A soil pH over seven was called sweet but is alkaline. Most vegetables, fruits, turfgrass, and many ornamental plants need a pH between 6 and 7. Adding lime raises the pH of the soil to make it healthy for these plants and was once known as sweetening the soil.

Soil pH is a measure of the acidity or basicity in soils. PH is the negative logarithm (base 10) of the activity of hydronium ions in a solution. It ranges from 0 to 14, with seven being neutral. A pH below 7 is acidic, and above 7 is basic. Soil pH is considered a master variable in soils as it controls many chemical processes that take place. It specifically affects plant nutrient availability by controlling the chemical forms of the nutrient. The optimum pH ranges for most plants are between 5.5 and 7.0. Many plants have adapted to thrive at pH values outside this range.