AgWatch


Soil pH And Lime Requirements Are Soil Health Assessments

DR. LARRY OLDHAM

MISSISSIPPI STATE, MISS.
   Two long-time soil fertility measurements are also soil health assessments. Soil pH and lime requirement are long recognized as the foundation of nutrient management as acid soils often need liming to aid crop growth and development. Soil organisms crucial to improving soil health also benefit from acidity management.
   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, other 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.
   Soils should be tested at least every three years, or more often if acidity issues are suspected. Growers should evaluate their lime options if it is recommended based on the soil tests. Three factors influence lime quality: particle size, ‘purity’, and moisture content.
   Without going into detail: dry lime spreads better.
In Mississippi, vendors of liming products are required to provide materials with a minimum 63 percent Relative Neutralizing Value (RNV).    The RNV calculations 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 a relevant time frame. All particles smaller than 50-mesh and half the particles in between the two sizes should react.
The RNV numbers allow 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. ∆
   DR. LARRY OLDHAM: Extension Soils Specialist, Mississippi State University

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