The activity of soil organisms

This is an important concept for understanding how soil organisms live and how soil biology is managed. You can imagine it being a bit like an Apple tree in winter. The leaves fall, but the tree is not dead - just dormant. However, if you look more closely, you will find the buds are still growing - just very, very slowly. The tree as a whole is dormant, but the buds retain a very low level of activity.

The situation with soil organisms is quite similar. Those that immediately grow under ideal conditions (generally about 7-10% of total soil biomass) represent the 'active' component of the population. This means that samples can be analyzed year round, since the microbes that are active in winter will still be present in spring or summer, but their level of activity will be different. Given the ideal conditions in laboratory cultures, these active soil organisms will grow much more quickly than others.

Also, it is important to remember that the soil is a complex ecosystem, but that ecosystems are more than just biological communities, they are the product of environmental factors, available resources AND biological interactions. Disturbance in one or more of these areas will effect what happens in the community. So while looking only at active soil organisms will avoid the effects of seasonal changes, test results can still be influenced by disturbances like recent fertilizer additions or cultivation. For best results, make sure you don't collect a sample for testing within 3 or 4 weeks of any such disturbance.

Of course, the biological community in the soil is also extremely diverse and you might expect that testing for a wide range of different soil organisms would be required. However, this is not the case. To get a good picture of soil 'health' it is important to look at the smallest possible number of "Indicator" groups that will give you the information required.

These groups will have basic requirements and will not be dependent on any other groups. For instance, Protozoa are predators and so their number and activity will depend on the availability of prey species. On the other hand, photosynthetic bacteria require only moisture, CO₂, minerals and light. In fact they are even capable of breaking down pesticide residues.

Soil Biology Indicator Organisms

Photosynthetic bacteria like Rhodopseudomonas spp and Bradyrhizobium spp require only sunlight, carbon dioxide and mineral nutrients to survive. They are important in recycling organic matter, particularly compounds that are difficult to break down - such as pesticide and petrochemical residues. They are also important for synthesis of bio-active compounds that are known to stimulate plant growth.

Yeasts such as Saccaromyces spp and Rhodotrula spp synthesize plant growth substances from amino acids and sugars that are produced by photosynthetic bacteria. These substances also promote the growth of Lactic acid bacteria and Actinomycetes.

Lactic acid bacteria such as Lactobaccillus spp and Pediococcus spp produce Lactic Acid from sugars and carbohydrates. Lactic acid is a strong bio-suppressive compound that helps control harmful micro-organisms. This effect, together with other trace nutrients produced by members of this group, is particularly beneficial to the growth of Photosynthetic bacteria and Yeasts.

Actino-bacteria such as Actinomyces spp and Streptomyces spp produce antibiotic compounds that are effective suppressants of pathogenic organisms. They have also been shown to produce plant hormones - especially when treated with kelp extracts.

Fungi such as Aspergillus spp and Rhizopus spp have many beneficial effects on plant growth. These include the production of enzymes, antibiotics and various growth regulators. They are also important in the conversion of organic matter to humic substances. Some of the less complex compounds produced from this process are also important food sources for some bacteria.

Cellulose Utilizers like Trichoderma spp require only minerals and cellulose for growth. They break down plant remains into organic materials that are beneficial to other micro-organisms.

Further Reading

Using materials that can stimulate the activity of soil organisms.
Materials like kelp extracts and fish emulsions have varying effects in terms of the different soil organisms they stimulate. Understanding these effects is important in choosing the right materials to apply.

How to incorporate soil biology into sustainable garden soil management.
Only the Mikhail System gives a fully integrated strategy for properly managing all three functional components of the soil - Physics, Chemistry and Biology.