Understanding Soil pH: Here's What Every Gardener Needs to Know

If you notice your plant's health is declining, one of the reasons could be that the soil pH is out of balance. The importance of soil pH cannot be overstated—the pH level of your soil can make or break the health of everything growing in your yard.

While you don’t need to have a background in chemistry, it is important to grasp the basics of soil pH, including why it is so important for plant growth, how to test the pH level of your soil, and how to lower or increase the soil pH if necessary. 

What Is Soil pH?

Soil pH is a measurement of how acidic (sour) or alkaline (sweet) your soil is. The abbreviation "pH" stands for "potential hydrogen” and indicates the concentration of hydrogen in the soil. The higher the amount of hydrogen in the soil, the lower the soil pH.

The scale ranges from 0 (most acidic) to 14 (most alkaline). It's rare to see extreme readings at either end of the scale in garden soils.

In the garden, acidic soils usually have a pH of 4-6.5, and alkaline soils have a pH of 7.5-9. A reading of 7 is considered neutral. The best pH for most soils or the most common is around 6.5. 

Why Is Soil pH Important to the Garden?

Plants obtain 14 of the 17 essential nutrients from the soil, and whether those nutrients are available for the plants depends on the soil pH.

Most nutrients are more soluble in slightly acidic soils than in neutral or slightly alkaline soils. The golden in-between for most plants to thrive is a pH range of 6 to 7. With this measurement, nutrients are the most available. Keeping the soil pH balanced is just as important as adding organic matter and nutrients to the soil. If there's a micronutrient deficiency in a plant, it's usually not because there isn't enough of the nutrient in the soil—it's because the soil pH limits the plant's ability to absorb it.

However, there is no one-fits-all pH for all plants. Most plants prefer soil that is either neutral, slightly acidic, or slightly alkaline, but there are also plants whose pH requirements are more pronounced. If the soil pH is a mismatch, acid-loving plants and plants that require alkaline soil will do poorly.

The other reason why the proper soil pH is key is that it affects the activity of beneficial microorganisms in the soil. In a strongly acidic soil, the bacteria that decompose organic matter in the soil cannot do their job, hence organic matter builds up in the soil and sequesters nutrients, especially nitrogen, so it’s not available to the plants.

What Determines Soil pH?

Several factors impact soil pH. In the western parts of the United States, surface soils tend to be naturally more alkaline due to lack of moisture and higher temperatures. In the eastern parts of the country, on the other hand, there is more precipitation that pushes lime into the subsoil levels, so the surface soil is more acidic.

In addition to climate and rainfall, there are also manmade factors that impact the soil pH. One of them is over-liming when, to raise an acidic soil pH, too much garden lime has been applied.

Accumulated leaves and pine needles from trees and other decaying organic matter also lower the soil pH. However, contrary to widespread belief, using a moderate amount of pine needles as mulch does not lower the soil pH. Pine needles can slightly raise the acidity of soil if they're worked into the soil, not just used as a mulch.

How to Test Soil pH

Without testing your soil pH, you have no way of knowing where your soil falls on the pH scale.

You can get your soil tested through your local county extension office. When you send it in the soil samples, make sure to specify what type of plants you are growing or planning to grow in the soil because, in addition to the test results, you will also get specific recommendations for raising or lowering the pH if needed.

You can perform a pH balance test at home using a kit or a pH meter from a garden supply store. It is even possible to conduct a test using only household materials.

How to Change Soil pH

After a pH test reveals mismatches between soil pH and your plants, here’s how to adjust the soil pH:

How to Raise Soil pH

Use garden lime to raise the soil pH. Garden lime consists of finely ground white limestone and it is sold by the bag at garden centers.

While effective, be careful to apply it in proper amounts: applying too much lime will eventually harm your plants.

After you test your soil in the fall, apply lime immediately if the results call for it. Rototill or turn over the soil with a shovel to the depths to which your plants' roots will be growing. 

How to Lower Soil pH

Lowering soil pH is easier than raising it. There are different amendments: sulfur, iron sulfate, aluminum sulfate, or ammonium sulfate. They differ in terms of how long they last, what plants they work on best, and when they are added to the soil.

Another alternative is to use a fertilizer for acid-loving plants such as azaleas, which feeds the plants and lowers the pH at the same time.