Whether or not a plant can absorb particular nutrients depends on the chemistry of its soil as measured on the pH spectrum. How the soil elements react produces ‘acidic’ or ‘basic’ conditions on a scale that extends from 0 to 14. A pH of 7 is considered neutral, higher is basic and lower is acidic. In nature, numbers higher than 7 can be found in the U.S. West and Southwest. Soils where we live in the Southeast, as well as the East and Pacific Northwest, are older, acidic soils. Plants deprived of nutrition suffer when the pH of the soil is too far out of their comfort zone.
For example, most of our favorite vegetables grow best in soil with a pH of 6.0-7.0, but warm-season lawn grasses like we grow need a pH nearer to 6.0. Since many soils in our regions are naturally acidic, you may need to add lime to successfully grow either vegetables or lawns. This need is greater for vegetables like asparagus that will not grow in acid soil, but thrive once the pH is raised above neutral with lime. Traditional vegetable farmers sometimes call this process “sweetening the soil” and spread lime on their fields about every three years in Mississippi soils to keep them at the proper levels. Container soils made of peat or pine byproducts may be too acidic for vegetables and some flowers. If these plants do not perform despite your best efforts, test the pH. If lime is recommended, use pelleted products (to reduce inhalation) at a rate of 5 lbs. per 100 sq. ft for established plantings and lawns, or about one-half cup for a 15 gallon container. Do another test 3-6 months after you put down lime to see how much the pH has changed. When putting in a new vegetable bed or lawn, lime 6 months before planting whenever possible.
Azaleas, hollies and gardenias are three plants that require an acidic soil to thrive. Gardenia and holly produce fewer blooms, so sweet fragrance and bountiful berries do not appear. Azaleas cannot take up iron from the soil if its pH is above neutral. You’ll notice yellow leaves marked by green veins and a lack of thrifty growth in an azalea so affected. This condition is called interveinal chlorosis because the areas between the veins are yellowed by lack of iron.
Two more notes on acid soil and your garden. No, it does not make tomatoes taste more acid to grow them in a more acidic soil. In fact, all tomatoes grow best in a soil near neutral and the taste quality of ‘acid tomatoes’ is more related to the tomato’s variety and one’s own taste buds. Second, it is soil pH that determines whether hydrangeas bloom in pink, lavender, light blue or dark blue: alkaline soils produce pink, acid soils produce blue shades. To maintain the color they prefer, many gardeners regularly add lime to the soil around pink hydrangeas and aluminum sulfate to maintain blues. Both are added to the soil at the base of the plants in fall and worked into the soil. Use half cup of either amendment for a four foot shrub. Aluminum sulfate can be applied again in the spring.