Chemical reactions lead to a degradation of the biological properties of the growing medium, described by the C:N (carbon:nitrogen) ratio.
Stable and relatively inert elements need to be present in the mix, therefore, because biological degradation leads to a degradation of the physical properties of the medium.
It is necessary in the first instance to make sure that the medium contains no toxic or harmful substances such as heavy metals, tannins, phenols or excessive calcium carbonate. With 'home-made' composts it is not easy to assess the real risks.
It is better to use commercial medium, the composition of which is better known.
The medium must also be free from disease organisms.
Unlike substances of mineral origin, which are relatively inert, organic materials are by their nature biodegradable and can thus change and lose some of their properties.
This degradation is the result of micro-organisms in the medium, such as bacteria and moulds. As organic molecules in the medium are broken down, they release carbon dioxide which reduces root respiration and the uptake of water and minerals.
More specifically, the breakdown of proteins produces ammonia, which raises pH. Some writers have also noted that ammonia may cause toxic effects and a greater susceptibility to certain diseases (Digat & Lemaire 1992, quoted by Urban 1997). The rise in pH due to ammonia may also impede the uptake of minerals.
The final stage of organic matter breakdown is mineralisation. This liberates ions such as phosphorus, potassium, calcium, and magnesium. If these are present in excess they will cause problems of salinity or in extreme cases plant poisoning.
Avoid, then, mediums which include materials that are too readily biodegradable.
The Carbon:Nitrogen (C:N) ratio is a good index of the biological stability of an organic medium. It is generally high early on in the decomposition process, and low towards the end, when the organic matter has become nearly stable. The C:N ratio is not however enough on its own to describe the biological stability of a medium (Lemaire 1993, quoted by Urban 1997).
In practice, peat and bark are both relatively stable (Lemaire 1993 quoted by Urban 1997).
Overall, the important point to remember is that mediums which are chemically inert and not highly biodegradable make possible the best adjustment of the plants’ supply of minerals.
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