Plants depend largely on 1 the mixture they are potted in for the nutrients they need for growth. If they are not given the correct compost they will either fail to grow well or not grow at all. Plants that do not thrive may be growing in a compost that is too acid for them, or too alkaline.
Alkaline composts
In general, alkaline composts are those that contain large amounts of lime (also know as chalk). Chalk or limestone soils occur naturally in many parts of Britain, and chalk is one of the minerals added to both peat- and soil-based composts.
Acid composts
Acid composts are the opposite of alkaline ones, as they are deficient in lime. While most plants that prefer an alkaline compost will also do well in any fertile mixture, there are certain plants, known as lime-haters, that cannot grow in anything but acid composts. Ericas and camellias are good examples.
Testing compost
The acidity or alkalinity of compost is measured on a pH scale, which ranges from 0 to 14. A pH of 7 indicates that the compost is neutral — that is, it is neither acid nor alkaline. Composts with a pH below 7 are acid, and the lower the pH number, the more acid they are. Similarly, those above 7 are alkaline.
Most plants grow best in compost with pH of 6.5-7. Those plants that are lime-haters will eventually die in alkaline composts.
Using a soil tester
A pH meter is very easy to use. Simply push it into moist soil and the pH number is shown on the scale. Cheaper soil-testing kits are litmus paper. A little soil is shaken up in water litmus paper dipped in the solution turns either red (acid soil) or blue (alkaline soil).
Acid-loving plants
- Calluna (Ling, or heather)
- Camellia
- Erica (varieties of heathers and heaths)
- Gentiana (gentians)
- Kalmia (Calico Bush)
- Lapageria
- Lilium (lilies)
- Magnolia
- Menziesia
- Pernettya
- Rhododendrons (including azaleas)
Special composts
Some house plants cannot grow in ordinary soil- or peat-based composts, but need one with added materials such as leaf mold. They include cacti, bromeliads, orchids and insect-eating plants.
Bromeliads such as the Urn Plant (Aechmea fasdata) need a compost that drains freely and is also free of lime. They will thrive in a compost made of equal parts leaf mold and peat. Insect-eating plants such as the Venus Fly Trap should be grown in a compost with plenty of organic material such as sphagnum moss, or equal parts moss and peat.
Cacti
Cacti and many succulents need a well-drained compost of 1 part coarse sand to 2 parts soil- or peat-based compost. Commercial cactus composts are available.
Orchids
Orchids, like cacti, also need a porous compost and you can buy commercial orchid composts. A good orchid compost should consist of equal parts peat, sphagnum moss and osmunda fiber.
