Soil Macrofauna
Soil Mesofauna
Soil Microfauna
Soil Macroflora
Soil Microflora
Soil Food Web

The study of the living component of soils is called Soil Biology. The classification (taxonomy) and study of soil organisms is largely based on physical similarities. Some organisms have proven difficult to classify. For example fungi (mushrooms) were often classified in the Plantae kingdom but sufficient differences exist that they now have their own kingdom. The five kingdom system of classification is in wide use: Animalia (animals), Fungi (mushrooms), Plantae (plants), Protista (simple algae, protozoa) and Monera (bacteria and blue-green algae). Many scientists, however, feel that this system of classification does not fully represent evolutionary patterns. The new field of cladistics is an attempt to correct this problem.

In this section we will take a simple approach to study soil organisms, dividing them first into two groups: Animals (fauna) and Plants (flora). Next we break down these groups further into more manageable divisions based on size of the organism: large, medium, and small or Macro- Meso- and Micro-. But exactly how big is Macro-? We have to define the limits for each size group. Macro organisms are considered to be larger than 2 mm in diameter greater than the thickness of a looney. Meso organisms are between 2 mm and 0.2 mm in diameter. Micro organisms are less than 0.2 mm and we normally need a magnifying glass or microscope to observe them. When you study soil organisms try to understand not only the individual organisms but also the complex interactions and processes that make them part of the soil ecosystem. Particularly important is the complex food chain or more correctly the food web in which energy passes from primary producers (plants) to consumers in a predatory cycle that forms a crucial part of life on our planet. Many soil organisms are beneficial but others are devastating to crops and animals. Beneficial organisms take part in many physical and chemical reactions that improve soil structure, break down organic matter and cycle nutrients. Others, however such as the tiny nematode, a worm,less than 0.1 mm long can cost farmers billions of dollars, every year, in lost production.

Soil Fauna Important soil macrofauna include: Vertebrates - gophers, mice ground squirrels Arthropods - insects Annelida - enchytraeid worms Mollusca - slugs and snails There are a number of small animals that burrow and live in Saskatchewan soils including mice, moles, ground squirrels and of course prairie dogs or gophers. In their search for food and nest building, these animals dig and burrow, bringing tons of subsoil to the surface. A valuable tilling service if it isn't your lawn or golf fairway. Important soil mesofauna include: Arthropods - mites, collembola Annelida - earthworms Many of the mesofauna are important for mixing the soil and breaking down the larger plant residues. Soil microfauna include: Nematoda - mites, collembola (spring tails) Rotifera - rotifers Protozoa - amoeba Unfortunately, a large number of these microfauna are parasitic and can cause devastating effects on crop yields Soil Flora The most important members of our soil macroflora group are the living plants or more importantly below-ground portion or the roots. Vegetation is an important factor in soil formation. We often forget that plants have underground root mass that can be equal or exceed the amount of material that we see above ground. The grassland soils of Saskatchewan have root masses greater 4000 kg per hectare in the top 120cm of soil. The roots of certain grasses can grow to depths of ten feet or more and trees can extend their roots to several hundred feet. For a few millimetres around the active roots is a special zone called the rhizosphere. As seen in the image, root exudates create a rich environment for the growth of microorganisms. The activity of the microorganisms in turn fixes nitrogen from the soil air, dissolves soil minerals and decomposes organic matter to enhance the ability of roots to pick up essential nutrients. There are very few organisms that we can put into the mesoflora group so the majority will fall into the microflora including the microorganisms fungi, bacteria, actinomycetes and algae. A single gram of soil (about 1/5 teaspoon) can contain over 100 million bacteria, 1 million actinomycetes and 100,000 fungi with hyphae if strung together would measure 5 metres in length. The weight of these organisms would only account for 0.05 percent of the weight of the soil. The size of a single bacteria is approximately 1 micron or 1 millionth of a metre - 100,000 bacteria placed end to end would measure 1 cm. The exact proportions of each of these organisms will depend on soil conditions such as available moisture, aeration, organic matter levels and the type of plants present. Chemical conditions such as acidity and alkalinity will greatly affect organism populations. Fungi like acidic soils, actinomycetes more alkaline conditions. Although very small, microorganisms have an importance in soil which far outweighs their size. They decompose plant and animal residues, synthesize humus, cycle nutrients such as carbon and nitrogen. The chemical by-products of microbial reactions bind together soil particles into stable aggregates that resist erosion. The association of special bacteria (Rhizobia) with the plant roots of legumes results in a symbiotic relationship in which the bacteria supplies nitrogen to the plant. A special kind of fungus called Mycorrhizae invades plant root cells and aid in transfer of soil nutrients and water into the plant. Although not directly associated with plant roots actinomycetes and algae are also important fixers of atmospheric nitrogen that can used later by soil organisms. Not all microorganisms are beneficial. Some organisms are harmful to plants and humans. Did you know that some of the most deadly bacteria come from soil such as the microbes responsible for tuberculosis, anthrax, leprosy and cholera? Food Web The survival of life on our planet depends on ultimately on the cooperation and interactivity of the various organisms. Especially important is the so called food chain or more accurately the food web. Energy is passed down the food chain from the primary producers such as plants to primary consumers such as ruminant animals to secondary and tertiary consumers which are predators. The body of one organism becomes food for another in a very complex interaction. When we take a soil sample for later study in the laboratory, we must always ask the question: what are we changing by doing this? Many organisms will die as the soil is removed and dries out while others such as insects worms and mammals may escape the sampling process. We must always be aware that the soil system includes the living organisms. Perhaps the only way to study soils without artifacts is in situ.