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Alabama Acid: Plants that love our sour soil
Azaleas belong under Alabama pines like a sprig of mint belongs in a mint julep. And just as you need the right ingredients to make a proper mint julep, you need the right soil to grow healthy azaleas. They're one of many species that require acid (sour) soil. Acid-loving shrubs and trees include many Alabama landscaping favorites: azaleas, rhododendrons, camellias, gardenias, red maples, sourwoods and dogwoods, just to name a few. Pansies, petunias, snapdragons and vinca are bedding plants that you may want to grow in the same vicinity as acid-loving shrubs and trees, since they also require acid soil. White potatoes, unlike most vegetables, thrive in acid soil, largely because potato scab bacteria do not. Rabbiteye blueberries are a favorite fruiting shrub, and require very acid soil. Positively particularAcidity is measured by pH (potential hydrogen), a scale ranging from 0 to 14 that measures the concentration of positively-charged hydrogen ions. Seven is neutral, while lower numbers are acidic and higher numbers are alkaline, or basic. Pure water has a pH of 7. Vinegar's pH is about 3, while household ammonia's pH may be 11 or higher. The scale is exponential; a soil with a pH of 5 is actually ten times more acidic than one with a pH of 6. Most plants thrive in soil that's just slightly acid, between pH 6 and 7. A few, such as some Mediterranean herbs, prefer alkaline soils. Some aren't very particular, thriving in a wide range of soils. The acid-loving plants are particular because the nutrients they need, especially iron, are more available in acid soils. Soil's parental influenceMuch of Alabama has naturally acid soils. The parent material—the rocks from which soil is formed—is often acid-forming sandstone or granite. Our abundant rainfall leaches alkaline-forming chemicals such as calcium from the upper layers of the soil, resulting in more acidic conditions over time. However, there are exceptions. The “black belt” that runs from west to east in the southern part of the state is named for its unusually dark soils. Derived from “Selma chalk,” a form of limestone, its pH ranges from 7 to 8. Valleys in the northeast quarter of the state are largely derived from alkaline-forming limestone. These and some coastline regions tend toward less acid soils. Take the testAcid-loving plants won't achieve their full potential if the soil isn't acidic enough. They may exhibit symptoms of iron chlorosis (sidebar) or other nutrient deficiencies. How acidic should the soil be? Recommendations vary, but generally fall in the pH range of 5 to 6 for acid-lovers. Rabbiteye blueberries perform best at an even lower range of 4 to 5.5. If you're unsure of a plant's pH requirement, ask your nurseryman or your County Extension agent. A soil test is an excellent investment. Auburn University will analyze your soil and provide detailed recommendations based on your plant choices for $8.00. You can get instructions and a box to mail your sample at your county agent's office. See http://www.aces.edu or http://www.ag.auburn.edu/agronomy//soiltest.htm for more details. Home test kits are reasonably accurate, moderately priced and easy to use. Avoid inexpensive pH meters, the kind you simply insert into the soil. They're temptingly convenient, but unless you're willing to pay around $100 for a professional-quality meter, your measurements will be unreliable. Create a planTry to group your acid-lovers. It's easier to acidify one large bed than half a dozen planting holes. When applying limestone to your lawn, avoid spreading it in your acidic beds. (Most turf grasses thrive in near-neutral soil, but if your lawn is Centipede, you can probably skip the lime since it prefers a soil pH of 5.5.) If you use acid-lovers as foundation plantings, keep in mind that concrete is made with limestone and can raise the pH of adjacent soil. Till acidic organic matter such as shredded pine bark or peat moss into the beds to a depth of at least eight inches. It will improve soil structure and provide nutrients in addition to lowering the pH. Pine bark nuggets and pine straw make excellent mulches, helping suppress weeds and retain soil moisture before eventually breaking down into acid-forming humus. Choose acid-forming fertilizers. Of these, “Ammonium nitrate is the fertilizer of choice,” says Dr. Charles Mitchell, Alabama Cooperative Extension agronomist and Auburn University professor. It creates acidic conditions and replenishes nitrogen (N) without adding phosphorous (P) or potassium (K), both of which are longer lasting and already abundant in many Alabama soils. Unlike ammonium sulfate, aluminum sulfate does not add nitrogen. It contains aluminum, which can turn your French hydrangeas blue, but aluminum can build up to levels that are toxic to your plants. Since most soils have enough aluminum, the secret to blue hydrangeas is to keep the soil acidic so that the aluminum is readily available. (Keep it above 6.5 if you prefer pink hydrangeas.) Elemental sulfur will lower the pH without adding any N, P or K. Application rates vary according to your soil type and pH, so follow package directions. If your soil is in the neutral to alkaline range, around 6.5 or higher, it may be easier to grow your acid-loving plants in pots or raised beds. Most potting soils have a pH near neutral, so mix them with equal parts of shredded pine bark to lower the pH, advises Dr. Mitchell. Raised beds should be at least ten inches deep and may include regular soil mixed with shredded pine bark or other acid-lowering amendments. Whatever steps you take to create acidic conditions, soil pH can change over time. You may have to test it yearly and continue using acid-forming fertilizers and mulches. You'll be rewarded with a landscape that inspires sitting on the front porch, sipping a frosty mint julep and tipping your straw hat to passers-by who stop to admire your awesome azaleas.
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