Acid soil is characterized by a low pH, typically below 7, with levels commonly falling between 4.0 and 6.5. Such soils are often found in regions with high rainfall, as water leaches away basic nutrients like calcium and magnesium, leaving behind more acidic conditions.

The acidity of soil is determined by the concentration of hydrogen ions (H⁺) in the soil solution. Acidic conditions may also arise from the breakdown of organic matter, fertilization practices (such as the use of ammonium-based fertilizers), or the natural composition of parent rock material, such as granite.

Plants adapted to acid soils often thrive due to specific mechanisms for coping with lower nutrient availability or higher concentrations of aluminum and iron, which can become toxic at very low pH. Common acid-loving plants include rhododendrons, camellias, heathers, and blueberries.

Gardeners working with acid soils can amend the pH by adding lime (calcium carbonate) to make the soil more alkaline or incorporate organic matter to buffer pH changes. For plants that prefer acid conditions, mulching with pine needles or peat moss can help maintain acidity.