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When fertilizer prices are high, it’s of most importance to make sure that the pH of our soil is in the 6.5-7 range. This makes sure that the plants can use the nutrients that we apply. With that being said it’s also important to know the quality of lime that you are using so you apply enough to make the needed changes to the pH of your soil. Agricultural lime quality can vary tremendously depending on the chemical properties and particle size caused by physical grinding of the stone.
Chemical properties determine the amount of acid that can be neutralized and are a function of the minerals in the stone. So that different sources of lime can be easily compared, the amount of acid neutralized by a lime of unknown purity is compared to the amount neutralized by pure calcium carbonate. If the unknown sample neutralizes only half as much acidity, then we say that it is equivalent to 50% calcium carbonate (or 50% CaCO3). Occasionally, when the limestone source has large amounts of magnesium, the percent CaCO3 can be greater than 100, because per unit of weight, magnesium carbonate neutralizes more acidity than calcium carbonate.
The important physical characteristic is the size of the particles in ground limestone. Agricultural limestone reacts more quickly when the particles are very small because the limestone has a better chance to dissolve and to neutralize soil acidity. In Kentucky, the size of limestone particles is described by the amount of material that can pass through a 10-mesh sieve, which contains 2-mm square holes (approximately 1/32 inch), and a 50-mesh sieve, which contains 0.3-mm square holes (1/200 inch). The particles passing through a 50-mesh sieve are considered small enough to completely dissolve in soil and be 100% effective. Particles passing through the 10-mesh sieve but not passing through the 50-mesh sieve are too big to be completely effective but are considered to be 50% effective during a three-year period. Particles not passing through the 10-mesh sieve are too large to dissolve in a three to four-year period, so these particles are considered to be 0% effective. The chemical characteristic, assessed as percent calcium carbonate equivalence, and the physical characteristic, assessed as the size of the particles, are combined into one value that quantifies the effectiveness of the limestone. This value is known as the relative neutralizing value, which is abbreviated RNV. The RNV is calculated as: CCE/100 * [(% between 10 and 50 mesh + % less than 50 mesh)/2]
Even when ag lime is applied according to soil test recommendations, the actual adjustment in soil pH is not always predictable, because the quality of Kentucky ag lime varies widely among quarries and over time. This variability is due to changes in the purity of the limestone strata being mined and normal wear of the crusher, which causes the fineness of limestone particles to vary. The University of Kentucky Soil Testing Laboratory bases ag lime recommendations on an RNV of 100%. The LaRue County Extension Office will adjust the recommended lime rates to reflect local lime qualities. The only lime that we have that would truly be 100% is pelletized lime that can be bought in bags and bulk at some fertilizer dealers. It’s also important to remember that when using these pellet lime products that they can work faster but it will still take the 100% RNV rate that will be recommended on soil samples. Applying 100 lbs to the acre of this product will not be enough to move the needle on the pH of your soils.
If you would like to submit soil samples to receive lime and fertilizer recommendations or have questions about liming your yard or fields, please contact the LaRue County Cooperative Extension Service at 270-358-3401 orDaniel.carpenter@uky.edu.
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