Agricultural limestone is the main product used to adjust soil pH. However there are also a number of lime by-products such as wood ash and various materials coming out of pulp/paper mills available. How does a farmer know if a lime by-product is sufficient to use on their crop land and if it is worth the money they are paying? To answer that, let’s take a look at what lime is and what types of characteristics make products good to use to raise soil pH.
Agriculture lime is a material that contains calcium and/or magnesium compounds that are capable of neutralizing the soil acidity. Ag limestone can be calcitic or dolomitic (contains at least 6 % magnesium) and regular ground and finely ground depending on particle size. There are three factors that determine limestone quality: chemical composition, particle size and moisture content. The chemical composition determines the materials acid-neutralizing value. Pure calcium carbonate is the standard for all liming materials. It has a Calcium Carbonate Equivalent (CCE) or neutralizing value of 100 percent. Looking at the magnesium content is also important. Particle size affects the speed at which the liming material dissolves. They measure the particle size by standard size sieve mesh. Each type of lime has different size requirements. You want a lime material with various sizes so you have some lime available immediately and some over the period of the growing season. The 100 mesh dissolve immediately, 60 to 90 mesh can take up to a year and the 20 mesh up to two years. The last factor is the moisture content. The percent moisture determines how much of the chemical reactive material has been replaced with water. High moisture reduces the effectiveness of liming material on a weight basis. Moisture also affects the ease of application; too low and it is like dust. According to the Georgia Department of Ag Lime law, limestone sold in Georgia cannot exceed 15 % moisture and most do not exceed 8 %.
Lime recommendations based on soil testing through the University of Georgia Ag and Environmental laboratory ion Athens bases the rate of lime required assuming regular ag lime is used. According to the Ga Lime law, ag limestone sold in Georgia must have a CCE of at least 85%. The Lime law was actually amended in 1996 to include “by-product” limes. By-product limes are any material other than Ag limestone that can be used as a liming source, for example wood ash and lime mud. Most by-products do not have as high a CCE as ag limestone and therefore require higher rates of application. For example if you have a wood ash with a CCE of 20 % then it would take approximately 4 tons/a to equal a ton of ag lime per acre that has a CCE of 85 %. Unusual sources may contain other elements or chemicals that may be harmful to plants. They can also have large chunks or be very wet which can affect the amount you will need and the ease of application. All lime by-products used in Georgia are supposed to be approved by the Ga Department of ag and state the CCE on the labels.
The bottom line is when looking to purchase lime for your farm you need to evaluate: available lime sources in the area, hauling cost, length of time between applications and planting, degree of soil acidity and the need for magnesium. To look at the Georgia Regulations on liming materials visit this link. To read more on alternative lime sources and how each one may affect your farming practice take a look at this publication from Virginia Cooperative Extension: Source of Lime for Acid Soils.