## Introduction

Due to the humid southeastern climate, South Carolina soils are highly weathered, resulting in a natural acidic (pH < 7) and low fertility status. As a consequence, there are relatively low levels of basic cations (calcium – Ca2+, magnesium – Mg2+, and potassium – K+) and relatively high amounts of acidic cations (hydrogen – H+ and aluminum – Al3+) present, especially in the high-clay sub-soil horizon (figure 1). Periodic lime applications to correct soil acidity are essential for optimum crop production. Application of dolomitic lime enhances the availability of essential plant nutrients in the soil by increasing pH through neutralization of acidity and adds Ca2+ and Mg2+ to maintain crop productivity.

Figure 1. A typical ultisols (highly weathered red soils with a clay-rich B horizon) soil profile of the southeastern Coastal Plain that contains an appreciable amount of translocated silicate clay in the subsoil horizon. Image credit: USDA NRCS.

All liming materials are not equally effective in neutralizing soil acidity. The goals of this publication are to inform growers, crop advisors, and Extension Agents about how to evaluate liming materials and choose the most effective and economical lime source and appropriate rate for their crop production system.

The South Carolina Lime Law requires distributors to provide

1. The net weight of the agricultural liming material.
2. The brand or trade name of the material.
3. The type of agricultural liming material. For example, if it is ‘dolomitc limestone’ or ‘calcitic limestone.’
4. Minimum calcium carbonate equivalence.
5. The minimum percentage of calcium and magnesium expressed as elemental calcium (Ca) and elemental magnesium (Mg). Calcium and magnesium also may be expressed as oxides or carbonates in addition to the elemental expression.
6. The minimum percent by weight passing through United States standard sieves.
7. The name and principal office address of the manufacturer or distributor.

Liming materials vary in their ability to neutralize soil acidity due to their physical and chemical properties. The physical property termed ‘fineness’, relates to particle size and identifies how easily the material will break down and dissolve. Chemical properties of a liming source reflects the neutralization value to soil acidity and is also called calcium carbonate equivalent (CCE).

## Calcium Carbonate Equivalence (CCE)

As a first step in choosing a lime source, growers should consult the label on the liming container and note the Calcium Carbonate Equivalence CCE because it can differ among products even with the same main source (table 1). The pure calcium carbonate (CaCO3) also called pure calcitic lime is given 100% CCE. A CCE greater than 100 indicates a higher neutralizing power than pure CaCO3. Soil testing labs make recommendations based on lime source and CCE. For example, the lime recommendations from Clemson University’s Agricultural Service Laboratory are based on 85% CCE of dolomitic limestone.

Table 1 lists the potential CCE of various liming materials. In South Carolina, ground dolomitic limestone is the most commonly used liming source due to its relatively low price and transportation costs from its source in Tennessee; it is the cheapest source of Mg which is prone to quickly being lost in sandy surface soils. Liquid lime, which is finely ground lime suspended in water, is not typically used in field production due to its high cost; however, it is a good source to treat low pH irrigation water.

Table 1. Composition and calcium carbonate equivalence (CCE) of selected liming materials.

 Fertilizer/Liming Materials Composition CCE Burnt lime CaO 150-179 Slaked/hydrated lime Ca(OH)2 120-136 Dolomite Limestone (Lawrenceburg, TN) CaMg(CO3)2 79.3 Dolomite Limestone (Austinville, VA) CaMg(CO3)2 107.3 Pelletized Dolomite Limestone (Austinville, VA) CaMg(CO3)2 96.4 Calcitic Limestone (Lacey’s Springs, AL) CaCO3 100.9 Calcitic Limestone (Arlington, GA) CaCO3 85.2 Slag CaSiO3 60-90

Adapted from Kunhikrishnan et al., 2016 and Yang et al., 2018.1,2

## Fineness Factor

The crushing and screening of agricultural limestone produces particles of different fineness which then determines the solubility of the material. Smaller particles are more soluble due to a greater surface area for reaction. The regulations set forth by the Fertilizer Board of Control in South Carolina require each container of liming material to have a minimum 90%, 50%, and 25% weight of the liming material to pass through mesh size 10, 50, and 100, respectively. The Fertilizer Board of Control provides additional information about liming material rules, regulations, and standards. Most companies provide guaranteed minimum chemical and sieve analyses in the following format (table 2):

Table 2. Guaranteed minimum chemical and sieve analysis example.

 Guaranteed Minimum Chemical Analysis (CaCO3) ………………………………………………………………………………………………. 86.00% Calcium carbonate equivalence (CCE)…………………………………………………….. 90.26% Guaranteed Minimum Sieve Analysis Screen/mesh size % Passing 8 mesh 100.00% 10 mesh 100.00% 20 mesh 100.00% 40 mesh 95.00% 50 mesh 93.00% 60 mesh 88.00% 100 mesh 80.00%

The proportion of material passing through mesh size 10 (figure 2) to 50 (figure 3) is considered half as effective (soluble) as finer material that passes through mesh size 50. A material coarser than mesh size 10 is expected to have no effective lime value, so its proportion is not considered in the calculations. The information in table 2 can be used to calculate the fineness factor for the liming material according to the following equation:

Fineness factor (%) – Equation 1 = ½ x (proportion of material between mesh size 10 and 50) + 1 x proportion of material passing mesh size 50.

