ISSUED: 5-61
Monroe Rasnake and Lloyd Murdock
Department of Agronomy

What are Acid Soils?
Soils that contain higher levels of active hydrogen and/or aluminum in relation to calcium and magnesium are acidic. The degree of acidity is expressed in terms of pH. A pH of 7.0 is neutral. Below 7.0 is acidic and above 7.0 is alkaline. Each change in pH unit represents a 10-fold change in acidity. For example, a soil with a pH of 5.0 has 10 times more active acidity than one with a pH of 6.0. Most crops grow best at soil pH values between 6.0 and 7.0.

Figure 1.-The pH Scale
3 4 5 6 7 8 9 10
Acidic Neutral Alkaline

The pH of Kentucky Soils
Most Kentucky soils are acid. Summaries of soil samples analyzed by the University of Kentucky Soil Testing Laboratories indicate that about two-thirds of the state's crop and pasture land needs to be limed. However, liming fields that do not need it wastes money and can lead to problems with nutrient availability. Therefore, lime applications should always be based on a soil test. Refer to Extension publication, AGR-16, Taking Soil Test Samples, or consult with your county agriculture Extension agent to learn how to take soil samples.

Benefits of Liming
Lime application neutralizes soil acidity, raises soil pH and adds calcium and magnesium to the soil. Liming also decreases the plant availability of elements such as aluminum and manganese which can be toxic to plants. Figure 2 illustrates this effect of soil pH. The figure also shows how increasing the pH of an acid soil increases the availability of some nutrients like molybdenum and phosphorus but decreases the availability of others like zinc and iron.

Figure 2.

The range of soil pH where nutrient availability is best balanced is between 6.0 and 7.0. Outside this range, one or more nutrients may become deficient. At low pH levels, calcium, phosphorus, magnesium and molybdenum may become deficient, while at levels of 7.0 or above, manganese and zinc may become deficient. For example, zinc deficiency of corn has been observed in Kentucky at high soil pH levels especially when available phosphorus is also high. Manganese deficiency of soybeans has been a problem on some high pH soils in West Kentucky. However, manganese toxicity due to low soil pH is a more wide-spread problem. This is particularly true in tobacco production where high rates of fertilizer can rapidly reduce soil pH, making soil manganese more soluble and plant available. An initial soil pH of 6.6 would avoid this problem in tobacco.
Liming acid soils improves the environment for beneficial soil microorganisms. Liming promotes a more rapid breakdown of organic materials in the soil, releasing nutrients for growing plants. Liming also promotes nodulation by nitrogen-fixing bacteria in legume crops like soybeans, clovers and alfalfa.
Liming may improve the activity of some herbicides. A problem sometimes occurs with weed control in continuous no-till corn. The use of surface applied nitrogen fertilizers for several years results in an acid layer at the soil surface. The effectiveness of triazine herbicides (ex.atrazine) can be reduced by this low surface soil pH. The problem can be corrected by adding lime or by tilling. Certain soybean herbicides are also sensitive to soil pH. Damage to the current crop as well as carryover damage to subsequent crops can occur if soil pH is out of the recommended range of the crop being grown and/or the herbicide being used.

When to Lime
Lime can be applied at any time and with adequate soil incorporation and moisture, a measurable pH change can occur within 4 weeks. However, it takes 6 to 12 months for a significant amount of the lime to dissolve and make the desired change in soil pH. For this reason, lime should be applied at least 6 months before the crop is to be planted. Fall is a good time to apply lime so it can be dissolving during the winter. Also, fall weather is usually better for getting on the land with spreading equipment.

Lime Sources and Quality
The most common source of lime for agricultural use is ground limestone. Limestone's quality is determined by its purity and fineness of grind. Kentucky farmers are fortunate to have limestone of high purity available in most areas of the state. How fine the lime is ground is just as important as stone purity. Kentucky regulations specify that "agricultural lime" must be ground fine enough that 90% will pass a 10 mesh screen and at least 35% will pass a 50 mesh screen. This is a minimum fineness for the lime to be effective in neutralizing soil acidity. Summaries of recent lime test results show that between 10 and 20% of the samples tested failed to meet the regulations.
A system was recently developed to combine the values of purity and fineness into a single value to indicate overall lime quality (see Cooperative Extension publication, AGR-106). This value is called the relative neutralizing value (RNV) and estimates the percent of added limestone that will dissolve in a 3 to 4 year period. The higher the RNV, the higher the lime's quality. Lime whose RNV is 80 will require a smaller amount to reach and maintain a desired pH than one whose RNV is 60. The average RNV in Kentucky is about 67. County Extension agents have information on the RNV levels of agricultural lime being sold in Kentucky.
Other liming materials are sometimes available in an area. These are usually by-products of industry or liquid suspensions of finely ground limestone. Use of these materials should be based on their purity (expressed as percent CaCO3) and fineness. With suspensions, the actual amount of lime in the mix determines the liming value. For example, a ton of lime suspension may contain only 1000 lb of lime. The rest is water and suspension agents.
Specialty products like bagged, finely ground limestone, pelletized lime, hydrated lime, ground oyster shells and others are available. These are usually more expensive but are convenient to use on small areas. Wood ashes can also be used to increase soil pH. Be careful in using these products so that an area is not over-limed.

Table 1. Characteristics of Some Liming Materials
Name Advantages Disadvantages RNV
Agricultural Limestone Inexpensive, long-lasting, handles easily, good quality Slow acting 50 to 100
Finely Ground Lime and Pelletized Lime Convenient, fast acting More expensive 80 to 100
Suspension Lime  Convenient, good quality, fast acting Expensive, doesn't last** 40 to 75*
Burned Lime  Fast acting, high RNV Difficult to handle, expensive, doesn't last** 150 to 185
Hydrated Lime  Fast acting, high RNV Difficult to handle, expensive, doesn't last** 125 to 145
Ground Oyster Shells Convenient Expensive, not widely available 90 to 95
Wood Ash Inexpensive, fast acting, contains potassium Danger of overliming, not commercially available 30 to 70
* Based on total weight of suspension.
** Lime sources are fast acting and usually applied at lower rates than agriculture lime which reduces their carryover effect.

Lime must be spread uniformly to do a good job of neutralizing soil acidity. Most lime in Kentucky is spread by trucks equipped with spinner blades. These do a fair job of spreading. Distribution can be improved by spreading half in one direction and the other half at a 90 degree angle. Trailer type spreaders that allow the lime to flow through by gravity do a good job of spreading but are slow and have a high labor requirement. Suspension applications result in a quite even distribution; however, this form of lime is usually much more expensive and requires very high rates of suspension for high rates of lime.

For maximum effectiveness, lime should be mixed with the soil. This is especially true when the soil pH is very low and a large application of lime (4 tons/acre or more) is needed. A good method in this case is to apply half the lime before tillage and the other half after. Lime cannot be mixed with the soil when topdressing pastures, hay fields or no-till fields. In those situations, surface applications will work but they will take longer to affect acidity below the surface.