Acidic soil is defined as a growing medium with a pH below 7.0, typically targeting a range of 5.5 to 6.0 for specialized plants. Commercially prepared acidic soil mixes are widely available for purchase. These specialized mixes are often packaged for container gardening or sold as amendments to be mixed into existing garden beds.
Identifying Commercially Available Acidic Mixes
Garden centers and nurseries stock specialized products clearly marketed for their low pH properties. When shopping, look for labels such as “Ericaceous Compost,” “Rhododendron/Azalea Mix,” or “Acid-Loving Plant Soil.” The target acidity for these commercial blends generally falls within a pH range of 4.5 to 6.3.
These pre-formulated mixes achieve their acidity through a specific blend of components. A primary ingredient is often sphagnum peat moss, which naturally possesses an acidic pH, frequently in the range of 3.0 to 4.5. Pine bark fines and composted sawdust are also common additions, as these materials decompose slowly and maintain the acidic environment. Some high-quality blends may also include granite sand to improve drainage and structure.
It is important to differentiate between an acidic potting mix and an acidic soil amendment. Potting mixes are lightweight and designed to be used alone for container gardening. Conversely, acidic soil amendments are intended to be blended with native garden soil to gradually reduce its overall pH for in-ground planting.
Understanding the Need for Low pH
The requirement for acidic soil is biologically driven, impacting how certain plants absorb nutrients. Plants that thrive in acidic conditions, such as blueberries, rhododendrons, azaleas, camellias, and certain hydrangeas, are collectively known as ericaceous plants. These species have evolved to efficiently take up micronutrients that become chemically inaccessible in higher pH soils.
In alkaline or neutral soils (pH 7.0 or greater), essential elements like iron and manganese react with other soil compounds. This reaction changes their chemical structure, locking them up and preventing the plant’s roots from absorbing them. This results in a condition known as nutrient-induced deficiency.
The most common visual symptom of this deficiency is interveinal chlorosis. This condition appears as a distinct yellowing of the leaf tissue between the veins, while the veins themselves remain dark green. The deficiency is most pronounced in the newest growth, causing the youngest leaves to show the most severe yellowing. Providing a medium with a pH around 5.5 ensures these plants can access the necessary iron and manganese for healthy chlorophyll production.
Amending Existing Soil to Create Acidity
If purchasing pre-made soil is not feasible, you can manually lower the pH of your existing soil using amendments. Chemical acidifiers are the most reliable method for achieving a significant and sustained pH reduction. Elemental sulfur is a common choice; it works slowly as soil bacteria convert it into sulfuric acid, making it suitable for long-term pH management.
Aluminum sulfate is a faster-acting chemical amendment because it lowers the pH through an immediate chemical reaction with water. However, it must be applied cautiously, as excessive use can lead to a toxic buildup of aluminum that may harm plant root systems. Before applying any chemical acidifier, perform a soil test to accurately determine the current pH and calculate the necessary application rate.
Organic materials can also be used, although their effect is often less pronounced and temporary. Sphagnum peat moss is an exception, as it is naturally highly acidic and can significantly lower soil pH when incorporated into native soil. Used coffee grounds and pine needle mulch are often cited as acidifiers, but they typically have a minimal long-term impact due to the soil’s natural buffering capacity.