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pH is a measure of the acidity or concentration of H+ ions in a potting mix. The higher this concentration, the more acid or
the lower the pH. A very high or very low pH directly damages delicate plant roots. Indirect effects of low or high pH include:
 | reduced availability of certain elements due to chemical changes;
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increased availability of other elements, e.g. manganese, iron and zinc, when pH is low, or phosphorus when pH is high, to such levels that they become toxic to plants. At high levels these elements can also interfere with the uptake of other elements, e.g. copper deficiencies can be caused by high levels of iron, zinc, manganese and phosphorus;
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| reduced proliferation of desirable microorganisms, e.g. symbiotic bacteria
(Rhizobium spp.) in roots of leguminous plants, while promoting undesirable
microorganisms, e.g. the fungi
Pithium spp. that cause damping-off of seedlings, and Fusarium
spp. that are responsible for wilts in numerous plant species. |
Lime (calcium carbonate), on the right,
is often added to potting mixes in order
to increase their pH.
The optimum pH range, which maximises nutrient availability to plants, is 5.0 to 6.0 for organic soils and potting mixes (>10% organic matter content), and 5.5 to 6.5 for mineral soils and potting mixes
(Handreck and Black
1984).
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Gio. W. Fichera
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