What is Soil pH?

The abbreviation pH, or “potential of hydrogen,” is a measurement of the concentration of hydrogen ions in a substance, which in turn defines its acidity or alkalinity. Why is knowing the pH of soil important to gardeners? The pH ultimately determines the availability of nutrients for uptake by plant roots and, therefore, directly affects a plant’s ability to thrive.

The pH scale ranges from 0.0 to 14.0, in which anything below 7.0 is acidic and anything above 7.0 is alkaline (or basic). A measurement of 7.0 is considered neutral. Most soils exhibit a pH range between 4.0 and 8.0. Soils with a slightly acidic to neutral pH (between 6.0 and 7.5) have an abundance of most of the nutrients that plants need. Conversely, few nutrients are available to plants in extremely acidic soils, while low amounts of certain micronutrients necessary for plant health are available in very alkaline soils.

The good news is that there are plants that will thrive at nearly every pH level. Some tolerate acidic, neutral, or alkaline soils – and sometimes others can manage quite well in all three. Plants that tolerate neutral to alkaline soils will often tolerate acidic soils, too. The majority of plants tolerate slightly acidic soils. That said, few plants tolerate very acidic soil (<5.0 pH).

• Among the plants that like acidic soils are: Azalea, Blueberry, Fothergilla, Mountain Laurel, Rhododendron, and Wintergreen.
• Among the plants that like alkaline ("sweet") soils are: Boxwood, Clematis, Dianthus, Hellebore, and Lilac.

The tolerance of plants to varying degrees of pH is an important factor when choosing varieties for specific sites. In fact, it is easier to select plants that will thrive at an existing pH than it is to change soil pH to suit specific plants. While it is possible to amend the pH of soil (see below), it is difficult to make continual, permanent change. We recommend testing your soil pH (see below) and, once the pH is known, making thoughtful plant choices before trying to change your soil’s pH. (Please note that soil pH may vary in different spots around your yard. A garden bed on, say, the side of your house may have a different pH from the soil along a sidewalk or the soil under Pine trees in your backyard. Many gardeners take and test samples from various beds around their yard.)

It is also important to note that many of us live in increasingly built-up or urban environments. The soils in our gardens are often near foundations, sidewalks, or paved areas. Due to the limestone in concrete and other building materials, these soils tend to have a higher (more alkaline) pH. Proper plant selection is critical to getting plants to succeed in these areas.

Soil pH Testing

It is by no means a requirement to test the pH of soil in your garden or yard. But pH testing will provide you with critical information about nutrient availability and the types of plants that will thrive in a given site.

Testing soil pH is easy. Home gardeners can procure soil pH-test kits from their local extension service. Soil samples may also be submitted for pH analysis (as well as for tests concerning a number of other important soil qualities, such as porosity and the presence of heavy metals). If you choose to do pH testing yourself, colorimetric test kits are simple to use. Just add a few drops of chemical reagent to a soil sample and compare the resulting color against a chart. Sometimes one or more tests, using different reagents, are necessary to determine the exact pH of your soil.

Technological advances have made other testing methods possible, too. Check online for digital meters that are pocket-sized or handheld.

Amending Soil pH

Sometimes it is desirable, even necessary, to change the pH of your soil. In general, add lime to raise the pH (make it more alkaline) or sulfur to lower the pH (make it more acidic). It is important to consider the various forms of chemical additives available and to apply them strictly according to package instructions. For small sites, it may be possible to remove the existing soil completely and replace it with a new mix.

As noted above, it remains difficult to make a permanent change to the pH of your soil. The conditions that made your soil acidic or alkaline in the first place, such as bedrock weathering or proximity to limestone-rich building materials, cannot be continually managed through soil amendments. Selecting plants that grow well at your soil’s existing pH, or that will tolerate a broad pH range, is far easier.

With thanks to the following resources:

• Trowbridge, P.J. and Bassuk, N.L. (2004). Trees in the urban landscape: Site assessment, design, and installation. John Wiley & Sons, Inc.
• Cornell School of Integrative Plant Science. (2020, October 29). Soil pH test demonstration [Video]. YouTube. https://www.youtube.com/watch?v=7t9ZVt9CAzk.