By Rick Thomason

University of Tennessee
Johnson County Extension Director

Although lime can be applied whenever soil, weather, crop and labor conditions permit, fall is an excellent time for spreading.  Fields are usually dry, lime dealers are less rushed and growers are not occupied with spring planting.  In addition, the physical condition of the soil is generally much better in the fall than in the spring. This will be advantageous in supporting the weight of the lime spreaders and minimizing the likelihood of doing structural damage to the soil as well as getting stuck in fields.  Any compaction or wheel tracking damage done in the fall will have winter freezing and thawing cycles to help alleviate some of these adverse effects.
Soil test results indicate that approximately 40 to 60 percent of the cropland in Tennessee is too acidic for optimum crop production.  Because of this, determining the need for lime by soil testing should be the first step in developing a sound crop fertilization program.  Soil testing should be done annually where high value row crops are being produced and once every 3 years on other fields (such as hay and pasture), lawns and gardens.
What is Soil Acidity? Soil acidity refers to the level of acids present in soils. As acid levels increase, the pH of the soil decreases.  While the pH scale ranges from 0-14, most Tennessee soils range in value from 4.5 to 7.5.  Soils with pH values greater than 7.0 are alkaline or sweet, and those with values of less than 7.0 are acid or sour.  As the soil pH decreases below 7.0, the amount of acidity rapidly increases.  For example, a pH of 5.0 is 10 times more acidic than 6.0 and 100 times more acidic than a pH of 7.0.    Acid levels increase as basic nutrients (calcium, magnesium and potassium) are replaced by hydrogen through soil erosion, leaching and crop removal.  In addition, the use of acid-forming fertilizers greatly enhances acid levels.
What Does Lime Do?  Applying lime to acidic soils provides the following benefits:
1.      Reduces amounts of soluble aluminum and manganese to non-toxic levels.  As soil pH increases, the amount of aluminum and manganese that can be toxic to plants decreases.
2.      Supplies calcium, and if dolomitic limestone is used it supplies both calcium (Ca) and magnesium (Mg), which are essential plant nutrients.  Also, the availability of secondary and micronutrients is about optimum in the pH range of 6.1 to 6.5.
3.      Increases the efficiency of nitrogen (N), phosphorus (P) and potassium (K) use by plants.  For example, the efficiency of applied phosphate may be more than doubled when soil pH is increased from 5.0 to 6.1 because of less fixation or tie-up in the soil.
4. Increases the availability of molybdenum (Mo), which is important for nitrogen fixation in legumes.
5.      Enhances microorganism activity.  Acidic soils slow the growth and multiplication of certain microorganisms, which in turn reduces soil processes such as the release of nutrients from organic matter decomposition (mineralization) and nitrogen fixation in legumes.
6.      Improves the effectiveness of certain herbicides when used at the right rate of application. Several herbicide families are soil pH dependent.
7.      Increases yields and profits.
*Source: UT Extension Publication PB1096, “Liming Acid Soils in Tennessee”