I’ve heard old-time gardeners describe soil as sweet or sour.  Sweet soil was used to describe alkaline soil, and sour soil was used to describe acid soils.  All of these terms refer to the pH of the soil.

Simply put, pH means “potential of hydrogen,” or the amount or concentration of hydrogen ions in a substance. It determines how acidic or alkaline a substance or solution is.

pH is measured on a numerical scale, with 7.0 being neutral. Above 7.0 means more alkaline, and below 7.0 means more acidic. Soil test kits or water test kits can be used to determine pH.

As growers, we are always watchful of pH levels in soils because different levels can affect how a plant can use the nutrients it is given.

pH levels that are too high can make nutrients such as iron, manganese, zinc and phosphorus less available to plants. pH levels that are too low can reduce the availability of calcium and magnesium. Very low pH levels can also increase aluminum and manganese to levels that may damage plants.

Nutrients are a balancing act. Too little or too much of one can cause problems with another. pH plays an important role in determining what nutrients are available to plants.

Think of pH as the gatekeeper. Your soil has nutrients, and pH regulates what nutrients are available to your plants.

Not all soil pH is acceptable to all plants.

Some plants prefer more acidic soil, like Camellias, Gardenias, Hydrangeas, Hollies, Blueberries, and Azaleas. Depending on the plant, these generally prefer soil pH levels between about 4.5 and 6.0.

Have you ever noticed that the old-fashioned mop-top hydrangeas can change colors from blue to pink?  This is an indication of soil acidity!  Pink usually means more alkaline soil, while blue flowers indicate more acidic soil. The reason is aluminum availability. In acidic soil, aluminum is more soluble and available to the plant, which helps produce blue flowers. As soil pH rises, aluminum becomes less available, and flowers tend toward pink.

Most general garden plants, flowers, herbs, shrubs, and trees do well in slightly acidic to neutral soils, generally ranging from pH 6.0 to 7.0.

Some plants can tolerate neutral to slightly alkaline soils. Many of these plants are found in areas that are drier, rocky, or limestone-based.  In areas where there is less rainfall, calcium and magnesium are less likely to be washed or leached from the soil. This can allow soils to remain neutral or alkaline.

Some common plants that tolerate neutral to slightly alkaline soils are Lavender, Rosemary, Thyme, and Sage.

So how do you know your soil pH?

You can’t determine pH by looking at your soil. You must have the soil tested to make sure.

There are several probe testers available on the market. Inexpensive garden-grade soil testers, in our experience as growers, are not extremely accurate, and this is one area where you need to be sure.  Commercial-grade soil testers can be more accurate, but the expense is often prohibitive for home gardeners.

For home gardeners, your local Extension office is one of the best options for testing your soil. The kits are easy to prepare, laboratory testing is reliable, and the results include recommendations based on what you are growing.

Your soil test will tell you the soil pH. If you list what you’re growing, the report can also provide recommendations for lime and fertilizer based on your soil test results and the plants you intend to grow.

UGA offers soil test kits you can order online. You can prepare the sample and send it back without leaving your home!

Visit the UGA soil testing resources for Georgia. The standard soil test kit is what you want.

Changing Your Soil pH

There are several methods for correcting your soil pH, but this is one task that needs to be done carefully. Once you’ve applied too much of something, you can’t simply take it back out of the soil. You have to correct the imbalance over time.

The amount of lime or sulfur needed depends on your current soil pH, the pH you’re trying to reach and the type of soil you have.

Sandy soils generally require less amendment to change pH than clay soils. Clay soils tend to resist changes in pH and may require more amendment.

This is another reason soil testing is important before applying anything.

The standard methods for changing soil pH are:

Soil too acidic → apply lime.

Soil too alkaline → apply elemental sulfur.

The chart below gives you more options. But a word of caution: anything you apply can have an effect on something else, so be sure of your soil pH before you apply anything.

Goal	What to Use	What It Adds	Speed	Best Use / Important Notes
Raise pH	Calcitic lime (calcium carbonate)	Calcium	Slow	Standard choice when soil pH is too low and magnesium levels are adequate.
Raise pH + add magnesium	Dolomitic lime	Calcium + magnesium	Slow	Best when soil pH is low and a soil test indicates magnesium is low.
Add calcium without significantly changing pH	Gypsum (calcium sulfate)	Calcium + sulfur	Moderate	Useful when calcium is needed but you do not want to raise soil pH.
Add magnesium without significantly changing pH	Epsom salt (magnesium sulfate)	Magnesium + sulfur	Fast	Corrects a confirmed magnesium deficiency. Should not be routinely applied without evidence of deficiency.
Lower soil pH long-term	Elemental sulfur	Sulfur	Slow	Usually the preferred amendment for gradually lowering the pH of in-ground soil. Microorganisms convert it into sulfuric acid.
Lower soil pH quickly	Aluminum sulfate	Aluminum + sulfur	Fast	Acidifies soil more quickly than elemental sulfur, but repeated or excessive use can increase soluble aluminum and salts. Best used selectively.
Lower pH temporarily / acidify irrigation water	Citric acid	Organic acid	Immediate, short-lived	Useful primarily for acidifying irrigation water or nutrient solutions. Its effect on soil pH is generally temporary because microbes break it down.