Liming Soils in Mississippi

The natural condition of most Mississippi soils is to become more acidic over time which reduces crop yields. From 1989 to 2009, just over half the soil samples analyzed by the MSU Extension Service Soil Testing Laboratory had pH values less than 5.9. Historically, most Delta soils have not been acidic or needed lime, however 36% of the samples from that region over the same period were less than 5.9.

Lime requirements are measured in a separate procedure from pH. On Mississippi State University Extension Service Soil Testing Laboratory reports it is the tons of lime needed per acre. Other laboratories may report a buffer pH value.

Annually, about half the soil samples processed by the Mississippi State University Extension Service Soil Testing Laboratory indicate a lime requirement.  Most lime used in Mississippi, other than marl (also called chalk), must be imported from other states, so liming programs can be expensive.

Why Liming is Important

Phosphorus and Mo are less available to plants in strongly to weakly acid soils; conversely Al and Mn can become available to the point of plant toxicity in strongly acid soils. Some micronutrients, including Fe, Mn, and Zn are less available as soil pH increases above 6.

Most crop plants are less productive in acid soils, so lime may be necessary to:

• Prevent aluminum and/or manganese toxicity,
• Increase phosphorus and molybdenum availability,
• Improve nitrogen fixation by legume crops,
• Improve the efficiency of applied phosphorus and potassium fertilizers, and
• Increase the volume of soil explored by roots.

Liming Materials

“Agricultural liming materials” are products containing calcium and magnesium compounds capable of neutralizing soil acidity. A limestone is a material that is primarily calcium carbonate or a combination of calcium carbonate with magnesium carbonate that can neutralize soil acidity. The lime essentially floods the soil with alternative cations.

Two types of hard limestone transported from other states are widely available in Mississippi. Calcitic limestone, or calcite, is calcareous rock composed of calcium carbonate. Dolomitic limestone, or dolomite, is calcareous rock containing both calcium and magnesium carbonates, and has at least 6% magnesium content. A softer material mined within the state, “marl” or alternatively “chalk” is granular or loosely consolidated earthy material that is largely sea shell fragments and calcium carbonate.

Other available lime materials:

• “burnt lime” is made from limestone that consists calcium oxide or a combination of calcium oxide with magnesium oxide;
• “hydrated lime” is made from burnt lime, that consists of calcium hydroxide or a combination of calcium hydroxide with magnesium oxide and/or magnesium hydroxide;
• “ground shells”  are ground mollusks;
• “basic slag” or other industrial byproducts that neutralize soil acidity; or
• “pelletized lime”, finely ground limestone coated with cementing agents.

Lime Neutralizing Value

Hard limestones are an expensive investment in Mississippi because of the transportation cost, whether by truck, rail, or barge, from rock quarries usually located in Alabama, Tennessee, Kentucky, and Missouri.

Lime quality or value depends on three factors: purity, fineness, and moisture content. The purity factor is Calcium Carbonate Equivalent (CCE) which expresses the acid neutralizing capacity of the material expressed as a weight percentage of pure calcium carbonate. Calcitic and dolomitic lime sources are not very soluble; hence smaller lime particles have more reactive surface area to interact with soil and soil solution, therefore there is more capability to neutralize soil acidity. The moisture content of liming materials affects handling and spreading ease, and influences the consumer perceived value of purchased lime.

Vendors of liming products are subject the Mississippi Agricultural Liming Materials Act of 1993. Regulations state that marl which is a “softer” material than calcitic or dolomitic limestones, must have a CCE of 70% or higher, and 90% or more of the material must pass a 10-mesh screen.

Calcite and dolomite sold in Mississippi must meet other criteria that use the quality factors to rate materials. Relative Neutralizing Value (RNV), alternatively known as Effective Calcium Carbonate Equivalent (ECCE), incorporates particle size and CCE to estimate lime value from required samples analyzed by the Mississippi State Chemical Laboratory. The particle size analysis determines the percentage of lime that passes 10-mesh and 50-mesh sieves. That data is combined with the CCE analysis to determine the RNV using the assumption that particles larger than 10-mesh have no effectiveness in neutralizing soil acidity in an agronomic timeframe. It is assumed all particles smaller than 50-mesh will dissolve to neutralize soil acidity, and half the particles in between these two sizes will react. Calculation instructions for RNV are available in MSU-ES Information Sheet 1587 Limestone Relative Neutralizing Value.

Comparing Lime Values

The RNV or ECCE allows you to compare value between materials from different sources. For example, suppose you find two agricultural liming materials.

• One has an RNV of 66 percent and costs $40 per ton. The other has an RNV of 85 percent and costs $55 per ton. Which is the better buy? By dividing the price per ton by the RNV decimal value, you can estimate the agronomic value of the materials.

($45/0.66) = $68, as compared to ($55/0.85) = $65. In this example, the material that is cheaper per ton actually ends up costing about $3 more per ton to neutralize the soil’s acidity.

Adjusting Lime Recommendations

The lime recommendations from the soil testing laboratory assume that the neutralizing value is 100%. The following example calculates the application rate based on the RNV of the material available for liming that is required to meet the 100% level:

• Purchased lime has an RNV of 70% and the recommendation is 2 tons per acre:

(2 tons recommended) / 0/.70 = 2.9 tons per acre is the adjusted rate to provide the recommended neutralizing value.

In reality, many spin spreader trucks used for lime application cannot apply at this level of precision, so the actual application rate is 3 tons per acre.