Limestone is often discussed in caring for yards and gardens, but oftentimes there is confusion on exactly what it does and how to best use it. Limestone does contain nutrients such as calcium and magnesium but ideally it is used for adjusting pH of yard and garden soils.

Discussion of using these types of soil amendments should start with understanding soil pH, which measures how acidic or alkaline soils are. Acidic soils have pH values below 7.0 while alkaline soils have pH values above 7.0.  Why soil pH values differ depends on multiple factors, starting with materials the soil originally developed from.

Soil pH itself does not directly impact plants. Instead, pH interacts with essential plant nutrients and alters their chemistry in soils. Nutrients have specific chemical forms (ions) plant roots accept from the soil. Depending on soil pH, adequate amounts may be available for plant use, or deficiencies may exist. Nutrients are readily available for most plants to remove (use) in slightly acidic soils, such as pH around 6.5. Blueberries and other acidic loving plants need soil much more acidic.

With soil test report results in hand, you can make accurate decisions regarding potential amendments. Do not routinely add materials such as limestone or sulfur to lawns or gardens without a recent soil test report indicating soil pH. Gardening shows and articles generated in the southeast United States often say to add limestone on a regular basis, but those soils are typically acidic in nature. Northern Wisconsin forest or lowland soils are often acidic, whereas soils in the southern part of the state are much more likely to be alkaline. In all cases, only soil test results confirm accurate pH values.

Specific materials can be added to adjust pH to a preferred value. To make soils less acidic, or raise the pH, limestone is suggested. Exact amounts are determined by how much of a change in pH is needed, along with soil texture. Information provided with soil test results will includes recommendations to help ensure the proper amount of limestone (or sulfur) is applied.

On the other hand, for soils more alkaline than desired, sulfur is suggested to be added to lower pH or make it more acidic. Elemental sulfur is preferred, as it is close to 100 percent sulfur. Fertilizers containing sulfur, such as ammonium sulfate, will also lower soil pH. However, sulfur content is lower, and these fertilizers also contain nitrogen. As a result, in trying to lower the pH an unnecessary excess amount of nitrogen is also potentially being added. Avoid using aluminum sulfate, as excess aluminum can be toxic to plants.