What is Soil pH and How to Work With It

We hear a lot about ways to alter our soil pH to suit certain plants, but what is soil pH and we should be messing with it? Dive in to learn the basics about how your soil pH level affects your plants.

Learning your pH level starts with a soil test. We compared home and lab soil tests here.

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Soil pH and Home Gardens

Here’s a little reality check. Most gardeners go their whole lives without ever knowing anything about their soil pH—or any other soil or general garden science—and seem to do just fine.

But there are some basics like soil tests, an understanding of your soil texture (sand, silt, clay), and the soil pH level that provide big, helpful clues about what our plants are dealing with and how we can better manage things.

Contents

• What is pH?
• Know Your Soil pH Level
• Altering pH Levels is Temporary
• Helping Individual Plants
• Amendments Used to Alter Soil pH
  • Increase Soil pH
  • Decrease Soil pH
• Audio Version
  • Resources

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What is Soil pH?

Want the audio version of this article? Listen here.

The pH scale is a measure of the concentration of hydrogen ions in your soil, or more specifically, in the soil solution—the water around the mineral particles in your soil—and, depending on these levels, some nutrients will be available and others will not.

For most of us, our soil pH number is between 5 and 8, although in some places it will be higher or lower.

Pure water has a pH of 7, so that’s what’s defined to be a neutral pH.

Lower than 7 is acidic, and higher than 7 is alkaline.

You’ll often hear it said that a slightly acidic level—around 6.5 to 6.8—is optimum. And that may mathematically be the range where, on average, the most nutrients are available. But that doesn’t mean that you need to get your soil pH to be in that range. Most plants grow fine with a pH between 5.5 and 7.5.

The pH scale works the opposite of what you might think. The higher the concentration of hydrogen ions, the lower the pH. In other words, there are more hydrogen ions in acidic soil.

And pH is a logarithmic scale, which means that a pH of 5 has ten times the hydrogen ion concentration of a 6 pH and 100 times that of 7.

But your plants don’t actually care about the concentration of hydrogen ions in the soil. Not directly, anyway. Hydrogen is one of the essential macronutrients for plants, but that’s not the issue here.

So why does pH matter?

The pH level determines which nutrients can be taken up by your plants.

Most plants tolerate a fairly broad range of pH levels and healthy growing conditions are key. It’s the extremes that may cause problems.

Here’s why.

Soil pH is all about chemistry, ions, and electrical charges.

Let’s do a quick high school science class refresher.

We all remember the periodic table of elements, and you may recall that at the top of the chart—atomic number 1—is hydrogen.

The hydrogen atom has one proton, one electron, and no net charge.

An ion is an atom or molecule that has an electrical charge, which means it does not have an equal number of protons and electrons. And, in the case of the hydrogen ion, we get that by taking away its electron. It still has its proton, but no electron to offset it, so the hydrogen ion has a positive charge. In fact, the hydrogen ion is essentially just a proton—as simple a positive charge as you can get.

Most of the nutrients that feed our plants are also ions. We call them by their elemental names: nitrogen, calcium, sulfur, and so on, but the forms that plants can take up and use are actually ions.

And—here’s the important part—the concentration of hydrogen ions in the soil—the pH—changes how these elements and molecules combine with each other, and with parts of our soil. It’s all driven by electrical charges. And that in turn determines whether or not these nutrients can be taken up by our plants.

At certain pH levels, it gets much harder for plants to get the nutrients they need.

At certain pH levels, it gets much harder for plants to get the nutrients they need. The nutrient may be in the soil, but they’re in forms that aren’t available to plants. Some elements even get to toxic levels in the soil, all because of what reacts with what and which compounds end up there.

This is why the solution to something like an iron or calcium deficiency in a plant may not be solved by adding more to the soil. They may already be in the soil but the plant cannot take them up because it’s not in an available form.

So what are our options in this situation? Keep reading.

Know Your Soil pH Level

Soil pH is another piece of information to help us select the right plants for our growing conditions along with climate, light, water and drainage, air, soil nutrients and fertility, and texture (structure).

