Microbes In Acid Soil


There is a direct impact of pH on the microbe but it is important to consider also the impact on (1) rhizodeposition and crop residue utilisation as the principal substrates for microbial growth and (2) the physico-chemical environment of soil as a water-stable structure which is a source of nutrients to micro-organisms and plants. Different microbial groups have different pH optima and therefore acid soil has a different population balance, or biodiversity, from neutral to alkaline soils. Whereas the biodiversity of soils is of increasing environmental concern, biochemical processes mediated by the populations are of most significance to crop productivity. One such process is nitrification which is inhibited at low pH, but this inhibition can be overcome by humic acids.

The toxicity of aluminium to rhizobia at low pH has been studied extensively in acid soils. Of more recent interest is the impact of pH on biocontrol agents, including those which have been genetically-modified (GMMOs). Particularly, the siderophore-producing bacteria are only useful in neutral to alkaline soils where Fe(III) is not readily available. Considering the biodiversity of acid soils and the evolutionary adaptive responses which organisms have encountered, it seems to be more likely that ‘ecologically-fit’ agents for acid soils will come from wild-types than GMMOs in the long term. However, as GMMOs are likely to be used increasingly in soils, it will be critical to estimate this behaviour in acid soils relative to neutral/alkaline soils.

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