by Stephen H

Created on: July 28, 2010   Last Updated: April 02, 2012

Gardeners need to know how acid or alkaline the soil is when they are planning the design of their gardens.  This is because some plants grow better in a more alkaline environment while others prefer a more acid location.  The level of acidity or alkalinity is measured on the pH scale, which runs from 1 to 14, with 1 being strongly acid and 14 strongly alkaline.  A neutral reading will be seen at a pH of 7.0, which is the pH value of distilled water.  It is unlikely that soil samples will measure at the extremes of the scale, with most soils falling in the range of pH 3 to pH 8.

Why soil pH is important

The acidity or alkalinity of the soil affects the nutrients that are available to growing plants.  Most plants thrive in a fairly neutral soil, or one which is very slightly acid, but there are exceptions to this.  For example, Azaleas need an acid soil whilst lilacs prefer a more alkaline environment.  One of the key nutrients for plant growth is phosphorus and by examining how the soil pH affects its availability to plants, it will apparent why this is an important consideration. 

Phosphorus is key to healthy plant growth.  It is used by the plant for several vital processes including photosynthesis, use of nutrients and forming of simple sugars for energy.  Plants take up phosphorous compounds through their roots from the soil, as one of its soluble compounds, usually a phosphate.  If the soil is either too acid or too alkaline, the phosphorous compounds that form are insoluble and so the plant is starved of this vital requirement for life and will become stressed and even die.

How you measure pH

Hydrangeas are a good indicator of the pH value of the soil, though will not provide an accurate value.  At low pH values, hydrangea flowers tend to be blue, as more aluminium is made available to the plant in acid soil.  As the soil becomes more alkaline, so less aluminium is available to the plant, the flowers tend to be pink.  It is not quite as straight forward as that, but it is not a bad rule of thumb.

A more accurate understanding of the pH of your soil can be obtained by testing a sample.  There are many home testing kits available from garden centres.  These tend to require you to put some soil into a liquid which contains an indicator (an indicator is a chemical which changes colour dependent on acidity levels).  The liquid will appear orange if the soil is acid, green

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