A first good step to understanding the nature of your soil is a soil test that includes the pH level.

You can get a lab test with soil analysis including pH (some extension offices offer this free) or buy a pH test kit like this one.

Test several samples from around your garden as variations are common. Soil may be higher in alkalinity around the foundation of your home.

Avoid testing when you’ve recently fertilized your garden as this can skew the results.

So, you’ve got your pH number. Now what?

You may not need to do anything.

For many of us our soil pH levels are between 5 and 8 and there are plenty of things we can grow in this range.

A range of 5.5 to 7.5 is considered acceptable for most plants.

A few plants like blueberries and azaleas prefer more acidic soil.

Soil pH Level	Plant Note
>8.3	Too alkaline for most plants
>7.5	Plants may not take up iron
5.5 to 7.5	Acceptable for most plants
<5.5	Soil microbe activity is reduced
5.0 to 5.5	Suits azaleas, rhododendrons, blueberries, conifers, pin oak, gardenia
<4.6	Too acidic for most plants

Altering pH Levels is Temporary

As the saying goes, pH levels can be managed, not cured.

We hear a lot of advice to use amendments to alter soil pH, but it’s not a simple fix. Ultimately, your pH is what it is.

The overall nature of your garden soil has a lot to do with the rock under the soil as well as climate—things greater than bags of amendments can alter long-term.

In housing developments, building materials like concrete, gravel, or limestone can shift soil to alkaline levels. The pH level of rain and ground water also plays a role.

Trying to change the pH level of an entire garden is a fight against nature that nature will win.

Yes, you can do short-term fixes, commonly with lime or sulfur, either annually or every few years—and lots of gardeners do—but ultimately the pH will revert back to what it was. And there are other side effects. .

My own approach with our alkaline soil is to add organic mulches which can offset it slightly and focus on plant selections that tolerate our growing conditions.

Helping Individual Plants

Help for Plants That Don’t Like Your pH Level

If you have existing plants known to struggle with your pH level, instead of applying pH-altering amendments, another option is to move them to containers or raised beds where you can provide soil that suits their needs without worrying about the rest of the garden.

We have alkaline soil and our blueberry bushes struggled until I moved them to containers with slightly acidic soil. You may see this marketed as ‘ericaceous’ soil or compost made for plants that do not tolerate alkaline or chalky soil.

Amendments Used to Alter Soil pH

This lists examples of amendments used to alter soil pH.

Application amounts and frequency depend on soil structure, pH, texture and other conditions. Any changes are not long-term.

Increase Soil pH

Garden nurseries and home improvement stores may offer two types of lime (calcium carbonate) or ground, agricultural limestone.

This may be pulverized, granular, pelletized, or hydrated. This process is referred to as ‘liming’.

Lime replaces hydrogen ions, adds nutrient(s), and makes phosphorus available. It can also increase the available nitrogen by speeding up the decomposition of organic matter.

• Calcitic lime (ground limestone) includes calcium as the name implies and is suited to soil with sufficient or high magnesium levels.
• Dolomite lime includes calcium and magnesium and suits soil with lower magnesium levels.

Decrease Soil pH

• Elemental sulfur | A slow-release amendment; soil bacteria gradually convert it to sulfuric acid.
• Aluminum sulfate | Alters pH instantly but can be toxic for plants.
• Iron sulfate
• Acidifying nitrogen
• Organic mulches
• Sphagnum peat | I see peat recommended due to its low pH (3.0 to 4.5) but I couldn’t find anything demonstrating how it could alter soil pH. There are also concerns about the environmental effects of peat mining (as with many other garden amendments).

Resources

Listen

NEW! Click play to listen:

• Soil pH: What it Means | SUNY College of Environmental Science and Forestry

• Acidifying Soil for Blueberries and Ornamental Plants in the Yard and Garden | oregonstate.edu (PDF format)

~Melissa the Empress of Dirt ♛