Controlling soil acidity is the most important fertility management decision. As we go into fall, growers should look at soil test recommendations for lime requirements.

Water is required for lime to react with soil, therefore a fall application is preferred so that the next crop gets the benefit.

If your last soil test was more than two years ago, now is the time to test again. Soils change over time with management. High levels of production and high rainfall require more attention be given liming programs because nutrients are removed from the soil through crop removal or because they leach.

While pH is the master variable to determine the acidity of soil, most labs determine the lime requirement separately. The reactive capability of a particular sample varies from soil to soil and even within a field.

Soils with a high sand content (have a low Cation Exchange Capacity) require less lime to correct pH, but they may require liming more often than soils with higher clay contents.

A few basic categories of lime are widely available. Calcitic lime is mostly or all calcium carbonate limestone. Dolomitic lime is calcium carbonates and magnesium carbonates, with at least 6 percent elemental magnesium content. Marl, a granular or loosely consolidated material, is comprised of sea shell fragments and calcium carbonate.

Byproduct materials, offered as liming materials in some locations, must be approved by appropriate environmental regulatory agencies as agricultural liming materials.

The Mississippi Agricultural Liming Materials Act was passed in 1993. A few years later, Mississippi instituted, through a change in regulations, a two-tier lime grading system for calcitic and dolomitic-based materials. Marl products were not affected.

In the change, lime that met previous regulatory requirements was Grade A. Grade B lime may have coarser particles or a lower Calcium Carbonate Equivalent.

Finer lime particles react more quickly than coarser particles, thus neutralizing soil acidity more rapidly. Mississippi Grade A lime may be difficult to locate due to transportation issues. Consultants should recommend and growers should use the best quality lime available.

Information about the relative values of different lime materials is available at local Mississippi Extension offices and on the MSU Extension Service Web site at http://msucares.com/pubs/infosheets/is1587.htm.

Because lime worked into soil reacts faster, apply it prior to any fall tillage. Lime in conservation tillage systems requires careful attention, especially if fertilizers have been applied to the surface for several growing seasons.

The soil surface should be sampled separately in long-term conservation tillage systems. Double-sample fields, one from the regular 0- to 6-inch depth, the other sample from 0 to 2 inches.

Use soil test recommendations from the regular sample for nutrient applications, and recommendations from the other sample for lime management.

Lime in much of the Mid-South is a significant investment due to the transportation costs. Producers are somewhat wary of large lime requirements. Where more than 2.5 tons per acre is recommended, it is acceptable to split the application over two years.

Some producers facing enormous lime bills in recent years and some researchers have found variable rate application based on intensive soil sampling and mapping is effective in correcting acidity while controlling costs.

Here’s what to do:

• sample fields for lime requirements,

• locate and purchase good quality lime, and

• apply as soon as possible.

Larry Oldham is a Mississippi Extension soil specialist.