By Marina Welham
Featured image above: AI-generated text-to-image of Virtual Marina and BB
working in the greenhouse.
The intensity of the acid/alkali reaction of soil or compost is commonly expressed in terms of pH units. pH stands for pondus hydrogenii, the concentration of hydrogen ions.
To do well, the majority of plants must be grown within the pH range to which they are most suited. Most cacti, for example, do best in soil with a slightly acidic reaction (pH6).
No two hobbyists use the same soil mix but common to all the mixes is their degree of acidity which is more important than the proportions of the various ingredients.
A pH meter is a valuable help in determining whether you need to adjust the pH of soil in which your plants are growing. These meters vary greatly in price from affordable to very expensive. The brands commonly sold in garden centers for around $10 are quite suitable.
pH VALUE | SOIL REACTION | SUITABILITY FOR PLANTS |
1 | Very strongly acid | Unsuitable for all |
2 | Very strongly acid | Unsuitable for all |
3 | Strongly acid | Suitable for very few |
4 | Acid | Suitable for some |
5 | Acid | Suitable for many kinds |
6 | Slightly acid | Suitable for most kinds |
7 | Neutral | Suitable |
8 | Slightly alkaline | Few species tolerate |
9 or 10 | Alkaline | Only highly specialized species tolerate |
11 or 12 | Alkaline | No growth |
13 or 14 | Strongly alkaline | No growth |
If the pH value of the soil is very strongly acid it is probably wise to prepare a more suitable soil mix After removing as much of the existing soil from the roots as possible, repot the plants into the new mix. Usually soil mixes with a very high acid reaction have too much peat in them. Many soil mixes sold today have a very high peat content.
If the soil is too alkaline, you can make it more acidic by adding a few citric acid crystals to the water (very little is needed to make a difference) and the plants watered with this citric acid mixture. You can buy these crystals at any drug store. As you add a few crystals to the water, insert the pH meter into the water and it will tell you the pH value so you know when enough crystals have been added. If you add too much just increase the water and test again with the pH meter. Or you can steep a handful of horticultural peat in a container of water and after it has soaked for a day or so, use this water to water the plants. Again the pH meter can be inserted into the water to test the acidity before you use it. Most horticultural peat has a reaction of about pH4.
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