Using Lime for Soil pH Correction

The pH range for optimal growth of most garden plants, showing the pH of various substances above and below that range.
The pH range for optimal growth of most garden plants, showing the pH of various substances above and below that range. | Source

Correcting pH in Soil

In order for plants to grow well, the soil pH should be within the range suited for each type of plant. Optimal pH levels vary for different garden plants. However, a pH range from 6.0-7.0 is suitable for most vegetable plants. At a pH range of 6.0 to 6.5, most of the plant nutrients in the soil are available to plant roots. Beyond this range, many nutrients become less available. So, it is important to know the pH of your soil if you are interested in getting optimal growth and yield.

Finding out how to "sweeten" acid soils with lime is not as simple as doing a pH test. As pointed out below, a certified laboratory can give you the information you can use to determine application rates for your soil. Once you have a value that you can use to calculate application rates, you can use a variety of materials to lime your soil. A table is provided below to compare materials.

Lime for Garden Soils

Lime is a mineral that consists mostly of calcium carbonate. It is mined from areas that used to be ancient seabeds, like in Central Texas, and it is crushed finely for making agricultural lime. This material is perfect for increasing the pH in acidic soils. The carbonate portion of the molecule replaces acidic hydrogen ions on soil particles to increase soil pH levels.

Another form of lime commonly used by gardeners is called dolomite. Dolomite is calcium magnesium carbonate. If soil tests reveal that your garden soil is low in magnesium, then this material should be used in place of lime. Dolomite is preferred in sandy soils with low organic matter.

A Limestone Quarry

Ancient seabed deposits provide a pure or nearly pure source of calcium carbonate.
Ancient seabed deposits provide a pure or nearly pure source of calcium carbonate. | Source

Application Rates for Lime

Lime application rates are based on a measure known as the exchangeable acidity (EA). Soil test reports provided by certified or state laboratories can provide this information for you. Ordinary pH values aren't enough to determine a soil's lime requirement. Pounds of calcium carbonate or dolomite needed per 1,000 square feet can be calculated from the EA value as follows:

  • For soils having an EA greater than 4, multiply the EA value by 19 to get the pounds needed to adjust the soil pH to 6.5 If you wish to adjust the pH to 7.0, multiply the EA value by 23.
  • If the EA value is less than 4 and the existing soil pH is less than 6.5, multiply the EA value by 46.

Maximum application rates, applied as calcium carbonate or dolomite, should not exceed 150 pounds per year. It is common to split applications over two years in cases where calculated rates exceed this amount. Sometimes applications are applied every 3 to 5 years.

Other sources of materials than calcium carbonate. If you are using other sources of lime, the application rates will vary. See the table below for equivalents. For instance, burnt lime is 1.5 to 1.8 times stronger than calcium carbonate, so you will have to divide the application rates calculated above by a number between 1.5 to 1.8.

Calcium Carbonate Equivalents of Other Materials Used for Increasing pH in Soils

Material
Calcium Carbonate Equivalent
Burnt lime (Ca0)
1.5 - 1.8 X
Hydrated lime (Ca(OH)2)
1.2 - 1.4 X
Ground oyster shells
0.9 - 1.0 X
Marl
0.7 - 0.9 X
Wood Ashes
0.4 - 0.5 X

Use Caution with Burnt Lime

Burnt lime is caustic, which means it can cause burns. When handling it, use gloves and eye protection!

Poll on Materials Used to Increase Soil pH

What material do you use to increase the pH of your soil?

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References

Clemson University: Soil Acidity and Liming.

Penn State Extension: Limestone Recommedation. Part 1, Section 2: Soil Fertility Management.

West Virginia University: Liming the Lawn.

Virginia State University: Sources of Lime for Acid Soils in Virginia.

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