Some, like rhododendrons, do not grow well in an alkaline soil, while others, such as clematis, thrive on alkaline, or “limy,” sites. The term pH is a way of expressing how acidic or alkaline a substance is: the lower the pH number, the more acidic the garden soil. Acidity and alkalinity depend on how much calcium is in the soil. Acidic garden soil lacks calcium and alkaline soil has it in excess, while neutral garden soil (7 on the pH scale) has just enough calcium to mop up acidity.
Alkaline soils have a pH of over 7. You cannot make them acidic in the long-term, and acid-loving plants will certainly fail. However, limy soil is often well-drained and quick to warm up in spring. Incorporate plenty of organic matter and a wide range of plants will thrive, but don’t over fertilize—this results in tall, floppy plants.
Acidic soils have a pH of 1–6. They are ideal for a wide range of plants, but acid-loving or “ericaceous” plants are almost impossible to grow elsewhere. If your soil is not acidic, you can grow them in containers filled with ericaceous compost. You can make acidic soil more alkaline by adding lime, but think carefully before using it. Most plants will put up with some acidity, and you can grow a wider range in mildly acidic soils. In theory, you can add acidic material to limy soils to lower the pH, but acidifying materials, such as sulfur dust, act slowly, are needed in very large amounts, and are expensive.
Testing Soil for pH Level
You can get a good idea of your garden’s pH with a test kit from a garden center or hardware store. Kits come with full instructions, are quick and simple to use, and are usually more reliable than the meters sold for home use.
- Take samples from different parts of the garden, as soil is seldom uniform. Make a note of where each sample comes from so that you can exploit any variations, such as an acidic patch, when you draw up your planting plan. Add the soil to the chemical solution in the test tube and shake to mix.
- Match the colored solution against the pH chart to assess what kind of soil you have. Dark green indicates alkaline soil, bright green is neutral, and yellow or orange indicates acidic soil.