Soil is a dynamic, living, natural entity. Living – The conclusion that soil is indeed vital is based on the fact that the number of living organisms in 10 g of soil could be as many as 9 billion – more than the people on this earth.

In the soil we find earthworms, insects, bacteria, fungi, algae, nematodes, etc. It is estimated that only 1 – 5 % of all micro-organisms present on earth are classified (Ward et al., 1992), most of which occur in the soil.

In many cases the role played by all these organisms in the soil is not fully understood. It may be stated unequivocally, however, that soil will not be able to support any other life without these organisms and their functions (Kennedy & Smith, 1995). The type, numbers, distribution and also diversity of organisms in the soil is therefore of the utmost importance.

Macro factors, such as climate and mother material, have a major impact on the occurrence and numbers of soil organisms. Other factors, such as practices applied by man, have a more direct bearing on local distribution of soil organisms. Factors in question are cultivation practices, levels of fertilisation, the use of chemicals such as fungicides, herbicides and insecticides, as well as crop rotation. Any of these practices will result in a temporary or permanent shift in the occurrence of micro-organisms in the soil. Although the number and diversity of soil organisms are often linked to the sustainability of crop production, diversity does not necessarily imply stability. It is interesting that while large and diverse microbial populations are encountered among organic cover crops, the biomass of soils treated with manure is large but not as diverse (Wander et al., 1995). In general, it holds true that minimal interference through management practices will increase the numbers, activity and diversity of soil organisms.

Why then are the soil organisms considered such an important indicator of healthy soil conditions and sustainable cultivation The reason is that soil organisms are primarily responsible for the circulation of nutrients, mineralisation and circulation of organic material and the formation and stabilisation of soil structure. Most farmers are familiar with the effect of earthworms and other larger soil organisms on soil structure and the topic has been thoroughly discussed by Lee & Foster (1991). In a nutshell, what it boils down to is that earthworm tunnels improve drainage and aeration, while stable excretions stabilise the soil on the whole. Organic material on the soil surface is also transported to deeper levels where it is made available again to other micro-organisms to be broken down further. Truly significant numbers of less well-known soil organisms are encountered in the meso- and microfauna. These organisms are the mineralisers and responsible for the circulation of nutrients by releasing them in the course of the mineralisation process. The mycorrhiza and rhizobium bacteria, living in direct association with the plant, occur in these groups, and this is also where various harmful organisms, such as plant parasitic nematodes, are encountered.

Wynboer - February 2002 - Soil biology - its importance to wine grape farmers Wynboer - February 2002 - Soil biology - its importance to wine grape farmers

The micro-organisms in the root area (risosphere) may contain both saprophytes and potential parasites. If a parasite establishes itself in the risosphere, it has a better chance of infecting the root of the plant. For the latter to take place, the pathogen has to compete effectively for nutrients with the saprophytes in the risosphere. The risosphere, being a microhabitat with higher microbiological activity than the rest of the soil as a result of associating and antagonistic reactions between the micro-organisms in the soil, is considered the “main line of defence” against plant pathogen infestation of the plant root. Plant performance is therefore largely influenced by the well-being of the microbiological ecosystem in the risosphere microhabitat. Under natural conditions, the plant root secretes a variety of substances that maintain this ecosystem. It is a known fact, however, that chemical sprays influence the well-being of the soil’s microflora, inter alia as a result of chemical substances being absorbed by the leaves and translocated to the roots, where the plant secretions no longer maintain the desired microbial ecosystem. For example, copper, sprayed on beans, reduced the amount of bacteria in the risosphere (Burgess, 1967). The plant’s ability to offer resistance to pathogenic infection is therefore seriously compromised.

Throughout the world, therefore, there is much concern about the sustainability of modern agriculture which is totally dependent on anorganic fertilisers, and also about the increasing use of chemical substances for pest and plague control. In the process the micropopulations in the soil are influenced negatively, with nd the producer ending up in a downward spiral where more and more anorganic additions are required to maintain production levels. The levels of organic material in the soil decrease and in addition to negative effects, such as an increase in run-off water and soil erosion, a decrease in fertility and increasing susceptibility to soil compaction, the release of CO2 during the breakdown of the organic material contributes to global warming.

There is a need for a long term vision and strategy in each industry where area specific management practices are developed through tests/measurements of the soil’s health to determine the short and long term effects of different management practices on the soil and its ecology. This is probably the only way to ensure sustainable agriculture. The following are a few examples of lacunae in our knowledge:

  • Can the microbiological diversity in the soil be controlled to manage the stability, function and quality of the soil system
  • What are the optimum levels of various organisms in the soil Is soil with fewer organisms or a smaller diversity necessarily poorer Is there is critical level, with regard to the soil’s biological diversity or activity, at which the ecosystem will crash
  • What interaction and dynamics occur between the various soil organisms Can they be manipulated beneficially
  • Are the current techniques of measuring soil fertility (soil quality) sufficient or should measurements to ascertain the soil’s health receive more attention
  • The ability of the ecosystem to handle large disturbances depends on the diversity of the system. How quantifiable is the importance of this and can it be evaluated in practice on farm level

References

BURGESS, A., 1967. Micro-organisms in the soil. Hutchinson & Co. Ltd., London, p 188.

LEE, K.E. & FOSTER, R.C., 1991. Soil fauna and soil structure. Aust. J. Soil Res. 29, 745-775.

KENNEDY, A.C. & SMITH, K.L., 1995. Soil microbial diversity and the sustainability of agricultural soils. Plant Soil 170, 75-86.

WANDER, M.M., HENDRICK, D.S., KAUFMAN, D., TRAINA, S.J., STINNER, B.R., KEHRMEYER, S.R. & WHITE, D.C., 1995. The functional significance of the microbial biomass in organic and conventional managed soils. In The significance and regulation of soil biodiversity. H.P. Collins, G.P. Robertson, M.J. Klug (Eds.), Kluwer Academic Publishers, The Netherlands, pp. 87-97.

WARD, D., BATESON, M., WELLER, R. & RUFF-ROBERTS, A., 1992. Ribosomal RNA analysis of micro-organisms as they occur in nature. Adv. Microbial Ecology 12, 219-286.

For further details contact Kobus Louw at 021 – 8093002 (fax) or kobusl@infruit.agric.za.

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