Print Version

Description:

Soil pH is one of the most important factors in soil analysis because it enables the gardener to anticipate the availability of the elements necessary for plant growth. It is known that soil microorganisms thrive in particular pH ranges. Bacteria, for example, break down organic matter to release Nitrogen and other plant nutrients and are usually less tolerant of acidic soil. Nitrogen fixing bacteria, by comparison, transform atmospheric nitrogen into a usable form for legumes at an optimum pH range of 7.0 to 8.5. This fact sheet provides introductory information related to soil pH.

What is soil pH

The acronym pH comes from the French words “pouvoir hydrogene” meaning hydrogen power. Soil ph is a numerical measure of soil alkalinity or acidity on a scale from 0 - 14 with neutral at 7. More precisely, it is a measurement of the concentration of hydrogen ions in a soil solution. The acidity or hydrogen ion concentration is greater as the readings decrease from 7 to 0. The alkalinity or hydroxyl ion concentration is greater as the readings increase from 7 to 14. At pH 7 the number of hydrogen ions equals the number of hydroxyl ions.

The pH scale is logarithmic. A difference of one unit represents either a tenfold increase or decrease in the availability of hydrogen or hydroxyl ions. For example, the acidity of a solution with pH 6 is ten times greater than one with pH 7. A difference of two units (e.g. 7 to 5) means the acidity is one hundred times greater.

Why it is important for the gardener to know soil pH

Soil pH is important to plant nutrition because if affects both the physical and chemical properties of soil. Slightly acid soil (i.e. pH 6.5 to 7) is best for most plants. Minerals tend to be more soluble in acid than in neutral or alkaline soils. Plants can only absorb nutrients in solution. The soil solution carries Nitrogen (N), Phosphorus (P), and Potassium (K), and all other required elements that plants need in order to grow, thrive and fight disease. The availability of these elements is directly affected by pH. For example, if the pH of the soil is above 5.5, Nitrogen, in the form of nitrate, becomes available. Phosphorus is available between 6.5 and 7 and Potassium most readily available at pH 6 to 7.

Strongly acidic soils can have a high concentration of aluminum and manganese that plants such as rhododendron, blueberries, and certain conifers are able to tolerate, but many of the vegetables, such as alfalfa, beets, and barley, cannot because they need more calcium or they find the aluminum toxic. Acid soils also slow down the activity of certain microorganisms such as the nitrogen fixing legume bacteria. By comparison, highly basic (alkaline) soils may not provide needed nutrients due to the high concentration of salts.

The presence of disease can be directly affected by soil pH. Fungi, for example, can tolerate acid soils. We also see that pH affects the manifestation of certain plant diseases. Potato scab is worse at pH 6.5 to 7.5 but potatoes thrive at pH 5 to 6.

Knowing the pH of soil is also important in the application of pesticides, herbicides and fungicides. If the soil is too acidic most plants will not absorb these chemicals and they will end up in the ground water and run off as a pollutant to our streams, rivers, and lakes.

N.B. “The City of Toronto adopted a new municipal by-law (spring 2004) that restricts the use of pesticides. For details about the by-law visit the Pesticide by-law page on Toronto's Public Health's Web site at www.city.toronto.on.ca/pesticides or www.toronto.ca/health. Questions may be e-mailed to pesticide@toronto.ca.”

How to test soil pH:

Using a pH meter provides the most accurate measurement of soil pH. These are available at most plant nurseries and are not expensive.

Most plant nurseries also sell inexpensive test kits that are easy to use. These often become available in the spring. Test kits usually consist of a test tube, some testing solution, and a colour chart. Soil samples should be taken after the growing season but before winter because some soil amendments take a long time to produce a result. Put a soil sample into the test tube, add a few drops of solution, shake it up and leave it for an hour or so to settle. Compare the colour of the solution in the test tube with the colour chart. Better kits will also provide a booklet that will help you to interpret your result.

University.of Guelph (Soil and Nutrient Laboratory, 95 Stone Rd. West, Guelph, Ontario, N1H 8J7) will perform detailed soil tests. Call 519-767-6226 for details related to cost.

How to lower or raise pH:

Soil pH may be lowered and soil made more acidic by the addition of organic matter such as compost, peat moss, pine needles, or oak leaves. Unfortunately very large quantities are necessary to change the pH measurement only a small amount. You can also use sulfur to lower pH at the rate of 0.5 to 1 kg. per 10 sq. meters (1 - 2 lbs. per 100 sq. ft.).

Aluminum sulfate or iron sulfate may also be used according to package directions. To raise soil alkalinity use lime in the form of crushed limestone or dolomitic lime at approximately the rate of 7 lbs. per 100 sq. ft. Do not apply this with manure or chemical fertilizers. Wood ash can also be used.

Ontario Soils

Soils in Ontario tend to be in the range of 6.0 to 7.5. The pH in southern Ontario is approximately 6.8. This means that it is difficult to grow azalea that require a pH of 4.5 to 6, or rhododendron that require a pH of 4.5 to 5.5, as well as other plants requiring highly acidic soil. Your soil will have to be amended.

References

The Toronto Botanical Garden Weston Family Library is an excellent source for horticultural information.

Books

• Miller, Raymond W. Miller and Roy L. Donahue. Soils-Sixth Edition 1990, Englewood Cliffs, New Jersey, U.S.A. : Prentice Hall Inc. 1990
• Reader's Digest. Illustrated Guide to Gardening in Canada. Montreal, Canada: The Reader's Digest Association (Canada) Ltd., 1989

Factsheets are produced by the Toronto Master Gardeners in association with the Toronto Botanical Garden. They provide introductory information about a broad range of horticultural topics and are intended for personal use and study purposes. Should your gardening group or organization wish to use multiple copies we ask that you inform the Toronto Botanical Garden at info@torontobotanicalgarden.ca.

Date revised: April 19, 2006