Soil Biological Activity
Good soil biological activity is a vital component of soil fertility. Some of the benefits have already been mentioned on this site including the release of nutrient elements from organic matter reserves. This section describes the types of organisms in the soil and some of the functions they perform. There is an extra discussion on earthworms reflecting their particular importance in soil fertility.
Soil organisms are generally more numerous around the root zone of plants. They increase the availability of mineral elements and, if in sufficient numbers, biologically control the diseases and pest problems caused by undesirable organisms. A good balance of elements present in the soil is important for optimum soil biological activity. They decompose organic matter; good soil biological activity causes a faster rate of turnover of organic matter.
The importance of the rate of organic matter turnover should not be underestimated. With sub-optimum soil biological activity, including earthworms, nutrient elements are locked up in old organic matter, dead roots, dead shoots and manure for months or years longer than necessary. A poor soil is a sluggish soil. Good soil biological activity also promotes good soil structure through higher quality organic matter particles with more active sites, soil particle aggregation from fungal growth and glue like earthworm mucus, and burrows and channels. As soil structure improves, biological activity is enhanced, which aids soil structure even further. The better soil structure allows deeper penetration of roots and reduced nutrient loss from leaching. A reduction in thatching also helps with rooting depth and water infiltration.
Bacteria
Bacteria are microscopic one-celled organisms, often numbering in the millions per gram of moist soil. They play an important role in decomposition. They also include species of nitrogen fixers, e.g. the rhizobia of legume nodules, and species that perform transformations of nutrient elements into forms that are more plant available.
Actinomycetes
Actinomycetes are related to bacteria but form a colony with branching similar to fungi. They are extremely important in decomposition with some being able to break down particularly resistant portions of organic matter. Numbers vary greatly depending on the level and extent of decomposition of organic matter, and are higher under a high pH and elevated calcium levels.
Fungi
Fungi generally form a branching network in the soil (sometimes visible). Their numbers are generally fewer than bacteria and Actinomycetes but their contribution to soil biomass is greater. Fungi are some of the best decomposers in the soil and can act on relatively resistant materials and on dry material. They also improve soil structure through binding soil particles together. Some fungi form a 'mycorrhizal' association with plant roots, improving the host plants ability to take up elements and drought tolerance. Excessive liming should be avoided, as high pH levels do not favor fungi. Also high use of phosphate fertilisers can reduce levels of soil fungi activity.
Algae
These are either single celled or form colonies. They are coloured green and are photosynthetic. The role of algae in good soil fertility include improving soil surface structure (through binding soil particles), nitrogen fixation (by some algae species) and being a food source - encouraging levels of other soil organisms.
Soil Animals
There are microscopic animals such as protozoa (single celled). One of the main roles of these animals is in 'grazing' the bacteria, Actinomycetes and fungi, thus promoting the rate of turnover of nutrient elements held in these microorganisms. Mites are also significant in consuming microorganisms. There are also many soil animals that decompose soil organic matter. The decomposing animals vary greatly in size and contribution to soil fertility, in poor soils there may be a predominance of small springtails whereas in high fertility soils, the earthworm is the most significant animal.
