Agronomist Explains the Basics of Soil pH and Lime

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Guest column by Jagathi Kamalakanthan, NCDA&CS soil lab agronomist

RALEIGH — Soils in our humid southern climate are naturally acidic and some old-timers often referred to soil needing to be “sweet” for good plant growth. This expression most often relates to the use of lime to correct our acidic soils.

Lime is commonly used by farmers and home gardeners to help soils obtain a proper pH level for optimal growth.

Acidity is usually referenced by the pH scale which indicates the intensity or concentration of acid. Values range from 0 to 14, with lower values being more acidic. Above a pH of 7, we refer to conditions being basic or alkaline. Pure water has a pH of 7, which is referred to as neutral, having an equal balance of acid and base. Vinegar, which is acidic, has a pH between 2 and 3. Bleach is a known basic/alkaline solution with a pH of around 12.

Lime is used to raise pH and lower soil acidity, plus it supplies calcium and magnesium depending on the source used. Too much soil acidity injures plant roots and affects their ability to take up nutrients and water. A lower than desired pH lessens nutrient availability and decreases populations of soil microorganisms that benefit plant growth.

The proper pH for plant growth is one that keeps the right balance of nutrients available for plant growth without being too acidic for root injury. This varies for different plants.

Tomatoes, boxwoods, and roses are plant species known to prefer pH near 6.5. Flowering annuals and most other vegetable plants grow well at a pH level at or near 6.0. Typically, a pH range of 6.0 to 6.5 is considered preferable for most plants, except for acid-loving ones.

Blueberries, azaleas, and rhododendrons are among the plants that desire a pH of around 5.0. Most turf grasses do well at pH level of 6.0 to 6.5, except for centipede where a 5.5 is preferable.

A question we often get asked is, “if I know my pH, how much lime do I have to add to get a pH of 6.0?” There is no way to accurately predict lime needs based only on soil pH. Soil pH is an indicator of soil acidity, but it is not a measure of potential acidity coming from the soil itself. The actual amount of lime will vary depending on the soil’s physical composition ─ sand, silt, and clay plus the abundance of soil organic matter. “An accurate lime rate can only be determined by a soil testing lab that measures the acidity associated with the surface of soil particles, in addition to soil pH,” Kamalakanthan said.

Lime can be applied at any time of the year. Since lime takes about a year to fully react, it is ideally best applied a few months ahead of planting. “It will start reacting as soon as applied, especially if mixed with the soil,” Kamalakanthan said. Here in North Carolina, you can apply lime any time of the year. If needed, liming ahead of planting a flower bed or vegetable garden is certainly advised. If soil pH is very low, surface applications of lime will not provide as much benefit as mixing into the soil.

Another misleading belief is that lime cannot be over-applied. Lime should typically not be applied every year and the correct rate is important. Usually, soils will need lime application every two to four years. This will vary depending on soil type and management. Soil acidity in very sandy soils typically re-occurs faster than in more clayey soils, so sandy soils need to be tested for lime needs more frequently (every two years) than clay soils (every three to four years).

Lastly, let’s discuss lime sources. Calcitic lime contains only calcium limestone and dolomitic lime contains calcium and magnesium limestones. Magnesium is an essential nutrient and can be needed especially in sandy soils of our coastal plain region. Calcitic lime can best be used in soils with more clay that better hold nutrients. Soil testing can help identify the need for magnesium to help determin which lime source is best. To correct soil acidity, consumers should purchase only products sold as lime or limestone. Products that contain calcium, such as gypsum, do not correct soil acidity.

Now is a great time to get your soil tested since your samples are analyzed for free and turnaround times are often less than two weeks.

More information on soil testing.

Written By:


Jagathi Kamalakanthan, agronomist
N.C. Department of Agriculture & Consumer Services’ Agronomic Services Division