The Gulf Coast clay soils in the southeastern United States are highly fertile and support the natural grassland vegetation of the coastal prairies. They are alkaline and low in organic matter, but treatment with sand, compost and gypsum soil amendments can improve the soil structure. Gypsum disperses sodium through the soil and allows clay to aggregate in small clumps, rather than being concentrated into a single layer impermeable to air and water.
Gulf Coast clay soils are fine examples of Grumusol, a tropical to subtropical Vertisol soil type that develops over clay deposits or calcium-rich rocks where alternate wet and dry seasons shrink and expand the clay during the year. In the dry season, the soil surface becomes deeply cracked and allows water to penetrate into the soil horizon. In the wet season, the saturated clay swells, the cracks are closed and the soil is impermeable.
Houston Black, the state soil of Texas, is soil science's model Grumusol. The Latin term "grumus" means a little heap, such as a crumb, and "sol" simply means soil. Clay soils have a crumb-like upper surface, and they are a dark color, even though their surface organic content is less than 3 percent and decreases with depth. Their sola, or upper layers, are more than 25 cm deep and may be as much 75 cm deep. Houston Black soil is 40 to 60 percent clay and predominately montmorillonite, a high sodium clay.
Gypsum in Soil
Gypsum is a naturally occurring hydrated calcium sulfate mineral that is abbreviated as CaSO4. Residential drywall is made of gypsum. Powdered gypsum ionizes in soil solution as positively charged calcium ions and negatively charged sulfur ions. The calcium ions are larger than sodium ions and have a stronger magnetic charge that can displace the sodium ions in the soil. The positive charged calcium ions flocculate, or clump, the tiny negatively charged clay particles into larger aggregates that are the same size as silt. The sodium binds to the sulfur and is washed away.
Clay particles are the tiniest soil particles and are more subject to wind and water erosion than larger particles. During heavy rainfall in summer months, dispersed clay in a sodium-dominated soil solution can form an impermeable crust on the soil surface that prevents seedling plants from emerging and decreases the total amount of water that infiltrates into the soil solution. The increased water runoff and smaller, transportable soil particle size combine to increase the total amount of erosion.
County agriculture extension services will test soil samples for small fees and advise on correct application of soil amendments. Gypsum is applied as 40 to 175 lbs. per 1,000 square feet, and may be applied more than once as needed. However, gypsum amendment is an industrial technique suitable for large-scale soil reclamation. For home gardening, clay soil can also be amended with an application of 2 inches of sand and 3 inches of composted organic matter turned under to a depth of 8 inches in late winter.