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
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.
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