros.
The micros are made up of archaea and bacteria (the really tiny micros) on the one hand and the larger algae, fungi, protozoa and most of the nematodes on the other. The tiny micros are the size of clay particles whereas the larger ones are the size of silt particles. Either way, they are far too small to see without a microscope.
At this microscopic scale different laws of physics dominate than at our macroscopic scale. Gravity, for example, is unimportant while Brownian motion – the random movement of water molecules – causes their microscopic environment to be in a state of constant flux.
All micros inhabit the thin film of water that covers all soil particles. Individual micros can’t move very far through the soil of their own volition (though colonies (or fungal mycellium) can expand over significant distances).
Two very important groups of micros are the protozoa and nematodes. These critters are primarily predators. In combination with viruses, these micro-predators provide top-down control over the number and species of bacteria, archaea and fungi in the soil.
Mesos are still very tiny, with diameters no bigger than a single grain of sand, but compared to the micros they are giants. Mesos are no longer stuck inside films of water, but rather live in the air pockets between soil particles or aggregates.
The most famous macro is of course the humble earthworm, but there are many others besides. Unlike the mesos, which live in the existing pore space in the soil, macros are big enough that they can make their own holes and burrows through the soil.
A final group of soil organisms are the so called mega-fauna. These are generally burrowing animals such as wombats, rabbits and bettongs. But actually most large animals have a direct impact on the soil. Hard hooves cut up and compact the ground while sharp beaks and claws dig away at the surface. Together these impacts can have significant effects or soil erosion, rain infiltration and important properties like bulk density (how fluffy the soil is).