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. The following table summarizes environmental conditions favorable for each group.

Environmental factor	Bacteria	Fungi	Algae	Protozoa	Actinomycetes
Carbon source1	Low C:N ratio materials that are easily degraded	Stable organic residues with high C:N ratio	Carbon dioxide (photosynthesis)	Bacteria	Stable organic residues with high C:N ratio
Moisture	Attach to materials with a film of water	Can grow to seek water	Moist but well oxygenated	Require moist soil	Can tolerate a range of moisture conditions
Aeration	Generally aerobic but some are anaerobic	Generally aerobic	Well-oxygenated	Aerobic	Aerobic
Temperature	Optimal at 25-30oC but can tolerate higher temperatures	Optimal at 25-30oC but can tolerate lower temperatures	Optimal at 25-35oC	Optimal at 18-22oC	Can tolerate a range of soil temperatures
pH1	5-9; optimum 7	2-7; optimum 5	6-9; optimum >7	5-8; Optimum >7	6.5-9.5; Optimum 8
1[1] 2[2]

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.

For more information, visit the following websites.

• Eubacteria
• Beneficial cyanobacteria and eubacteria synergistically enhance bioavailability of soil nutrients and yield of okra
• Eubacteria facts
• Soil bacteria - USDA

Fungi

Fungi are eukaryotes, organisms with a true cell nucleus. Most fungi are aerobic heterotrophs. Although bacteria are more numerous in soil, the total mass of fungi and bacteria in soil is similar. The fungi are divided into five main groups, all of which occur in soil as plant and insect pathogens. The majority of soil fungi consist of a filamentous mycelia made up of hyphen, and sexual reproduction takes place via fruiting bodies produced from the mycelium.

Fungi have several important specialized functions in the soil, as plant pathogens, decomposers and as mycorrhizal symbionts with plants. The soil hosts numerous fungi that are plant pathogens, primarily attacking the roots. Some are very host specific and can be suppressed by avoiding susceptible crops for a number of years. Certain decomposition processes of highly complex and recalcitrant structures such as lignin are almost exclusively carried out by a limited group of fungi.

Fungi are capable of moving to their substrate by hyphen growth. Consequently, colonization and decomposition of litters on soil surfaces is mainly carried out by fungi. Fungal decomposers are also capable of acquiring nutrients from one part of their hyphen and carbon substrates from other parts, thereby enabling them to grow on physically separated nutrient and carbon substrates. Some fungi form symbiotic relationships with plants by attaching themselves to the roots. The plants supply the fungi with carbohydrates, and the fungi hyphen act as an extension of the plant roots, enhancing uptake of soil nutrients.