Soil pH
The pH of a soil is an indication of its acidity or alkalinity. The pH ('parts Hydrogen') scale covers a range from 0 (very acidic) to 14 (very alkaline), with a reading of 7 being neutral.
The Importance of pH
It is very important to know the pH value of your soil as many plants and soil micro-organisms prefer either acid or alkaline conditions, and the pH level can encourage particular pests and diseases to thrive. The pH also affects the availability of nutrients in the soil and consequently their availability to your plants. Alkaline soils tend to be deficient in boron, manganese and phosphorus, whilst acidic soils have excess aluminium and manganese, and lack phosphorus.
Calcium and pH
In general, the pH of soil is controlled by its calcium level. This alkaline element is lost when rain or irrigation carries it down through the soil and out of the root zone (termed 'leaching'). Soils that lay on chalk or limestone remain calcium rich, whilst other soils gradually lose their calcium and become more acidic. This particularly applies to free-draining soils such as sand or silt.
The majority of soils in the UK range from a pH level of 4.0 (acid) to pH 8.0 (alkaline).
Soil pH Ranges
A pH range of 5.5-7.5 will produce good plant growth, although a level of pH 6.3-6.8 is considered to be the optimum range preferred by most plants and soil bacteria. In addition, earthworms, which benefit the soil by aerating it and by speeding up the decay of organic matter, also prefer these near-neutral conditions. Most ornamental plants will tolerate a wide pH range, although some, such as rhododendrons, prefer acidic conditions (calcifuge - or ericaceous - species) and others, such as fuchsias, prefer an alkaline soil (calcicoles). Most vegetable crops produce their highest yields in neutral soils.
Adjusting Soil pH
Soil that is significantly acidic may be adjusted by the addition of lime or lime-rich material such as mushroom compost. Whilst this may be useful to raise the pH of an acidic soil enough so that vegetable crops may be grown, in general, it is more practical to select plants that are suited to pH conditions of your soil. It is never worth trying to make alkaline soil more acidic. If the pH of your garden soil prevents you from growing your favourite plant, consider building a raised bed and filling it with the appropriate soil.
The presence of certain wild and garden plants can help the observant gardener determine their soil pH:
Alkaline soil
* ash (Fraxinus)
* beech (Fagus)
* rhododendron
Acidic soil
* birch (Betula)
* foxgloves (Digitalis)
* gorse (Ulex europaeus)
* heathers (Calluna, Daboecia, Erica)
Testing Your Soil
The pH of your soil may be tested using either an electronic pH meter or a soil test kit; both should be available at your local garden centre. As soil can vary even within a small space, you will need to take several tests across your garden.
When using a soil testing kit, use a clean trowel and container to achieve untainted results. Take several samples from across the area in which you intend to plant, discarding the top 5 cm (2 in) of soil. Place the samples into a clean container, mixing each sample with the others. Any large objects such as twigs or stones should be removed, and any lumps broken down using the trowel or a clean teaspoon. Mix the soil together and then take a small sample from the container and add it to the powder in the test tube supplied. Follow the test manufacturer's instructions as to quantity and mixing methods. After you have added the correct amount of water, place the stopper on the test tube and shake the contents well. The tubes should normally be left for a short time to allow the contents time to settle. As the soil settles, the water changes colour to reveal the level of acidity or alkalinity; this colour may be matched against the supplied chart to give you the pH level of your soil sample.
- Dark green: alkaline
- Bright green : neutral
- Yellow or orange: acidic