Lime Need and Value

Acid soils often need liming to aid crop growth and development. Fall applications provide more time for the lime to react with the soil, less stress on the human component, and better field conditions  for equipment operation.

Soil acidity problems rarely have dramatic visual symptoms. Affected areas are less hardy or vigorous in growth; the size of the areas increases within fields over several growing seasons. If lime is needed, the benefits include:

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

Vendors of liming products are subject to the Mississippi Agricultural Liming Materials Act of 1993 and the regulations under that law. Those regulations were amended in 2005 to eliminate a previous grading system for calcitic and dolomitic materials (aka ‘hard limes’), and to require a minimum 63% Relative Neutralizing Value (RNV). The RNV “must be shown prominently on the front face of the label, sales invoice, delivery ticket, or bulk ticket”.

Under the Mississippi statute, RNV’s are based on particle size analyses of the lime using the percentage of lime that passes 10-mesh and 50-mesh sieves and the Calcium Carbonate Equivalent data. It is assumed particles larger than 10-mesh are not effective neutralizing soil acidity in an agronomic timeframe. All particles smaller than 50-mesh and half the particles in between the two sizes should react.

The RNV allows value comparisons between materials from different sources. For example, of two agricultural liming materials, one has an RNV of 66 percent and costs $50 per ton. The other has an RNV of 85 percent and costs $60 per ton. Which is the better buy? The agronomic value of the materials may be estimated by dividing the price per ton by the RNV decimal value ($50/0.66) = $76, as compared to ($60/0.85) = $71.

Is there much difference between lime materials? The Mississippi Bureau of Plant Industry collected 20 samples of lime available at retail locations in the state while preparing for the regulation change. Nineteen of the twenty CCE’s were above or near 90, however the RNV values varied about 20% because of the different particle sizes.