Every species of plant has a particular soil pH level that it favors. Microbes, or microscopic bacteria that live within soil, breakdown organic matter, raising and lowering pH. The abbreviation "pH" stands for potential Hydrogen, which in gardening terms refers to a plant's ability to attract hydrogen ions during its growth cycle. Acidity, and alkalinity levels in soil, dictate this attraction. Unless a plant is growing within its ideal soil pH range, it will be unable to derive the macronutrients it needs to survive.
Gardeners use the pH scale to determine soil pH levels. The pH scale runs from 0 to 14, and a reading of 7.0 is considered neutral. Readings below 7.0 are considered acidic, and readings above a 7.0 pH fall into the alkaline category. Unit changes within the pH scale are tenfold, meaning that a pH of 3.5 is ten times more acidic than a pH of 4.5. Approximate soil pH level readings can easily be determined with a soil pH testing kit available for purchase in most garden centers and plant nurseries.
Most test kits rely on probe or electrode, which measures the pH once it is inserted into the soil. A battery powered meter provides visual feedback, displaying the soil pH reading on a screen located above the probe within one-half-hour or less. Alternatively, gardeners who do not have access to a soil pH testing kit may bring soil samples to their local University’s cooperative extension for accurate testing. While some cooperative extensions test the soil samples free of charge, others charge a nominal fee for this service.
Once a soil reading is identified, it can be compared against that of a particular plant’s preferred soil pH range. Much like sun and watering specifications, this preferred range can be found on the nursery tag. If the soil pH levels are deemed to be higher or lower than those specified for a particular plant species, the plant will fail to thrive. The addition of an amendment can help alter the pH, making the soil more suitable for planting.
Lime and peat moss are two common amendments used to raise or lower soil pH levels. When lime is added to acid soil, the pH rises. Peat moss, which is highly acidic in nature, will lower the pH of alkaline soil. The amount of lime or peat moss necessary to alter the soil depends on how much of a pH adjustment is needed. Allocation instructions and application methods are printed on the amendment’s packaging label.
It is important to keep in mind that while all plants have a certain soil pH level that they prefer, it does not mean that they cannot tolerate a pH that is slightly higher or lower. For instance, while most common plants flourish in soil with a pH between 6.0 and 7.2, most will also tolerate a pH as high as 7.8. Acid-loving plants such as rhododendrons, some berries and azaleas are usually the exception to this rule; tolerating nothing less than acidic growing mediums.