Most lawn grasses grow best in a slightly acidic soil with a pH (the measure of potential free hydrogen) of 6.0 to 7.0. Native plants may adapt to more acidic soils; St. Augustine grass, for example, tolerates a 5.0 pH. Most homeowners want a choice of more than native grasses, though. The cause of acidic soil often helps identify a cure for the condition.
Most plants, including lawn grasses, grow best in slightly acid soil. Too much acidity, however, kills bacteria and organisms like worms and insect larvae that aid soil development and health. Healthy soil results from a balance of basic processes. When the balance of soil composition, decomposition of organic matter and availability of water is disturbed, the pH of the lawn can become too acidic.
Minerals like nitrogen, phosphorus, potassium, sulfur, calcium, magnesium, and molybdenum come from soil as it breaks down or "weathers" into ever smaller pieces. It also provides sulfur that can combine with hydrogen to form acids. Soil source and type contributes to its acidity or alkalinity. When large amounts of limestone were present in the original rocks, acidity will be slight. If large amounts of sulfur were present in them, acidity will be greater. Sandy soil or loam, typical near coastal areas or in river valleys, often allows fast leeching of calcium, potassium and magnesium, leading to acidic soil.
As dead leaves and plant matter decay, they release nitrogen, carbon and other elements and compounds that formed the plant. Excessive organic matter like that found on forest floors creates acidic soil by using the hydrogen from rain water to form carbolic acid and sulfuric acid in the ground beneath it. One of the reasons that gardeners wait until compost and manure is "well rotted" before use is to minimize the "fast-release" of available nitrogen so that nutrients leech slowly through the compost.
Water provides hydrogen and oxygen to carry carbohydrates through plant tissue and utilize soil-borne nutrients. The pH of rainwater ranges from 5.0 to 5.5, meaning that a larger percentage of hydrogen is bound in acids. The pH of "acid rain," the result of excess sulfur in industrial areas, ranges from 3.0 to 5.0. Too much water, whether from flooding or irrigation, leeches potassium and other nutrients from the soil and replaces them with excessive sulfur and other acidic compounds. When heavy rains cause leeching of calcium and magnesium, iron and aluminum can rise to toxic levels. The end result is a dropping pH and increasing acidity. As plants and other organisms die in acid soils, their decomposition contributes to the acidity.
Sometimes human intervention contributes to soil acidification. Fast-release nitrogen fertilizers leech calcium, magnesium and potassium when used in combination with heavy irrigation. Heavy watering to keep a lawn green during hot summers depletes calcium, magnesium and potassium from the soil. Both can lower pH and increase acidity, particularly if there is not enough limestone present in the subsoil. The best way to find out if soil amendments are needed to correct acidity is to complete a soil test at your local state university extension service soil laboratory.