Fall 2014 Lime Programs

With harvest beginning, it is time to think about the next crop. Fall liming allows time for the soil to react before the next summer crop is planted, and takes advantage of the (usually) drier conditions for field operations.

Soil pH measures soil acidity, the master variable of soil fertility. Technically pH is the concentration of hydrogen ions reported on a logarithmic scale that goes from 0 to 14. The number is actually the power that 10 is raised, therefore pH 7 means there are 10 to the seventh power hydrogen ions in the solution. Because of the exponential nature of pH, a pH 6 soil is 10 times more acid than a soil at 7; pH 5 soil is 100 times more acid than pH 7, and pH 4 is 1000 times more acid than pH 7. Mississippi soils continue to acidify over time due to rainfall, parent material, fertilizers, and management.

Many soils require liming to maintain production. The need for lime is reported as lime requirement, and is based on laboratory procedures that measure the reactivity of a soil sample. Annually, about half the soil samples processed by the Mississippi State University Extension Service Soil Testing Laboratory indicate lime is needed.

Three types of liming materials are generally available in Mississippi: calcitic limestone, dolomite limestone, or marl. Calcites and dolomites must be transported into the state, so liming programs are significant investments that require attention to quality. The quality or value depends on three factors: purity, particle size, and moisture content.

Moisture content usually reflects conditions during handling and transportation, however some by-product materials have significant water contents. Chemical purity is described by Calcium Carbonate Equivalent (CCE), or the acid neutralizing capacity of the lime expressed as a weight percentage of pure calcium carbonate. Calcite and dolomites are not very soluble; so smaller lime particles which have more reactive surface area are preferred.

Regulations developed under the Mississippi Agricultural Liming Materials Act of 1993 use the quality parameters to determine criteria that calcite and dolomite sold in Mississippi must meet, and must be provided to the end users of the materials. The Relative Neutralizing Value (RNV) or alternatively the Effective Calcium Carbonate Equivalent (ECCE), incorporates particle size and CCE to estimate lime value from official samples analyzed by the Mississippi State Chemical Laboratory through the MDAC Bureau of Plant Industry.

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.

This information is then used to calculate the RNV of that material. More information about this subject is available in MSU-ES Information Sheet 1587 Limestone Relative Neutralizing Value. All agricultural liming materials, other than marl, sold in Mississippi must have a minimum RNV of 63. The value of different lime materials can be also compared using the information provided in Information Sheet 1587.

Some confusion exists about adjusting the recommended rates based on the RNV. Lime recommendations often assume that the neutralizing value of the material to be applied is 100 percent. This value is rare, so adjustment is needed.

If the recommendation is 2 tons per acre, and the lime has an RNV of 70 percent, divide 2 by 0.70. This equals 2.9 tons per acre of lime to meet the recommended neutralizing value. In reality, most spin spreader trucks used for lime application are not this precise, so the actual application rate is 3 tons per acre.

Sometimes soil testing after liming shows the pH is the same or maybe lower, or the lime requirement is unchanged or higher. This can be perplexing to someone who spent significant money to raise the pH. What happens is the complexity of soils. The propensity to change pH or resist is the ‘buffer capacity’ of a soil. It is what we attempt to measure in the laboratory to determine lime rates. Workers have found up to tenfold difference in buffer capacity within single fields in southern Georgia. The chief contributors to soil buffer capacity variability are soil organic matter and clay content. Generally, soils with higher levels of clay are more resistant to change.