Soil biologic properties and processes
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| Overview of soil biological properties and associated activities affecting soil biological properties and processes. Click on links to go to a specific section. | |||
| Property | Effects | Desired value1 | Management strategies |
Soil biological properties, unlike chemical and physical properties, are difficult to define. Various sources provide different lists of what comprises soil biological properties, ranging from considering only soil biota to including biological processes such as respiration, nutrient cycling, mineralization, and organic matter cycling.
We've chosen to list only soil biota and microorganisms as soil biological properties, but we provide a discussion of the processes affected by soil organisms and methods for measuring these processes. We include a discussion of effects of human activities, discussion of stormwater applications, and links to related topics, including information on sampling, testing, and soil health assessments.
Soil biota
There are different ways of classifying soil biota, such as classification based on function, size, or method of producing energy (i.e. autotroph or heterotroph). The following discussion classifies biota by size, using the scheme given by de Neergaard.
Microorganisms
Microbiota are less than 0.2mm and consist of bacteria, actinomycetes, fungi, algae and protozoa.
Bacteria
Bacteria are the most diverse and abundant biotic group in soil. Bacteria are usually in the range of 0.4-2 μm and vary in shape. Some bacteria have flagella, making them mobile, but most often they are attached to surfaces by ion exchange. Bacteria include archaebacteria and eubacteria. Archaebacteria are considered an ancient class of bacteria, often found in harsh environments. Eubacteria are a large group typically having simple cells with rigid cell walls and often flagella for movement. The eubacteria comprises the “true” bacteria and cyanobacteria. Eubacteria play important roles in nutrient and chemical cycling, including nitrogen fixation and organic matter decomposition. Actinomycetes are bacteria, but they have traditionally been considered as an intermediate group between bacteria and fungi. Actinomycetes in soil are particularly specialized towards decomposition of organic matter, including more complex substrates as chitin and hemicellulose, particularly under adverse conditions as high pH, temperatures and water stress.
Considering functionality, the United Stated Department of Agriculture (USDA) divides bacteria fall into four functional groups. Decomposers consume simple carbon compounds, such as root exudates and fresh plant litter. In doing this, bacteria convert energy in soil organic matter into forms useful to other organisms in the soil food web. A second group of bacteria, called mutualist, form partnerships with plants. These include nitrogen-fixing bacteria. A third group of bacteria is the pathogens. A fourth group, called lithotrophs or chemoautotrophs, obtain energy from compounds of nitrogen, sulfur, iron or hydrogen instead of from carbon compounds.
The following factors significantly affect bacteria populations in soil.
- Availability of carbon sources. Organic matter concentrations of 2-8 percent, by weight, are ideal. Soil organic matter can be increased through the addition of materials such as compost, biosolids, and wood chips.
- Moisture.
- Aeration.
- Temperature.
- pH.
- Availability of nutrients.
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