Soil pH: What Every Gardener Needs to Know About Soil pH

Small plant being planted in soil.

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Unless you happen to be a gardener who also has a special fondness for chemistry, soil pH may not exactly be the most exciting topic for you. And the chemical foundations of the topic are apparent from the very name: "pH" stands for "potential Hydrogen." Indeed, your eyes may be glazing over at this point if plants are your chief passion in life, while chemical elements leave you utterly unmoved. But the pH level of your soil can make or break your garden. Fortunately, you don't have to get lost in the weeds of soil science to grasp the basics of soil pH. Let's find out what every gardener needs to know about soil pH; we'll leave the rest for the aspiring chemists out there.

What is Soil pH?

Soil pH is a measurement of how acidic (sour) or alkaline (sweet) your soil is. The scale used for the measurement ranges from 0 (most acidic) to 14 (most alkaline). Extreme readings at either end of the scale are rare in garden soils. In the garden, acidic soils usually have a pH of 4 to 6.5, alkaline soils 7.5 to 9. A reading of 7 is considered neutral.

Why Is Soil pH Important?

Soil pH is not a nutrient, but it governs the availability of nutrients to your plants. Just because nutrients are present in a patch of ground, that doesn't mean that your plants will make use of them and thrive. The nutrients must be available to your plants, and, to be available, they must be soluble. They will not be soluble if the soil pH is a mismatch for the plant in question.

Most plants fall into either of two categories:

  • Those that prefer soil that is neutral to slightly acid
  • Those that prefer soil that is neutral to slightly alkaline

But some plants need to be matched up with an alkaline soil to be happy, while others are acid-loving. The plants that are fussier about soil pH may require a little more work on your part: You will have to correct the soil pH for them to thrive. But a little extra work is a small price to pay to avoid the letdown of being disappointed with a plant's performance.

What Causes High Soil pH?

There are many possible factors behind the raising or lowering of soil pH. Two of them are climate and rainfall, and you can't control either of them.

When it comes to the possible causes of a high soil pH, there is one that you can certainly control: over-liming. "Over-liming" refers to an instance where, to correct overly acidic soil, a gardener in the past has applied too much garden lime to the soil to try to raise the soil pH.

But sometimes a high soil pH isn't your fault. Even geography can be to blame. If you live in parts of the western U.S., your soil can be alkaline simply because of the chemicals present in the ground there. Likewise, if you're forced to use hard water to irrigate with, this can increase soil alkalinity.

What Causes Low Soil pH?

As with high soil pH, so with low soil pH: Sometimes, it isn't your fault; other times, it is. There may be naturally occurring chemicals in the ground that are to blame. On the other hand, the problem could also be that you've over-corrected a soil that is too alkaline.

The ground in regions that get a lot of rain can be acidic because the rain leaches calcium, magnesium, potassium, and sodium out of the ground, after which the acidifying elements, hydrogen and aluminum, become predominant.

In the past, even very knowledgeable gardeners have beaten themselves up over a false alarm: They thought that using pine needles as a mulch led to the lowering of soil pH. This belief is now widely rejected.

How to Test Soil pH

Unless your soil pH is tested, you have no reliable way of knowing where your soil falls on the pH scale. And unless you have such a number to work with, you have no definite path forward to correct any possible mismatches between soil pH and your plants.

One way to have soil pH tested is to send in a sample to your local county extension office. They'll perform the test for you. When they return the results, they'll also provide recommendations (if applicable).

But you can also perform your own test. Make it a regular part of your fall garden checklist. You can buy a test kit at your local home improvement center. But, to save money, you can even conduct a test using only household materials.

How to Change Soil pH

Once you have a number to work with, you can change the soil pH if it is a mismatch for the plants that you are growing.

How to Raise Soil pH

Use garden lime to raise soil pH. This substance is a white rock powder and is sold by the bag at garden centers. While effective, be careful to apply it in proper amounts: Applying too much lime can eventually harm your plants.

After you test your soil in fall, apply lime immediately if the results call for it. Rototill it into the ground to the depths to which your plants' roots will be growing. 

How to Lower Soil pH

Lowering soil pH is easier than raising it. The substance you need to add to the ground is aluminum sulfate. Conveniently, this substance is found in fertilizers designed for acid-loving plants such as azaleas (Rhododendron spp.). So, in the normal course of fertilizing such plants, you will automatically be lowering their soil pH.

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  1. Myth vs. reality: What’s the truth behind some common gardening practices? Oregon State University Extension Office.