Soil animals

This topic contains 11 questions:

  1. What are soil animals and what are the main types?
  2. Why are soil animals important?
  3. How do soil animals affect decomposition of organic matter?
  4. How do soil animals affect soil structure?
  5. What do soil animals need to survive in soil?
  6. What are mites?
  7. What are the main types of mites in soil?
  8. Where do they live in soil?
  9. What do mites do in soil?
  10. How many mites are there in Australian soils?
  11. How can I collect mites that live in my soil?

Question 1. What are soil animals and what are the main types?

Soil animals are an extremely diverse group of organisms. Soil animals are grouped into three groups roughly according to their size. The first group is the microfauna. These are the smallest of the soil animals ranging from 20 - 200µm. The main soil animals in this group are protozoa. The mesofauna is the next largest group and range in size from 200µm - 10mm. The most important animals in this group are mites, collembola (or spring tails) and nematodes. The macrofauna contain the largest soil animals such as earthworms, beetles and termites. Generally, the most common soil animals are protozoa, nematodes, mites and collembola.

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Question 2. Why are soil animals important?

Soil animals perform several functions in soil that make them a vital part of all ecosystems, including agriculture. Soil animals are involved in:

  • decomposing organic matter and mineralising nutrients,
  • controlling populations of pathogens,
  • improving and maintaining soil structure and
  • mixing organic matter through the soil.

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Question 3. How do soil animals affect decomposition of organic matter?

Soil animals contribute directly to nutrient cycling in soil when they release mineralised nutrients in their excreta. However, most of their contributions are indirect by:

  • grazing on the microbial biomass, which can increase the rate at which they decompose organic matter
  • fragmenting organic matter and increasing the surface area for microorganisms to decompose,
  • controlling the grazing pressure of nematodes on microorganisms,
  • mixing soil and organic matter and
  • introducing microorganisms onto fresh organic matter.

More on Organic Matter

More on Organic Matter Breakdown

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Question 4. How do soil animals affect soil structure?

Soil animals have an important role in the formation of soil structure. Soil animals improve soil structure by forming channels and pores, concentrating fine soil particles together into aggregates and by fragmenting and mixing organic matter through soil.

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Question 5. What do soil animals need to survive in soil?

Soil animals are found in almost every environment on earth, including Antarctica! Different species require different conditions to grow and survive but all soil animals, require sufficient carbon, nutrients, water, oxygen and an optimum pH and temperature. The optimum pH and temperature varies between species. Some organisms do not survive dry or very cold conditions, but they may leave eggs in the soil that hatch when conditions become more favourable. Other soil animals remain in the soil in an inactive state and become active again when conditions become favourable. Larger soil animals such as earthworms may live deeper in the soil when there are unfavourable conditions near the soil surface.

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Question 6. What are mites?

Mites are very small animals, usually less than half a millimeter long. They are related to spiders, so they have 8 legs and also pincers (chelicerae) that are bound to look scary to the other animals in soil. Some mites only eat bacteria or fungi, while others are carnivores and eat other mites or nematodes. 

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Question 7. What are the main types of mites in soil?

There are four suborders of mites in soil prostigmatids, oribatids, astigmatids and mesostigmatids. Prostigmatids are the dominant group in soils in the south west of Western Australia. They are usually soft bodied and different species have a great variety of feeding habits. Over 14,000 prostigmatid species have been identified throughout the world, but very few species have been described in Western Australia.

Oribatid mites are found throughout the world, including Antarctica. In forest soils, they can be the most dominant small animal. In New South Wales agricultural fields up to 23 oribatid species have been found. In studies at several sites in Wetern Australia only 6 species were identified. One of the species (belonging to the genus Zygoribatula) is dominant. This lower diversity reflects the contrast between the Western Australian environment and the richer soils in the eastern states of Australia. Many oribatid species have been found in native vegetation in very dry parts of Western Australia. The range of different life histories and feeding biologies of oribatid mites means that their presence in agricultural fields may enhance decomposition processes and therefore benefit your farm.

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Question 8. Where do they live in soil?

Mites are mainly found in the top 5cm of soil where the great majority of soil organisms live. This is why protecting top soil is so important: it harbours the life needed to perform vital ecosystem functions. However, mites are still found at great depth, and the deeper soil layers may act as a refuge for mites when conditions at the surface are inhospitable. 

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Question 9. What do mites do in soil?

Through their activity mites take part in the vital processes that occur in soil. For example, mites can introduce microbes to fresh residues so that decomposition proceeds. Their grazing on microbes can stimulate microbial activity, just as sheep grazing can stimulate pasture growth. The movement of mites in the soil may also mix the soil up so that microbes can have greater access to residue. Studies have shown that when mites are excluded from residue, its decomposition rate is slowed by an average of 23%. Research in the south west of Western Australia has demonstrated that mites are integrally linked with an important part of the nitrogen cycle - immobilisation.

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Question 10. How many mites are there in Australian soils?

A study at Newdegate in Western Australia in winter found 2,500 mites per square metre in soil following lupins and 9,500 mites per square metre in soil following canola. More than 35 species of mites were found in cropping soils at Newdegate, Avondale and Cadoux in Western Australia. 

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Question 11. How can I collect mites that live in my soil?

  1. The soil should be collected from the top few centimeters.
  2. The best time to collect mites from your soil is in the winter when the soil is moist as mites move to deeper soil layers when the soil is dry.  
  3. Place some gauze over the top of a funnel and place some freshly collected soil on the gauze.
  4. Under the funnel place a collecting jar with some water in it.
  5. Position a lamp so that the light (and heat) are shining on the top surface of the soil sitting in the funnel. The heat will help move the mites downwards towards the collecting jar.

After a day or two use a hand lens to look through the collecting jar for animals. There may be animals floating on the top of the water which are likely to be springtails. Springtails are an important group of animals closely related to insects.

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