Gardening Basics 3: Soil pH and What It Means

Rhododendrons are one example of a plant that requires a certain soil pH. They prefer acidic soil.

Rhododendrons are one example of a plant that requires a certain soil pH. They prefer acidic soil.

by Tammie Painter

Too often it is assumed that just because a plant grows in a gardener’s region it will do well in every garden within that region. Unfortunately, even native plants grown in the wrong conditions will become weakened and troublesome over time, while plants grown in their preferred conditions will flourish with less care. The right choices will allow you to spend time enjoying a healthy garden rather than tending to a weak one.

This post, the third in a three-part series, covers soil pH: what it means for plants and how to test and treat it.

Key condition #3: Your Soil’s pH
Soil pH may seem too scientific for the home gardener, but it’s a vital matter for your plants.

At its most basic level, soil pH determines your plant’s ability to access nutrients. In the right pH conditions, a plant’s roots are able to absorb nutrients from the soil. The wrong pH locks up those nutrients, making them unavailable to the plant. This has to do with pH affecting the ions on nutrient molecules in a way that either gives the nutrients a free pass into the roots or puts up a wall to the nutrients’ entry.

A common pH problem can be seen when acid-loving plants, like blueberries, are planted in neutral or basic soil. At first the shrub does fine, but over time its leaves yellow as it goes into iron chlorosis. Your immediate reaction is to give it a dose of fertilizer. The nitrogen in the fertilizer will give the blueberry a spurt of leaf-producing growth, but this spurt places additional stress the nutrient-deficient plant. Left in neutral soil, the shrub's health and stamina suffers, you get few blueberries and eventually the plant dies.

Extension services and better gardening books can tell you a plant’s pH requirements. You can also make some general assumptions. The majority of plants do well in near-neutral soil (pH 6.5 to 7.5); plants from rainy areas tend to prefer acidic soils (pH 4.5 to 6.5); and plants from arid regions do well in the alkaline range (pH 7.5 to 8.5). Keep in mind that these are broad generalizations and they don't apply to every plant.

Tests for soil pH range from full-analysis kits you send to a lab to inexpensive litmus-strip kits available at most garden centers. If you suspect your soil has serious problems, opt for the lab-tested kits, because many of these will also tell you what nutrients your soil is lacking. For most home gardeners, though, the inexpensive home tests will suffice.

Once you have your soil’s pH numbers, you can either choose plants to match or you can adjust it by amending the soil (see box). This change is best done slowly over a season or two, and it will need to be maintained on a regular basis. Trying to push your soil beyond what is the normal range for your region will be an uphill and continual struggle.

It’s much easier to simply use plants that prefer your soil’s natural pH, but if you want to attempt something different, it’s possible to adjust the pH. Bear in mind it will be an ongoing process as the soil will shift back toward its natural pH over time.

You can raise the pH of acidic soil by working in lime (calcium carbonate) or pulverized oyster or clam shells into the soil. Liming agents work most efficiently when they wash into the soil soon after application, so time this project when rain is predicted.

Liming agents generally require three to four months to change pH. Carefully follow all application directions, as this change is difficult to reverse if you swing the pH too far.

Wood ashes also raise pH, but not as strongly as liming agents. They are good if a small pH increase is desired.

Adding any kind of organic matter, such as compost, shredded leaves, conifer needles, or bark mulch, will lower soil pH as they decompose. Plan to add organic matter at least a month or two ahead of planting time to give the amendments time to work.

Image credit: gehringi/iStock/Getty Images

Read “Gardening Basics 1: Understanding Light Conditions.”

Read “Gardening Basics 2: Your Garden’s Soil Type.”