ATLANTA, Georgia — Radiant sunlight bakes the Georgia red clay all summer long. Many herbs evolved in the bright sunlight of the Mediterranean and the near East, so they can handle the strong light of the deep South. But in order to have a beautiful herb garden in Atlanta, something must be done about the red clay.
Actually, clay is a great soil type, much richer than sandy soil, where water and nutrients quickly drain away or leach below the root zone. Different soils are distinguished mainly by particle size. Large sand grains are visible, as are the large spaces between the grains. Clay particles are too small to see, and the spaces in between are also minuscule, causing the heavy, compacted condition and poor drainage associated with clay soils. By adding plenty of organic matter, clay soil is loosened, and roots of lavenders, sages and thymes can easily penetrate, growing fragrant and lush.
The organic matter can come from homemade compost, well-rotted manure, well-rotted wood chips, or you can buy compost by the bag. I use all types, at different times, and cover my herb bed with 5 to 6 inches of organic matter before forking it in deeply. The soil microbes and the rain go to work on the humus, creating humic acids that cause the tiny clay particles to clump together and create larger spaces that allow for better drainage. Organic matter breaks down quickly in the long Georgia rowing season, so I must add more routinely. I find that by forking in several bushels of coarse river sand at the same time, the effects of the job are considerably prolonged, as inert sand does not decompose and disappear.
Acidity is the next condition of Georgia red clay that must be addressed for a great herb garden. Atlanta-area soils are very acidic in nature, which is why my region is known for its beautiful azaleas, dogwoods and blueberries. But herbs grow best in neutral or alkaline soils. Lime, chemically known as calcium carbonate, does the job, converting the acid to a more basic soil. The Georgia Cooperative Extension Service recommends 5 pounds of lime per 100 square feet, but that is for a standard plow depth of 8 inches. I prepare my garden patch to twice that depth, so I add triple the amount of lime. I use pulverized lime, which is finely ground and gets into the soil more quickly.
Because Atlanta soils are typically deficient in magnesium, which plants need for good growth, I choose pulverized dolomite lime, also known as magnesium calcium carbonate. Adding the nutrients magnesium and calcium while adjusting the soil pH provides excellent conditions for rosemary, yarrow, tansy and all the herbs I love.
Wherever you live, you’ll find the cooperative extension service in your area to be invaluable for learning about your soil, its deficiencies and what amendments will make it better. I was amused to learn that in the making of the motion picture "Gone With the Wind" in the 1930s, David O. Selznick shipped crates of Georgia red clay to Hollywood. It was used to dust Scarlett’s skirts, adding authenticity as she ran through the fields of Tara. I wouldn’t trade my Georgia red clay for anything.
If you suspect your soil is to blame for plant maladies, consider sprucing it up. Check with your local Cooperative Extension Office to learn about the soils in your area or have it tested by a laboratory. A soil test will show the percentage of necessary elements needed for good plant growth. A soil lab will give readings for each element on a high, adequate or low margin and recommend additives to amend soil. Organic soil amendments such as aged manure, compost or sphagnum, for example, can correct a deficiency in phosphorous or iron. The report will also give the pH of your soil. A pH above 7.0 is alkaline; below is acid.
Geri Laufer is a horticulturist, lecturer, author, and herb gardener in Atlanta, Georgia.