The pH scale is a measure used by chemists to rate a substance's acidity on a scale of 0 to 14. The human body measures at a neutral 7. The more acidic a substance or environment the lower the number. The more alkaline the higher the number. Most soil's are slightly acidic, measuring within a range of 6 to 6.5. This pH level is suitable for many plants, although some thrive better in acid or alkaline soils. Gardeners adjust soil pH levels by adding amendments to the growing medium.
Just as the human body needs to maintain a pH level of 7 in order to function at optimum levels, plants require their growing medium to maintain a certain pH level in order to complete their vital chemical functions. When acid levels are too high, concentrations of aluminum and zinc increase creating a toxic environment for plant life. Gardeners amend soil with lime compounds to counteract high acid levels. When acid levels are too low, plants are incapable of taking in nutrients like calcium and phosphorous. Such deficiencies result in unhealthy, lackluster plants. Growers with alkaline soils who desire an acidic boost provide their garden with urea, or ammonium sulfate. Compost also releases acid as it decomposes, thus increasing soil's acidity.
Soil tests are the most efficient way to determine your soil's acid levels and fertilization needs. The type of plant you choose to grow also influences whether or not to amend soil pH. Avoid damaging soil structure and causing plant death by testing soil before amending it with lime. Some plants prefer acidic conditions. Azalea, rhododendron and blueberry bushes do well in pH levels of 5 or less making lime an unnecessary amendment. Limestone, in pulverized, granulated, pelleted or hydrated forms, is incorporated evenly into moist soil. Moisture and uniformity of application is essential for limestone to effectively lower pH levels.
The bulk of a soil's particulate matter is derived from the parent rock beneath the topsoil. Limestone is a bedrock found in Pennsylvania, Florida, Arkansas, Michigan and Kentucky, among other areas. Although limestone bedrock can lessen topsoil's acidity in certain conditions it will not. Clay soils, for example, can be so dense and compacted that minerals build up within its matrix effectively increasing acid levels regardless of any limestone bedrock lying beneath it.