  The Cultivation Page
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The intensity of the acid/alkali reaction of soil or compost is expressed in terms of pH units, representing the negative logarithm of the concentration of hydrogen ions.
Most plants grow at their best when grown in soil with a pH range which best suits them. Most cacti and other succulent plants grow well in slightly acidic soil around pH6. A few succulent plants grow naturally on limestone soils with a basic reaction, but an alkaline soil is more likely to kill plants than a slightly acidic one. Some lime-hating succulents e.g. Echevieria and some South American cacti e.g. Echinopsis, Gymnocalycium grow naturally in peaty soils and should not be grown in compost with added lime. On the other hand, some North-American cacti, e.g. white-spined Escobarias, Ariocarpus fissuratus, Lophophora williamsii, seem to prefer limestone soils and a handful of ground dolomite of lime may be added to the potting mix. A few cacti are reported to grow on gypsum (Calcium sulphate) which should have a neutral pH reaction while providing high Calcium.
If you wish to test the pH of your soil mix or water supply, affordable pH meters can be obtained in many garden centres.
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| Very strongly acidic soils can be improved by adding dolomite of lime (ground limestone). Very alkaline soils can be improved by watering with slightly acidified rainwater. Add very small amounts of citric acid crystals or one or two drops of hydrochloric acid. Check with a pH meter before use. Alternatively add some horticultural peat (typically pH4) to the mixture,although my prejudice is that peat encourages root meally bugs. Rainwater in or near cities may be naturally acidic.
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