Soil Chemistry

An important characteristic of soil is its level of alkalinity or acidity, this is known as the pH. The pH scale runs from 1, which would be extremely acidic and would not support plant life, to 14. which would be extremely alkaline and no plant would survive. The range found in most soils is from 4 to 8. Some plants prefer the soil to be more acidic, e.g. azaleas and rhododendrons while others prefer a slightly more alkaline soil, e.g. brassicas.The pH scale is logarithmic which mean that the difference between two adjacent points on the scale is not double, or half the value before or after it, but a factor of 10 different. e.g. moving from a pH of 6 to 7 does means that the pH has doubled, it has changed by a factor of 10!Before planting it is useful to pH test on your soil. There are several ways to do this:

  • Send a sample to a soil analysis laboratory. They can give you an accurate pH reading and may also be able to offer advice about fertility. A sample of soil should be taken from various parts of the garden and thoroughly mixed before sending. This will give you an average reading for the whole plot. Use an organic service if possible.
  • Buy a simple pH meter from a garden centre and test the soil yourself. This will give you a rough and ready reading with the advantage that you can take spot reading at various points in the garden but may not be very accurate.
  • Use a pH test kit that uses a liquid colour change to indicate pH. These are a little more involved than the simple probe but can give reasonably accurate results. The procedure is the same as sending a sample to a professional laboratory,

Taking a soil sample
There may be local variations of pH that will depend on the land use prior to gardening. When new houses are built it is not unusual to find quite large variations of pH caused by builders rubble and discarded mortar so it is worth checking as many sites on the garden as possible.

Soil pH and plants

pH 3 4 5 6 7 8 9
Most plants fail Acid-loving plants such as camellias, heathers,
conifers, blueberry, phlox, hydrangea – blue and rhododendrons thrive
Most fruit thrives Most plants thrive Tolerable to most plants especially brassicas
and legumes.
Some plants thrive such as spinach and clematis Most plants fail
Substances toxic to plants released into soil Some

nutrients washed out of the soil


Iron and manganese become less available to
Phosphorous becomes less available to plants
Earthworms generally absent Potato scab disease cannot survive Most soil life thrives Clubroot disease less rampart

Correcting pH If your soil is too acidic you need to add lime. The rate will depend on the type used and the level of pH change required. Organic gardeners use ground lime or dolomitic limestone rather than slaked lime, hydrated lime and caustic lime as the effects are gentler (and more controllable.) As a general rule add 200g/ sq m (7oz/sq yd) of ground lime or dolomitic, annually until you reach the desired pH.Making a soil more acidic is more difficult; the best way is to use garden compost and manure annually until the soil is more acidic but it is a slow and unpredictable process.

As a general rule always test your soil before planting, especially on a new plot. Grow the plants that are suited to your particular garden rather than trying to make large changes to the pH.

The table below shows the effects of pH on nutrient availability.ph_nutrient_chart