Figure 2. Mesh size 10 (2 mm sieve holes). Image credit: Bhupinder S. Farmaha, Clemson University.

Figure 3. Mesh size 50 (0.297 mm sieve holes). Image credit: Bhupinder S. Farmaha, Clemson University.

Example fineness factor calculation using Equation 1 and data from table 2:

Fineness factor (%) – Equation 1 = ½ x (100 – 93) + 1 x 93 = 3.5 + 93 = 96.5

Equation 1 can be simplified, becoming Equation 2 as follows:

Fineness factor (%) – Equation 2 = ½ x (proportion of material passing mesh size 10 + proportion of material passing mesh size 50)

Fineness factor calculations based on Equation 2 and data from table 2:

Fineness factor (%) – Equation 2 = ½ x (100 + 93) = 96.5

The results using Equations 1 and 2 are the same, but there is an advantage in using Equation 2 because liming products contain the % material passing through mesh sizes 10 and 50.

## Relative Neutralizing Value

The CCE and fineness factor are combined into the following equation to calculate relative neutralizing value (RNV), an index that indicates the relative performance/value of a liming material to correct low soil pH.

For a lime with CCE of 85% and fineness factor of 96.5%, the RNV will be:

For a lime with CCE of 107.3% and fineness factor of 96.5%, the RNV will be:

It might be necessary to make an adjustment to lime recommendations. For example, assume your soil test report recommends two tons per acre of agricultural limestone that has an RNV of % based on 85% CCE and 96.5% fineness factor. However, you want to use a different liming material that has an RNV of 103.5% based on 107.3% CCE and 96.5% fineness factor. In this case, the rate of purchased lime should be adjusted as per the following equation:

Adjusted lime rate – Equation 4 = Recommended rate of agricultural limestone per acre x RNV of agricultural

limestone / RNV of purchased lime source = 2 tons per acre x 0.82 / 1.035 = 1.6 tons per acre of purchased lime

## Comparing Lime Prices

It is important to accurately compare lime prices to make a cost-effective purchase. Continuing the example from above in the previous section of “Adjusting Lime Recommendations” to compare lime price, if the cost of the agricultural limestone that has an RNV of 82% is $45 per ton, and the lab soil report recommends two tons per acre, then the cost of material is$90 per acre. The other product, with an RNV of 103.5%, is $55 per ton, but now only 1.6 tons per acre are needed. The cost of higher RNV material is$88 per acre. The economic comparison of the two liming materials shows a savings of \$2 per acre by choosing the lime source that has an RNV of 103.5%, relative to the standard lime source that has an RNV of 82%.

## Summary

Lime application to manage low soil pH is important in South Carolina, considering elevated acidity of soils in the state. This practice neutralizes potentially detrimental soil acidity, supplies Ca and Mg for the plant depending on source, and improves availability of essential plant nutrients. To get correct lime recommendations from a soil test, a good representative soil sample from the recommended soil depth for the given cropping system is important. The Agricultural Service Lab at Clemson University provides guidelines on how to collect a soil sample. We encourage use of a sweatless soil sampler to take soil samples from the precise depth.3 In the selection process, growers need to evaluate lime quality before purchase. If needed, adjustments to the application rate should be made using the above RNV calculations showing in Equation 4. Additionally, growers should also find out soil-test Mg levels, crop Mg needs, and determine if a preferred material will meet Mg demands for the crop.

## Acknowledgments

We would like to extend our utmost appreciation to the following volunteer reviewers of the original version of this document:

Dr. Carl Crozier, Professor
North Carolina State University, Vernon G. James Research and Extension Center (Plymouth, North Carolina)

Mr. Charles Davis, Certified Crop Adviser and County Extension Agent,
Clemson University (St. Matthews, South Carolina)

Dr. David Hardy, Section Chief
North Carolina Department of Agriculture and Consumer Services, Agronomic Division (Raleigh, North Carolina)

Dr. John Grove, Professor and Director
University of Kentucky Research and Education Center (Princeton, Kentucky)

Dr. Shannon Alford, Director Agricultural Service Laboratory, Clemson University (Clemson, South Carolina)

## References Cited

1. Kunhikrishnan A, Thangarajan R, Bolan NS, Xu Y, Mandal S, Gleeson DB, Seshadri B, Zaman M, Barton L, Tang C, Luo J, Dalal R, Ding W, Kirkham MB, Naidu R. Functional relationships of soil acidification, liming, and greenhouse gas flux. Adv. Agron. 2016; 139:1–71.
2. Yang R, Mitchell CC, Howe JA. Relative neutralizing value as an indicator of actual liming ability of limestone and byproduct materials. Commun. Soil Sci. Plant Anal. 2018; 49:1144–56.
3. Smith WB. Depth control for sweatless soil sampler. Land-Grant Press by Clemson Extension. 2019; LGP 1004. http://lgpress.clemson.edu/publication/depth-control-for-sweatless-soil-sampler/.