Soil Animals

  1. What are soil animals and what are the main types?
  2. Where are soil animals found in soil?
  3. What do soil animals need to survive in soil?
  4. What is the soil food web?
  5. How many soil animals are there in the soil?
  6. Why are soil animals important?
  7. How do soil animals affect decomposition?
  8. How do soil animals affect soil structure?
  9. How do agricultural practises affect the size of the soil animal population?

Question 1: What are the main types of soil animals?

Soil animals are an extemely diverse group of organisms. It is believed that there are twice as many species of organisms living in the soil than there are in tropical rainforest canopies. Soil animals are grouped roughly according to their size, into three groups. 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 most common soil animals are protozoa, nematodes, mites and collembola. The macrofauna contain the largest soil animals such as earthworms, beetles and termites, that are less common.

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Question 2: Where are soil animals found in soil?

Most soil animals (and microorganisms) occur in the top 5 cm of soil, although some occur at depth as well. Soil animals may move to lower soil layers when conditions at the surface are harsh. Most soil animals occur in the surface layer because this layer contains the most food (carbon and nutrients) in the form of organic matter and other organisms. A study in an agricultural field at East Beverly in Western Australia found that 75% of soil organisms in the soil were in the top 5 cm.

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Question 3: 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, like soil microorganisms, require sufficient carbon and nutrients, moisture, oxygen and an optimum pH and temperature. The optimum pH and temperature levels vary between species. In Western Australia conditions during the summer months are not suitable for many species of soil animals. Some organisms do not survive these conditions, however 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 during these times.

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Question 4: What is the soil food web?

The soil food web is a way of relating soil organisms to each other based on what they eat. The soil food web ‘starts’ with organic matter. This could be crop residues, pasture or any plant material in the soil. Bacteria and fungi consume organic matter breaking it down in the process. Bacteria and fungi are in turn consumed by nematodes, protozoa, earthworms, collembola and some mite species. Nematodes and protozoa are consumed by mites. Mites and collembola are eaten by beetles and ants.

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Question 5: How many soil animals are there in the soil?

The number of individual soil animals in the soil is enormous. In all but the driest environments there are billions of protozoa per square metre (m2), millions of nematodes/m2 and 100 000’s of mites/m2. Data from east Beverly in Western Australia, found that there were approximately 800 million protozoa/m2, 900 000 nematodes/m2 and 130 000 mites/m2 in a soil under pasture.

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Question 6: 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 with:

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Question 7: How do soil animals affect decomposition?

Soil animals affect decomposition and mineralisation in a number of ways. They contribute directly to mineralisation when they release mineralised nutrients in their excreta. However, most of their contribution is indirect. Indirectly they affect organic matter decomposition by:

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Question 8: 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 organic matter and mixing it through soil.

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Question 9: How do agricultural practises affect the size of their population?

Agriculture generally decreases the number of individuals in the soil and decreases the diversity of soil animals (number of species) compared to native vegetation. Soil animals require certain soil conditions to grow and survive. Agricultural practises alter soil conditions, making them them harsher than those of native vegetation:

The harsher environments mean that some of the original species are not able to survive. This decreases the species diversity and may cause different organisms to dominate.

Within agriculture more intensive tillage practises decrease the diversity of soil animals less than minimum tillage. Tillage can interupt the life cycle of soil animals and affects the larger soil animals more than smaller ones. Crop type also has an effect on soil animals with the number of soil animals decreasing from soil under continuous pasture to wheat/lupin rotation to continuous wheat.

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