7/2/2013 - By Harrell's

Some of us think the importance of lime is to turn the standard rum and coke into the island version… a “Cuba Libre”. If that were the case, I would hold three PhD’s in the subject. But the lime I am discussing today is the sedimentary rock formed by millions of years of calcareous sea creature deposits – limestone. And more specifically its importance in horticulture.

There are several forms of “lime”, their differences basically dependent on the other minerals in the stone. High cal lime being pure unadulterated calcium carbonate, and dolomite, a mix of mostly calcium carbonate and some magnesium carbonate. The latter is widely used as a soil amendment in the US and Europe. Latin America typically has to rely on high cal as dolomite is scarce.

Lime is added to soil mixes for more than one reason. Usually we think of it being added to adjust the pH of soil. Remember though, calcium is an essential secondary nutrient that is largely responsible for building cell walls in plants – sometimes we seem to forget that one. Many organic soils, especially those with peat and/or bark start off with a fairly acidic pH often in the mid 4’s. By adding dolomite or limestone, we can bring the pH up to where nutrient availability is optimal.

The more acidic the pH, the more Hydrogen ions are in the soil or water. When calcium carbonate meets these ions, they react with one another and form water (H₂O) and carbon dioxide (CO₂). By doing that, the free hydrogen has been essentially neutralized and the pH rises. Dolomite also has magnesium in it, so this is a bonus.

Dolomite also comes in different textures which affect the longevity of the lime.  At the Harrell’s soil plant (Harrell’s Horticultural Products or HHP), we have an Ohio lime type dolomite that is coarser. Some nurseries will add both fine and course, so the fine works up front and the Ohio type works at tail end giving more longevity.

In potting soils, most foliage and flowering plants prefer a pH in the 6.0-6.5 range. To accomplish this, we will add 8-10 lbs of lime per cubic yard of soil. Many woody plants enjoy a pH in the 5’s so here we might add 4-5 lbs. of lime per cubic yard. Azaleas and camellias want an even lower pH so we might add gypsum instead of lime, as it influences pH less.

In existing potted plants that need to have pH brought up, you can top dress dolomite. A good rule of thumb is to apply 1 tablespoon of dolomite per gallon of soil i.e. a 3 gallon pot would get a 3 tablespoon application. Not easy to do in a 4” pot?! Well dolomite can also be drenched. Harrell’s carries a product called Limestone F that is a flowable dolomite. This is used at 1 gallon per 100 gallons of water. Again, great on small containers or in situations where pH needs to be brought up immediately.

Field nurseries need to watch their pH levels, as well. Pure sand or heavy organic soils are often acidic.  The best time to check is before it is ever planted and then dolomite can be broadcast as a pre-plant application, but it is never too late to add. Often a field can have a ton to the acre applied if pH is very low and you need to bring it up a notch, say 0.5 points.