Soil Management: Soil Biological Properties

Understanding the Basics: Soil Biological Properties

Soil structure is greatly affected by the animals and microbes in the soil. For example, the chemical and physical nature of the soil is changed as it passes through the intestines of worms. Soil animals and microbes can directly impact the availability of certain nutrients.

There are more organisms in a teaspoon of topsoil than there are people on Earth. Soil organisms are an intimate part of the organic fraction of soil, and contribute significantly to soil fertility and soil structure.

Plant residues have little value in the form we return them to the soil. The soil organisms, whether large (macro) or small (micro), feed on this residue and break it down in a continuous process.

Virtually all topsoil has passed through the gut of soil animals. Although we might think of burrowing animals such as groundhogs, moles, and shrews as having a large impact on soil because they are relatively visible, they are far less important to soil processes than the much more numerous, tiny animals and microbes.

There may be billions of protozoans (one-celled animals) and bacteria, tens of millions of nematodes, and hundreds of thousands of mites in a square metre of plow layer.

The living organisms of soils can be divided into two broad categories:

• microorganisms
• includes fungi, bacteria, actinomycetes and algae
• macroorganisms
  • includes protozoa, nematodes, earthworms, arthropods (insects, spiders, etc.), and rodents.

Earthworms and Ontario Soils

Earthworms are common in fine- and medium-textured soils (clays and looms), but rare in coarse-textured soils (sands). You can see this in a soil landscape where worms are rare on the sandy ridges, but abundant in depressional areas where fine soils, organic matter, and water accumulate.

One species, the dew worm or Canadian Nightcrawler (Lumbricus terrestris), is very abundant and widely recognized by most people. It's the species that's harvested from golfcourses, parks, and pastures. Hundreds of millions are annually exported to the USA for fishing bait.

Smaller earthworms play an important role in organic matter cycling. They live within the top 4-8 centimetres, while the much larger dew worm can burrow 1-2 metres deep.

Earthworm populations will dramatically increase in number:

• over two or three years when fields are converted to no-till or forages
• from regular manure applications
• especially if conservation or no-till techniques are used.

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Soil Organisms and Soil Structure

The effect of soil animals on soil structure is considerable. Topsoil is basically composed of animal feces of varying ages. Soil animals ingest organic matter and mineral components of soil, and mix them together before depositing the combined material as fecal pellets or casts.

Reduce tillage and add organic matter: this will increase soil organism populations and improve soil structure.

Highly specialized microbes, mostly bacteria, are involved in the transformation of nitrogen through the N cycle. Nitrogen is essential for plant growth and microbial activity. The rate of the decomposition is governed by the relative availability of a few key nutrients: carbon (C) and nitrogen (N) . (The processes of this nutrient cycling are discussed more fully in the Best Management Practices book, Nutrient Management.)

Available in Published Version of Soil Management

• Soil Organisms
• Common Types and their Importance - Chart
• The Carbon and Nitrogen Cycles and C:N Ratios
• Carbon to Nitrogen Ratios of Common Materials - Figure
• Impact of Human Activities on Soil Organisms

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