If you have a spot in your yard that is always muddy or sparsely populated with stubby plants, you may have clay soil. Soil in poor enough condition will not even support weeds, and clay is known to be a bad host for vegetation. Clay soil has a reputation for getting wet, then solidifying into a thick mass with a hard crust on top. It is one of the most problematic soils because several of its features--bad enough when occurring separately--work together to create an inhospitable home.
Clay soils are slick when rubbed between the fingers, leaving a streaked track and a somewhat sticky feel on your hand. Clay can appear in shades from yellow to brown to grey. The soil packs tightly and can be difficult to break apart.
Where Clay is Lacking
Clay soils require amendments to support most types of plants because their properties render them unfriendly to root systems. They retain large quantities of water, are dense and lacking air pockets and lose nutrients quickly. The Colorado State University Extension recommends adding fiber to clay, including wood chips, bark or straw. It further suggests adding mulch on the surface and tilling both organic and inorganic amendments into the clay as methods of improving the composition. This will increase the soil's ability to hold on to and provide nutrients for plants and develop pockets of air needed by roots and microbes.
What to Avoid
In dealing with clay soils, there are a few amendments to avoid. Do not add vermiculite. It worsens the degree of water retention. The University of Missouri Extension also warns against using sand. A very high volume is needed to make a significant change in clay soil, and smaller amounts can further complicate the existing problems, binding the soil to a greater degree or sitting on top in a separate layer, giving you the troubles of both sand and clay to handle.
Getting Around the Problem
The Ohio State University Extension suggests tilling clay in the fall or turning it by hand with a shovel to take advantage of the changes in winter temperatures to help break apart the soil. The University of Missouri Extension recommends raised beds if you have difficult clay soils. This allows gardeners to grow on top of the poor soil, bypassing the problem. After a few years of use, the bed can be dismantled and the soil tilled into the clay to improve its overall condition and productivity.