1.5 Soil biota are extremely varied and numerous

In about 5 cubic centimetres of agricultural soil you're likely to find:

  • At least 5 billion bacteria.
  • 5 million protozoa.
  • 5,000 nematodes (about 0.3-1.5 mm long) - the most common multicellular animals in soil.
  • About 6 mites and other microarthropods: this equates to up to 600,000 per square metre.

For larger organisms we have to look at quadrats of about a square metre:

  • Earthworms - maybe 300 per square metre (earthworm casts add more bacteria back to the soil than the worm eats. More bacteria mean healthier soil).
  • There may be around 20,000 kilometres of hyphae per square metre. Above ground, a meadow may look like separate plants. Underground, the plants are interconnected by their fungal associates (mycorrhizas) so they all belong to a single web of living things.
  • Small mammals; mice, voles, shrews, moles, which depend on the earthworms, arthropods and fungi for their nutrition, and in their turn feed predators; owls, foxes, etc., so the food web extends from microbes to large animals.

Resources Box

Life in the soil

Thomas E. Loynachan, Emeritus Professor of Agronomy and Microbiology at Iowa State University, created a set of 16 short digital videos showing the scope of life in the soil.

CLICK HERE to visit a page providing access to these.

Life in the soil on YouTube

 Deep Down & Dirty: the Science of Soil. A close-up of creatures living beneath the soil, made by the British Broadcasting Corporation: https://www.youtube.com/watch?v=gYXoXiQ3vC0

The Living Soil Beneath Our Feet. Made by the California Academy of Sciences: https://www.youtube.com/watch?v=MlREaT9hFCw

Development of communities of soil biota is characterised by progressive addition with many pioneer species remaining throughout soil development. Size and diversity of soil biota communities increases rapidly during the first 20 to 50 years and then more or less stabilises after hundreds of years, while plant biomass and soil organic matter content do not reach a peak for many hundreds or even thousands of years (Haynes, 2014).

Development of communities of soil fauna is less rapid than that of the microbial communities because dispersal is slower, and some faunal species require a certain depth of organic topsoil and/or litter layer before high populations can develop. With increasing time, the food web, which is based on organic detritus, becomes increasingly complex (Haynes, 2014). Organism abundance, diversity, and activity vary in a patchy fashion both horizontally across a landscape and vertically through the soil profile. Different groups of soil organisms exhibit different spatial patterns, with the spatial heterogeneity in microbial properties being an inherent feature of soils (Frey, 2015; O’Leary et al., 2020).

Updated January, 2021