What You Need To Know About Soil pH

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Whether you're a veggie or flower gardener, knowing the basics about your soil's pH will help you select and grow healthy, flourishing plants.

Understanding the science behind your soil’s pH will help you grow healthier plants that produce more flowers or fruits, so it’s worth your time to dig into the details. It’s easy to learn but not so easy to change. We’re here to help, so read on.

What Is Soil pH?

Technically, pH measures the potential (p) Hydrogen (H) ions present in your soil. The pH scale ranges from 1 to 14. Any number below 7 signifies acidic; numbers above 7 are alkaline. The number 7 is neutral.

Each number represents a tenfold difference. That means a pH of 5 is ten times more acidic than a pH of 6, and 100 times more acidic than a pH of 7. That’s quite a spread!

What Makes Soil Acidic or Alkaline?

Soil’s natural pH ranges significantly across North America, primarily because of soil composition. If your garden sits on top of a limestone base, it will naturally be more alkaline. If your soil formed from acid rocks or near a peat bog, it is more acidic.

Other factors can affect a soil’s pH. Areas where heavy rainfall is common typically have more acidic soil because rainfall’s pH is about 5.6. Frequent applications of general-purpose fertilizers containing ammonium or urea can also make the soil more acidic over time.

How Does pH Affect My Plants?

Soil pH is important because it affects the availability of many essential nutrients. Imagine all of those healthy nutrients waiting in the soil for the plant roots to soak them up. If the pH is too acidic or too alkaline, the door shuts so the nutrients can’t get into the roots. Even if you add more nutrients to the soil, the plant still can’t get them because the door is shut.

You can open those doors by adjusting the soil’s pH to between 6.5 to 7.5. The three main ingredients of general purpose fertilizers — nitrogen, phosphorus and potassium (N-P-K) — are easily absorbed in this range. When the soil becomes significantly more acidic or alkaline, those doors begin to close.

Secondary nutrients, like iron, manganese, boron, copper and aluminum, are more easily absorbed by plants when the soil’s pH is between 5.0 and 6.5. This is why acid-loving plants can look sickly when the soil isn’t acidic enough for them to absorb iron. It’s also why bigleaf hydrangeas (Hydrangea macrophylla) turn blue in acidic soil. When the soil’s pH is below 6.5, they can absorb the aluminum, which turns their flowers blue.

What Is the Ideal Soil pH?

That depends on what you’re trying to grow. For most ornamental and vegetable gardens, a pH between 6.0 and 7.3 works best because the plants can easily absorb the most nutrients in this range.

There are several exceptions to this rule. Some commonly grown plants prefer a more acidic pH, like blueberries, raspberries, rhododendrons and azaleas, conifers, birch and oak trees. A few that enjoy more alkaline soil include clematis, lilacs, forsythia, garlic and asparagus.

Understanding what soil pH your plants prefer will help you keep them growing strong, and know what to do if they start to decline. If they seem to be growing well and staying healthy, your soil’s pH is probably right where it needs to be.

How Do I Know My Soil’s pH?

Measuring your soil’s pH is easy — just use a simple pH meter sold at garden centers, hardware stores and online. It consists of a probe attached to a small box with a dial. Depending on which meter you choose, the package instructions may tell you to moisten the soil, or sand the probe lightly with fine-grit sandpaper to obtain a more accurate reading. Within a few minutes, you’ll know your soil’s pH.

More complex soil testing kits that test for specific nutrients are also available. If you want a detailed analysis of your soil, you can send a soil sample to your local University Extension office. For a small fee, they’ll analyze your soil and send you a detailed report of the nutrients it contains and its pH.

Can I Change My Soil’s pH?

It’s possible, but not always feasible, to make your soil’s pH more acidic or alkaline. Remember that tenfold difference between each number on the pH scale? Changing your soil’s pH from 8 to 5 means a difference of 1,000 units. In such cases, it would be better to select different plants that will naturally thrive in your alkaline soil. This is especially true for heavy clay soils which are the more resistant to pH changes.

If you only need to shift your soil’s pH by one to 1.5 units, add lime to raise the pH or sulfur to lower it. Don’t expect the change to be immediate. It often takes months for the shift to occur, so prepare your soil for spring planting by amending it in the fall or early spring (if the ground isn’t frozen). This will be an ongoing task. Annual applications will be necessary to maintain the pH where you want it.

Use aluminum sulfate or garden sulfur to make your soil more acidic. Don’t confuse this with fertilizer like Miracid or Holly-tone for acid-loving plants. Those products will not shift your soil’s pH as much as aluminum sulfate or garden sulfur will.

Pelletized lime is the easiest to apply to make your soil more alkaline. Work lime into the top few inches of your soil around individual alkaline-loving plants, or spread it across your lawn.

Knowing the science behind your soil will bring a new level of understanding and appreciation of how your plants grow, and help you help them thrive for years to come.

Susan Martin
Susan Martin is a lifelong gardener who enjoys sharing her passion for plants, gardening and the business of horticulture with fellow plant enthusiasts across North America. She has spent over two decades working in the horticulture industry on new plant development, garden design, sales, marketing and consulting. Susan has received visitors from around the world in her home garden which has been featured in numerous gardening publications. Her goal is to inspire and educate people about how to garden every day.