"Lime Time" Again

By Wiley C. Johnson, PhD, Institute Agronomist

  I keep track of the questions we get here at the Whitetail Institute, and there is still a lot of confusion concerning lime products and liming. The reason for liming is to correct soil acidity. This allows the plants to uptake the nutrients in the soil that are needed for growth. Acidity is measured by determining pH, which is the soil’s hydrogen (H) content.
This is best done from a soil sample in a laboratory. A pH of 7.0 is neutral, less than 7.0 is acid, and more than 7.0 is basic or alkaline. Most crops grow best in a neutral or slightly acidic soil.

Most soils that we use for wildlife food plots are acidic and tend to become more acidic as they are fertilized and subjected to increasingly more acid rainfall. Soils become more acid from the surface down, not from the subsoil upward to the surface. Most cultivated soil acidity comes from fertilizer nitrogen. A general rule of thumb is that one pound (unit) of applied fertilizer nitrogen causes soil acidity that requires three pounds of lime to neutralize. Since lime reacts where it is put and does not move in the soil, periodic application of a relatively small amount (one ton or less per acre) of lime goes a long way toward correcting the acid-forming effect of fertilizer and acid rain on the soil’s surface.

At planting time the required amount of lime, as determined by soil test, is best applied by mixing about half deeply (6-8 inches) with initial tillage and the remainder incorporated later more shallowly (upper 2 inches). If this is not practical for you, put on the recommended amount of lime whenever you can and however you can. This is much better than not enough lime or no lime at all.

Several products will reduce soil acidity, but ground limestone rock is by far the most commonly used. There are two kinds of limestone, dolomite and calcite. They are generally equal in neutralizing effectiveness. Impurities reduce limestone’s neutralizing effectiveness but all commercial agricultural lime is at least 90% pure CaCO3 equivalent. So, it comes down to fineness being the critical factor in determining how good different liming products are. Fineness is measured by how much passes through screens of different sizes (mesh). For example, a 10 mesh screen has 10 divisions per inch. Lime held on a 10 mesh screen is almost gravel. Generally, lime must pass a 60 mesh screen to be effective in neutralizing soil acidity. State laws define minimum fineness standards for agricultural lime. Soil test recommendations are based on the minimum fineness standards. Comparisons have shown that particles larger than 10-20 mesh have little or no practical effect, very fine material (100-200 mesh) is the most effective initially, the 60-100 mesh material is just as good by the third year, and the 20-60 mesh material is definitely inferior to the finer particles.

Several products that use ultra-finely ground (100- 200 mesh) limestone are advertised as being more effective than ordinary ground agricultural lime, thus requiring less material to get the same result. This may be true initially, but after three years ordinary lime is just as good. In fact, this sustained effect could be quite an advantage. I recently bought several bags of ordinary ground limestone that was only guaranteed to meet minimum fineness standards that actually was as fine as flour. For most purposes, very finely ground limestone is not necessary, and the neutralizing ability of ordinary “ag lime” is plenty good.