Lime Abuse and Neglect - A Serious Problem

I am often asked, "Does this crop need lime?" I have also heard such statements as "I'm going to lime my yard this year because so and so down the road did his and it looks a whole lot better" or "I'm gonna throw some lime by my tomato plants this year because I've been having trouble with blossom end rot." It frustrates me to hear such statements because the use of lime is such a basic practice to the successful growth of plants, yet it is so often misunderstood. I hope that with the following information, you will gain a better understanding of the purpose of lime and how to use it correctly.

The only way to correctly answer questions, like the ones above, is to have the soil tested to determine the need for lime and/or the amount of lime to be applied. In North Carolina, we can get our soil tested with no fee by the NCDA. Even though we have the benefit of this service, too many people don't take advantage of it. Perhaps if people understood why we lime, what lime is and what determines the liming recommendation, then maybe they would take advantage of the soil testing service.

Lime is not a fertilizer! Unlike fertilizer, lime is not a material that we apply to the soil to be taken up by the plant. Instead, we apply lime to the soil in order to change the soil from an acid to a near neutral condition. Yes, it's true that various forms of liming materials will provide some minor nutrients, however, you should only use lime for this purpose if and only if the soil pH needs to be raised.

Lime is an alkaline material and when alkaline and acidic materials come in contact with one another, each material tries to neutralize the other. A unit of measurement called pH is used to record whether a substance is acidic, alkaline, or neutral, and it also determines the strength of acidity or alkalinity. The pH scale reads from zero to fourteen with seven being neutral, zero to seven considered acidic, and seven to fourteen considered alkaline or base.

Most plants, but not all, grow best in soils with a pH range between 5.0 and 7.0. As pH levels in the soil begin to drop below 5.0, toxic elements such as aluminum and manganese become more available to plants. Essential elements such as nitrogen, potassium, phosphorus, and others become less available to plants. Beneficial biological activity is hindered as well. As pH levels rise above 7.0, elements such as phosphorus, copper, and zinc are less available.

At least five factors are used to determine the lime requirement and to make a lime recommendation. These factors include the present pH, desired pH, soil acidity, soil type, and the crop. The pH of the soil is measured and if this reading is below the desired pH level, then a lime application is recommended. The desired pH is based on the soil type and the specific crop requirements.

Soils are placed into three classes, based on the amount of organic matter that is present - mineral, mineral- organic, and organic. Soils that have more organic matter tend to have lower aluminum content. Thus, for most plants, a lower pH level can be allowed. Mineral soils contain more aluminum, therefore, a higher pH is recommended to avoid aluminum toxicity. For most plants, the desired pH level in a mineral soil is 6.0, 5.5 for a mineral-organic soil, and 5.0 for organic soils.

Some crops have specific pH requirements that are also taken into account when determining desired pH.

Acidity is the term used to express the quantity of hydrogen and aluminum in the soil. Along with the present pH and the desired pH, the acidity is used to determine the amount of lime required to raise the pH to the desired level.

Because of heavy rainfall, North Carolina soils are highly weathered and are naturally acidic. Therefore, correcting soils with low pH levels is more common than correcting soils with high pH levels. Occasionally, a soil with a pH reading that is higher than the desired pH level will be found in North Carolina. This can happen when someone overlimes their soil. Sulfur is the material recommended to bring the pH levels down to an acceptable level. Sulfur is more expensive than lime and overliming can easily happen when it is applied without knowing how much it needs, therefore, all the more reason to encourage people to have their soil tested to determine its needs.

For more information about soil acidity and proper lime use, contact the Cooperative Extension Service and ask for publication number AG-439-17.

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For questions regarding lime, e-mail me!

Date Created 10/15/97.
Last revised on 2/26/02.