Potting soil, like regular gardening soil, has a pH level. pH is the measure of how alkaline or acidic the soil is. pH lower than 7.0 means the soil is acidic, while 7.0 and above is alkaline (7.0 is neutral). Acidic soil needs lime to balance out, while alkaline soil requires sulfur. Testing your potting soil using a home pH testing kit will inform you whether and how your soil is adjusted for optimal plant growth.
Remove a handful of the potting soil from the pot and place it into a small bowl or container. Four inches of soil will do, says Mark Highland of Organic Mechanic Soil.
Add four parts water to one part soil until it is a dark, murky mixture.
Pour a portion of the liquid into the container that came with the pH test kit (available at nursuries and hardware stores) and add the chemicals. Put the lid on and shake the container until the liquid changes color. Compare the color to chart provided with the testing kit to get your pH results.