Soil organisms

The soil is home to a quarter of all living land organisms from a wide range of taxa. Nearly all soil organisms can’t make their own energy, as plants do from the sun, and so live on plant residues, soil organic matter or other soil organisms.

Find out more - Soil organic matter – biological fuel and carbon store

Most soil organisms are invisible to the naked eye, they are often observed by measuring what they do. However, earthworms are the most beneficial macrofauna in UK soils, they are ecosystem engineers making pores within the soil and also mix organic materials into the soil.

Find out more – Earthworms

Soil organisms can also be pests. Most soil organisms are beneficial but some have the potential to be plant pathogens and pests, especially insect larvae.  

Find out more – Pests and pathogens in the soil

Soil biota are often grouped by organism size (micro-organisms, microfauna, mesofauna and macrofauna). These organisms interact with one another – larger organisms often prey on the smaller organisms and there is a complex food web below ground.

Classification

Body width

Examples

Microflora
(micro-organisms)

<10µm

Bacteria

Fungi

Actinomycetes

Algae

 

 

 

Microfauna

<0.1 mm

Protista

Nematodes

Mesofauna

0.1mm to 2.0 mm

Enchytraeids

Mites

Springtails (Collembola)

Macrofauna

>2.0 mm

Earthworms

Insects and their larvae

Millipedes

Slugs

Snails 

Bacteria are single-celled prokaryotes. The large majority of bacteria existing in soil (> 95%) are not culturable and so for a long time could not be studied.

We are now beginning to get new insights from molecular methods which extract DNA from soil and analyse the patterns formed plasticity and capacity for change are a very important characteristic of all soil prokaryotic populations because of their capacity for rapid growth through binary fission.

The other key defining characteristic of the soil prokaryotic population is its ability to ‘slow down’ metabolic activity and maintain activity in a dormant state, even under conditions of very low energy and nutrient availability.

At any time a high proportion of the population is dormant and can remain this way for a long time until conditions for growth improve.  

Find out more - N-Fixers – getting legumes to work for you.

Fungi are eukaryotic and have a mycelial morphology with a mass of hyphal tubes enclosing multi-nucleated cytoplasm. Fungal hyphae are usually 2-10 μm in diameter, but can extend to m/km in length. Fungi are involved in a large number of interactions and processes in soil and are part of many complex relationships with other soil organisms.

Mutualistic relationships between arbuscular mycorrhizal fungi (AM fungi) and crops are widespread with only a very few crop species not forming such associations.

Find out more - mycorrhizal fungi - extending the reach of plant roots

Protists are unicellular water-dwelling organisms, which are predators of the microbial (mainly bacterial) populations in soil Changes in species balance and biomass within protistan populations have been related to soil conditions and the impact of soil management practices.

Nematodes are microscopic roundworms with a diameter of < 50 μm. Nematodes occupy central and diverse trophic positions within the soil food web, with at least three different functional groups identifiable:

i) plant feeding/root herbivore species are primary consumers;

ii) bacterial and fungal feeding nematodes, which are secondary consumers;

iii) predatory and omnivorous species (tertiary consumers).

The presence of different nematode species in the soil is being considered as an ecological soil quality indicator

Enchytraeids are related to earthworms (class Oligochaeta) and they look like small, white or transparent earthworms. Functionally, they breakdown organic matter and are also microbial feeders and are therefore an important component of the decomposition system in soils.

Mites are the smallest (usually less than 1 mm), and also the most diverse group of arthropods in soil and therefore show a very wide range of feeding habits and lifestyles. The presence of these microarthropods markedly increases decomposition rates across a range of environments.

Collembola, also known as springtails, also have a central role in soil food webs and affect decomposition processes. They are small (less than 6 mm in length) wingless insects in the subclass Apterygota. Different collembola species are specialised for different microhabitats in soil and litter and are quite susceptible to desiccation unless they remain in a moist environment.

A wide range of insects inhabit the soil for all or part of their life cycle. In many case insect species simply use the soil for the egg or pupal stages of the life cycle. Larvae of beetles, flies and ants are common; in addition woodlice, centipedes and millipedes are found in all life stages in soil. A number of these species are root herbivores and thus affect a range of above ground plant processes. Find out more about pests and pathogens in the soil