Yes, it is a citrus fruit used in certain drinks but the lime we are talking about is a form of a chemical called calcium oxide (CaO). Pure calcium oxide is called quick lime which is very caustic and is sometimes used to help decompose dead animals. This is NOT the type of lime we are talking about either.

The most common form of lime used in horticultural (and agricultural) crops is calcium carbonate (CaCO₃). Another often used type is dolomitic lime (CaCO₃.MgCO₃) which also adds magnesium when applied.

Over the years, I have heard lime credited with doing a lot of things from eliminating grubs to curing fungal diseases. As far as I know, adding lime to your soil does only two things. It adds calcium to the soil and, in the process, raises the pH number i.e. makes the soil less acidic i.e. alkaline.

The pH is a measurement of the hydrogen ion concentration found in a substance. It is represented on a scale up to 14. A pH of 7.0 is neutral (neither acid nor alkaline) while numbers below 7.0 are said to be acid while those above 7.0 are alkaline. For more on pH...

Anyway, with a few exceptions (so-called acid loving plants like Rhododendrons, azaleas, boxwood, etc.), most landscape plants perform best at a pH which is slightly acid i.e. between 6.0 and 7.0. If the pH is too high (alkaline) or too low (acid), for the plant's requirements, it will not be able to absorb necessary nutrients even though they are present in the soil around the roots. It will show deficiency symptoms even though a soil test says there are plenty of nutrients in the soil.

So, the only reason you as a home gardener should add lime to your soil is because the pH number is below (too acid) the required range for the plants you are trying to grow.

You need to have your soil tested at a reputable soil lab either commercially or through your land grant university. Trusting an inexpensive home pH kit might or might not give you good information. Professional labs use high tech machines that are always properly calibrated to assure a quality result.

Also, the home kit, even if it works properly will only give you one of the two pieces of information you need. Knowing only the pH will only tell you that you need to adjust the acidity. What a quality soil test result will tell you is how much lime to apply to make the proper adjustment in pH.

There are certain chemical processes present in soils that impact the amount of lime needed to make the desired change. There are buffers and other factors such as the amount of clay versus the amount of sand which will impact the recommendation. A reputable soil test recommendation will take all of this into consideration and tell you how many pounds of lime (calcium carbonate) to apply per 1,000 square feet of area to meet your pH goal.

So, don't guess...soil test!

It takes a lot of lime to make a change in pH. The soil test recommendation may call for many pounds to be applied per 1,000 square feet of area. The more water soluble the product, the quicker it will work its way into the soil.

Even so, it can take 2 or 3 years for the total effect of the lime to be measurable in the soil. You should run another soil test about the second or third year after your application to see what is happening. If additional applications are needed, the new test recommendations will tell you.

Note: We have provided some general information and observations on this topic aimed at the home gardener. Before you take any serious action in your landscape, check with your state's land grant university's Cooperative Extension Service for the most current, appropriate, localized recommendations.