Gardeners who have moved to the Lowcountry from other states have asked me how to work with the soil we have here. Two critical features of soil that gardeners need to manage are soil organic matter and drainage.
Soils in the southern United States are notoriously low in organic matter. Our warm temperatures promote rapid decay of plant (and animal) remains in soil. Without regular additions, the percentage of organic matter declines steadily.
The only way to remedy the deficiency is regular additions of composted organic matter to all soil types. Gardeners should mix compost with their soil every time beds are prepared or plants are set in the ground.
The maximum amount of organic matter to add at once is a 2-inch layer of compost mixed into a 6-inch layer of soil. This is actually a lot of compost, 25 percent by volume. Plants will respond noticeably to even half as much.
To simply amend soil, purchased organic amendments, such as mushroom compost and various products generally labeled as soil amendments, work fine.
Fresh compost, purchased from Bees Ferry Landfill in Charleston County or the city of Columbia or self-generated in a home compost bin (https://bit.ly/2KfW5LN), includes active microorganisms along with organic matter. Some of these microorganisms stimulate plants’ root and shoot growth, offering extra benefits for the extra effort to buy or produce fresh compost.
Gardeners who are not convinced that amending soil is worth the work should do this simple comparison. The next time annual flowers or vegetables are planted, add compost to only one-half of the bed. Both halves should otherwise be watered and fertilized the same. If the soil is deficient in organic matter, the plants in the compost-amended half will grow larger, faster or more vigorously or be less prone to wilting in dry periods than the plants in the non-amended soil.
Knowing the type of soil in a yard is important for managing water. The Yonges loamy fine sand at the Clemson Coastal Research and Education Center is considered “poorly drained,” even though it is a sand. The fine sand particles and the loam in Yonges sand hold water more tightly than other sandy soils, such as Hockley loamy fine sand. Even if the surface topsoil is a type of sand, clay subsoil underneath can impede drainage.
The best way to manage poorly drained soils in the Lowcountry and Grand Strand is to construct raised beds by adding topsoil and compost to native soil to increase the height of the area to be planted. Making raised beds is a standard recommendation when planting annual and perennial ornamentals, vegetables, fruits and shrubs. Plants not set in raised beds will struggle their entire lives, which may be shortened due to root rot and lack of oxygen in poorly drained soils.
One fact from my introductory soil science course that stuck with me over the years is uniformly mixed soil drains better than soil made up of different layers. When making raised beds, there is little benefit to piling topsoil on top of poorly drained native soil or even on top of sandy soil. Little water will flow from the top to the bottom layer until the top layer is saturated, when extra force from the weight of the accumulated water pushes it downward. Saturated soil, even saturated good quality topsoil, does not support healthy root growth.
When making raised beds, the native soil must be loosened with a spade or rototiller and mixed with topsoil and compost. The resulting mixture is mounded into a bed that is four to six inches higher than the surrounding soil. The edges of the raised bed should slope a bit to allow water to run off.
One occasional problem with raised beds is that water may accumulate on the flat soil at the edge of the raised bed, which presents a problem when a raised bed adjoins a lawn. A more gradual slope may be needed to lead water away from the edge of the raised bed. Another solution is to dig a narrow trench at the edge of a raised bed to catch run-off rainwater.
Raising beds and adding composted organic matter will improve plant growth in the coastal plain.
Anthony Keinath is professor of plant pathology at the Clemson Coastal Research & Education Center in Charleston. His expertise is in diseases of vegetables. He is also an avid gardener. Contact him at firstname.lastname@example.org