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Winter heath, in the Ericaceae family, requires more acidic soil
Photo: Jardin botanique de Montréal (Gilles Murray)

Healthy soil has an adequate pH. The pH represents the amount of hydrogen ions (H+) present in the soil. The more hydrogen ions there are in the soil, the lower the pH, and the more acidic it is.

To obtain an accurate pH reading for your soil, you can have it tested by a laboratory. You can also use a pH meter or buy a soil test kit from a garden supplier.

No matter what method you use, pH is always measured on a scale from 0 to 14.

  • pH 0 to 6.9 = acidic soil
  • pH 7 = neutral soil
  • pH 7.1 to 14 = alkaline soil

This scale is exponential, which means that:

  • Soil with a pH of 6 is 10 times more acidic than soil with a pH of 7.
  • Soil with a pH of 5 is 100 times more acidic than soil with a pH of 7.

The pH affects the availability of nutrients. When soil is too acidic or too alkaline, plants cannot absorb all the minerals they need, even if they are present in the soil. Some elements, like aluminum, iron, zinc and manganese, may be abundant enough to be toxic to plants.+ Inadequate pH also interferes with the structural stability of the soil, promotes fungal diseases and destroys many different beneficial organisms.

What pH is best?

Optimal pH varies depending on the type of soil and what plants you intend to grow. Most plants do well with a pH from 6 to 7, although there are some exceptions. Plants in the Ericaceae family (rhododendrons, heathers, bog rosemary, etc.) and blue-flowered hydrangeas, for instance, require more acidic soil (pH from 4.5 to 5.5).

Most soils in Quebec tend to be acidic. There are alkaline soils in some parts of the Island of Montréal, Laval and nearby, however. The only way to find out exactly how acidic your soil is to have it tested. The test results will tell you what quantities of amendments you need to add to adjust the pH.

A good way to keep your soil’s pH balanced is to regularly apply organic matter in the form of compost or composted manure. When the soil is too alkaline or too acidic, however, you will need to add other amendments.

Overly acidic soil: Add lime or wood ashes.

Overly alkaline soil or for growing acid-loving plants: Amend with sulphur, iron sulphate or pine needles.

N.B.: It is always best to lower or raise the pH gradually, instead of making any drastic changes that will upset the biological balance of your soil and interfere with its fertility. Ideally, the pH should not be changed by more than one unit a year.

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