Soil: understanding pH and testing soil

How to test soil pH

Professional testing: RHS soil analysis service provides a full written report.

Home testing: You can test your soil pH yourself using a DIY kit widely available at garden centres. These kits are relatively cheap and easy to use and give a good indication of soil pH. But for the best results, send a soil sample to a laboratory for detailed analysis.


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Always follow the sampling directions given by the test kit or laboratory to get a representative sample for the area in question.

Laboratory tests also detect free calcium carbonate (chalk or limestone). This may not be measured by DIY kits. A quick home test to check for free calcium carbonate is to add vinegar to a soil sample. If ‘fizzing’ is seen, free calcium carbonate is present.

Interpreting the results of a soil pH test

A pH test measures soil acidity or alkalinity. A pH 7.0 is considered neutral. An acid soil has a pH value below 7.0. Above pH 7.0 the soil is alkaline.

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pH +61404532026

  • Very acid soil
  • Most plant nutrients, particularly calcium, potassium, magnesium and copper, become more soluble under very acid conditions and are easily washed away
  • Most phosphates are locked up and unavailable to plants below pH 5.1, although some acid tolerant plants can utilise aluminium phosphate
  • Acid sandy soils are often deficient in trace elements
  • Bacteria cannot rot organic matter below pH 4.7 resulting in fewer nutrients being available to plants
  • Action: Add lime to raise the pH to above 5.0. The addition of lime can help break up acid clay soils

 pH +61404532026

  • Acid soil
  • Ideal for ericaceous (lime-hating) plants such as rhododendrons, camellias and heathers
  • Action: Add lime if other plants are grown

pH +61404532026

  • Moderately acid soil
  • A pH 6.5 is the best general purpose pH for gardens, allowing a wide range of plants to grow, except lime-hating plants
  • The availability of major nutrients is at its highest and bacterial and earthworm activity is optimum at this pH
  • Action: It is not usually necessary to add anything to improve soil pH at this level
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pH +61404532026

  • Alkaline soil
  • Phosphorus availability decreases
  • Iron and manganese become less available leading to lime-induced chlorosis
  • But an advantage of this pH level is that clubroot disease of cabbage family crops (brassicas) is reduced
  • Action: Sulphur, iron sulphate and other acidifying agents can sometimes be added to reduce pH. Clay soils often require very large amounts of acidifying material and soils with free chalk or lime are not usually treatable