pH stands for "potential of Hydrogen" and is a measure of the relationship of hydrogen ions (H+) to hydroxyl ions (OH-). Soil pH is a measure of hydrogen ions (H+) in the soil. In other words, a soil pH value is a measurement of the concentration of ions held to soil particles and organic matter. The pH scale ranges from 0 to 14, with pH 7.0 being neutral (levels of H+ and OH- are equal at pH 7.0). Readings below 7.0 indicate the soil is "acidic," and readings above 7.0 indicate "alkaline" soil conditions. Most of the plants we grow in our home gardens require a soil that is slightly acidic, usually within a pH range of 6.2 to 6.8. Soil pH is very important because it directly affects soil nutrient availability. Plant roots can only absorb nutrients after they have been transformed into certain ionic forms. Only within certain pH ranges can sufficient amounts of these nutrients be transformed into these ionic forms. When the soil pH is outside the desirable range (6.2 to 6.8), the nutrients are literally "tied up in the soil" and are not in a form useful to plants. Soil pH can be lowered by adding sulfur if it is too alkaline or increased by adding limestone if it is too acidic. The best way to determine your soil pH and the necessary corrective measures (if any) is to submit a soil test sample to your local Cooperative Extension Office. Contact the office for instructions. When submitting a soil sample, make sure you specify what crop you intend to grow, since different crops require specific pH levels. You will receive a report noting your soil pH, and the type and amount of additive to correct any problems. The best time for a soil test is the fall. You can work the additive into the empty garden, as well as plan fertilization needs for the coming year.
For information on other issues affecting soil management, visit the eXtension website: Soils and Composting.