Soil amendments for clay soil should help loosen compacted soil, improve soil drainage and increase soil porosity (the amount of space in the soil available for air and water). Roots need oxygen in order to survive and grow. If soil becomes too compacted or too wet, roots will die from lack of oxygen.
The best amendments for clay soils are pine bark humus (less than half an inch in diameter) and composted leaf mold. Pine bark humus, also known as bark fines, milled pine bark or finely ground pine bark mulch is preferred since it is cheap, readily available and easy to work with.
These products are often sold at local garden centers as “soil conditioners” or “soil amendments” under a variety of brand names. They are not the same as potting soils, which are often blended with materials like peat moss and vermiculite that help retain moisture.
Potting mixes are fine for potted plants, but not for clay soils in a flowerbed. Clay soils already hold plenty of moisture and have the highest water-holding capacity of any soil in the world.
Amending the soil for annuals
Annual flowers can benefit greatly from soil amendments. These plants only live for one season, so gardeners have a very short window of time to get them established to produce flowers.
Moist, well-drained soils are key.
Most annual flowers prefer moist, well-drained soils. This sounds like a contradiction, but it makes sense when you consider annual plants have a very limited root system and require consistent soil moisture to survive.
Till the soil several inches deep in the area intended for planting annuals. If the site is already excessively wet, plant in a raised bed 6 to 12 inches high. The raised bed can be enclosed using either treated lumber or a mound of soil called a berm.
Add soil amendments at a minimum of 25 percent by volume and a maximum of 50 percent. This is roughly equivalent to the addition of 3 to 4 inches of soil amendments incorporated 8 to 12 inches deep in the clay soil. This will provide an ideal environment for root growth.
One cubic yard (nine, 3-cubic-feet bags) of soil amendment will cover an area about 100 square feet. Again, be sure to mix amendments thoroughly with the underlying clay soil. You may wish to add a starter fertilizer to annuals immediately after planting. Based on the results of a soil test, you may need to add lime or sulfur to adjust soil pH.
Amending the soil for perennials, shrubs, and trees
Research shows that there may be little or no actual benefit to amending the soil when planting most perennials, shrubs, and trees in the landscape. The exception, however, includes those plants that must have extremely well-drained soils, such as azaleas, rhododendrons, mountain laurel and blueberries.
In Georgia, most any type of organic material or compost added to the soil will likely break down and decompose within a year or two. This is primarily due to the amount of rainfall and high temperatures the state receives.
Organic matter would have to be added every year to have any benefit, which is not practical for most perennial plants. Most native soils in Georgia contain less than 1 percent organic matter on average.
Plants that live longer than a year will eventually outgrow their planting hole. The sooner trees, shrubs and other perennial plants’ roots get acclimated to Georgia’s red clay, the better off they will be in the long run.
I do know, however, that some gardeners are not comfortable with the “no amendment” rule and they add soil amendments when planting all types of plants. I also tend to agree, especially when I am planting perennials, shrubs, and trees in extremely poorly-drained, rock-hard red clay soils.
According to UGA research, the greatest benefit you can provide for most perennials, shrubs and trees is to dig a wide hole, at least two or three times larger than the original plant’s root system; and loosen up the clay soil with a shovel or garden tiller.
For more information on soil amendments, see the University of Georgia Extension publication “Flowering Annuals for Georgia Gardens” at pubs.caes.uga.edu.