Don't worry, it's not all that bad. Most of us in this area of NC are "blessed" with this type of soil. It has its advantages. It tends to hold water rather well once it is soaked, making it possible to water less often than one would have to water a sandy soil. It is also very rich in minerals. The key is to working the soil so that the plants can take advantage of the minerals and nutrients, and in using plants that will be able to tolerate the soil type.
What is Clay Soil?
Clay is a fine-texture soil made of tightly packed organic and inorganic particles. It is generally poor in drainage and in airflow, but high in water content. It takes much longer for water to be soaked into a clay-based soil than that of sand, so this makes it possible for a plant to dehydrate under the surface despite being given lots of water. The key is the infiltration rate, which can be changed by adding larger particles of organic matter or pea gravel/permatill to the soil. Anything that will create more airspaces in the soil will help the infiltration rate. The opposite problem of a plant drowning is also possible. If the top level of soil has an adequate infiltration rate but there is a hard slab of compacted clay underneath the plant, the water will build up under the plant, creating a fishbowl effect. This is why Master Gardeners do not recommend digging a hole for a plant and replacing all the dug-out clay with "good" soil. The top will drain just fine but the water will sit underneath the plant roots, denying them of airflow.
Amending Clay Soil
If you decide to grow plants that are fussy about their growing conditions, it will be necessary to amend the soil. There are several ways to do this:
1. Double Digging
2. Use of Permatill
3. Incorporating Leaf mulch, pine bark humus, or compost into the bed
According to the NCSU CALS Horticulture department, the best amendments for clay soils are pine bark humus of less than ½" in diameter, composted leaf mold, or small pea gravel/permatill of less than 3/8" in diameter. Leaf mold should be fully composted. If it is not, the bacteria will compete with the plants for the Nitrogen in the soil to do their work. The department also does NOT recommend incorporating the following into clay soil: peat moss, sand, hardwood bark, and sawdust, as well as pine straw, and wood chips. They recommend that additions should be at least 25% and up to 50% of the total soil volume in order to have an effect.
Drainage
You will need to pay attention to drainage with clay soil. How do you check
drainage in any soil? From Ehow.com:
1. Dig a hole large enough to hold a one-gallon container.
2. Fill the hole with water and go on about your business.
3. Check the hole in an hour. If it's empty, you have sandy soil.
4. Come back an hour or two later. If it takes between two and four hours for
the water to drain from the hole, you have loam: the gardener's dream soil.
Look again four hours after you poured the water into the hole. If some of it
is still there, you have clay soil - and poor drainage.
Ok, So I Want Less Fuss!
Your options are to create a raised bed or to purchase plants such as these
listed below that are better adapted to perform in clay soil. Most of these
plants have deep taproots or aggressive root systems.
These plants can be hard to dig up once established, so think about where you
want to plant them! Otherwise, you will end up in the chiropractor's office,
like yours truly.
When planting trees, it is best to dig a hole twice as wide as the root ball
and just as deep as the plant was sitting originally. This will break up the
soil without creating a fishbowl effect.
Plants for Clay Soils in North Carolina
These plants listed below will tolerate or even thrive in clay soils. Although it is best to try to work your clay soil well with organic matter, it is still a good idea to use plants in your landscape that will work well with your original soil type. In other words, you won't have to fight nature too terribly much in this case!
Pyracantha (Firethorn) - shrub grown for berries
Solidago (goldenrod) - Fall-flowering perennial (no, this is not the 'sneezy'
one!)
Weigela (flowering shrub)
Baptisia (Blue False Indigo)-large perennial, flowers look like lupines
Grape Vines
Viburnum
Cotoneaster
Hemerocallis (daylily)
Hosta
Leyland Cypress
Heliopsis (oxeye sunflower, false sunflower)
Liatris (Gayfeather)
Monarda (bee balm)
Echinacea (Coneflower)
Black- Eyed Susan (Rudbeckia sp.)
Sedum "Autumn Joy"
Ironweed (vernonia n.) - tall flowering plant for butterflies, found in wildflower
mixes
Yucca (Adam's Needle)
Forsythia
Honeysuckle
Willow
Thuja (arborvitae, white cedar)
Norway and Silver Maple
River Birch
Hickory
Common Hackberry
Common Honeylocust
Apple, Crabapple
Pear
Swamp White Oak
Bur Oak
Eastern Pin Oak
Elm
Eulalia grass, Zebra Grass (Miscanthus sinensis)
Switch Grass (panicum virgatum)
Fountain grass (pennisetum)
Black Chokeberry "Autumn Magic" (aronia melanocarpa) - shrub grown
for foliage
Buttonbush (Cephalanthus occidentalis) - a water-loving butterfly-attracting
shrub
Grey Dogwood (Cornus racemosa)
Red Osier Dogwood (Cornus sericea)
Russian Olive
Acanthus (Flame Acanthus, Hummingbird Bush, Texas Firecracker Plant)
Achillea (yarrow)
Aconitum (monkshood)
Hollyhock (Alcea rosea)
Amsonia (Blue Star)
Anchusa (Bugloss)
Anemone
Bergenia (Pigsqueak)
Coreopsis verticillata (forget the rest of them!)
Foxgloves, by seed
Echinops (Globe Thistle)
Geranium
Helenuim (Sneezeweed, Helen's Flower, Dogtooth Daisy)
Hellebore (Lenten Rose)
Heuchera (Alum Root, Coral Bells)
Flag Iris
Lysimachia (Creeping Jenny, Moneywort)
Dentaria (toothwort)
Rattlesnake Master
Cup Plant (Silphium perfoliatum)
Wild Quinine (Parthenuim)
Aster
Asclepias (Butterfly Weed)
Sea Holly (Eryngium y.)
Meadow Rue (Thalictrum sp.)
Rose of Sharon
Roses love clay!! (if drainage is checked)
Cosmos
Hardy Hibiscus
Ajuga (great groundcover for difficult compacted places in part shade and shade)
Serviceberry (irregularly-shaped shrub)
Liriope
Boxwood