What We Mean When We Talk About Soil pH

(News article for May 15, 2021)

When writing about growing various types of plants, I often mention soil pH. I thought it might be helpful to talk a little bit about what that means and some practical considerations.

For those not interested in the details, I’ll go ahead and make this point: You need a soil test to know what your soil’s pH is and how much, if any, lime or sulfur is needed to change pH to bring it into the optimal range for whatever type of plant you’re growing.

Soil pH is a measure of the acidity or alkalinity of the soil. Soil with a lower pH is more acidic, while soil with a higher pH is more alkaline. The pH scale ranges from 0 to 14. A pH value of 7.0 is neutral.

Plants have pH ranges within which they grow best. Many grow well between pH 6.0 and 6.5. Some plants common to our area grow best at a lower pH, or in a more acidic soil. Azaleas, blueberry plants, and centipedegrass are some plants that fall into the latter group.

Soil pH affects how available nutrients are to plants. Of the nutrients that plants take up from the soil through their roots, each nutrient has a range within which it’s most available to plants. For example, if soil pH is too high, iron is not sufficiently available to blueberry plants, and they show symptoms of iron deficiency (yellowing of new growth).

If pH is too low, plants can experience toxicity due the high availability of certain micronutrients and aluminum. For example, cantaloupe plants may show symptoms of manganese toxicity when soil pH is below approximately pH 5.8.

Besides affecting general plant growth and nutrient availability, soil pH affects the incidence or severity of some plant diseases. While white (“Irish”) potatoes can grow well in less acidic soils, they’re often grown at a lower pH (below pH 5.2) to reduce problems with the disease potato scab. On the other hand, tomato plants are more likely to experience problems with southern blight and southern bacterial wilt when pH is too low.

In most parts of the southeastern US, the pH of soil that has not been either limed or otherwise treated to change the pH is relatively acidic, with soil pH values between approximately 4.5 and 5.5. There are exceptions. In Louisiana, soil pH is naturally higher in some areas along the Mississippi and Red Rivers.

To make soil more favorable for the growth of specific types of plants, we can adjust soil pH. We often use ground limestone to raise pH and sulfur to lower pH.

Ground calcitic limestone is made up of calcium carbonate. Calcium carbonate is the liming material to which others are compared. When soil is low in magnesium, using dolomitic lime (ground dolomitic limestone) is suggested, as it contains both calcium carbonate and magnesium carbonate. It generally has a similar effect on pH, per weight, as calcitic lime.

More finely ground limestone changes pH more rapidly than more coarsely ground limestone. Gardeners often find pelletized limestone for sale. This tends to work more quickly than regular calcitic or dolomitic lime, because the particles that make up the pellets are very finely ground.

Other liming materials are available. Their rates must be adjusted to account for their greater or lesser effect on pH.

It takes a period of months for lime to have its full effect on soil pH. For planting in the spring, soil testing in the fall will allow time for lime application well in advance of planting. Likewise, for fall planting, you can do a soil test in the spring and go ahead and apply lime if needed.

Lime rates are typically much higher than fertilizer rates. An application rate of 1 ton per acre, or 44 pounds per 1000 square feet, is not uncommon.

When soil pH is higher than ideal for a type of plant, sulfur can be used to lower pH. As with lime, it’s best to apply and incorporate sulfur into the soil before planting. If adjustments are made after planting, one must be careful about the rate of sulfur used so that it does not injure plants. When reducing pH of established lawns, for example, it’s suggested to not apply more than about 9 pounds of sulfur per 1000 square feet at one time, even if more is needed to bring the pH to the recommended level.

A soil test is necessary to know what the pH of soil is and how much lime or sulfur, if any, is needed. Applying lime or sulfur without a soil test is not recommended, since applying either when not needed, or applying too much, can make the situation worse.

The same amount of lime or sulfur can change pH by different amounts depending on soil characteristics. For this reason, LSU’s Soil Testing and Plant Analysis Lab titrates soil samples to determine the expected pH with the addition of various rates of calcium carbonate or sulfur, to determine how much lime or sulfur to recommend.

Let me know if you have questions.

Contact Mary Helen Ferguson.

6/2/2021 5:12:49 PM
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