The structure of clay soil can make it difficult to till.
The structure of clay soil can make it difficult to till and cause it to drain poorly. Additionally, it can create an undesirable environment for seedlings to emerge and for roots to deeply penetrate. The structure, however, can be improved with the addition of wood chips. compost and other organic material.
Soil Structure
Soil structure is the manner in which soil particles stick together, or aggregate. The clay particles in clay soil are usually arranged horizontally in a plate-like structure. Over time, they stack high and become consolidated. This causes the soil to become tight and sticky. In clay soils, the clay particles make up more than 40 percent of the total soil. Soils with a high clay content often aggregate into large chunks, called peds.
Importance of Structure
Improving the structure of clay soil means achieving an equal balance of sand, silt and clay particles. This results in looser, more crumbly or more granular soil, better water-holding capacity, better absorption and lack of crust. It also would improve the tilth of the soil, or the ease with which it can be tilled, along with the quality of the seedbed, ease of seedling emergence, and depth of root penetration.
Soil Amendment
Fine Gardening recommends adding organic matter to improve the structure and tilth of clay soil. This includes the application of compost, manure, leaves or other organic yard waste. The organic matter coats the soil particles, separating clay from the other aggregates. Microorganisms then break down the organic matter and produce glomalin, which reduces crusting, erosion and runoff, and increases the rate of water absorption.
Wood Chips
The organic matter should be incorporated by deeply tilling it into the soil. This, however, can be difficult with clay soil. Horticulturist Linda Chalker-Scott, an associate professor at Washington State University, recommends adding wood chips to the soil's surface about a month before it is tilled. This allows the soil to retain moisture, reduce the impact of foot traffic and regain its structure, making it easier to penetrate.
Other Methods
Adding sand to clay soil also would improve its structure, but only if the sand makes up about 50 percent of the total soil volume, according to Chalker-Scott. Fine Gardening also suggests ''cover cropping'' to improve the structure of clay soil. This means planting a ''cover crop,'' such as grasses or legumes, when the soil would not normally be planted. The cover crop smothers weeds and allows the organic matter to further decompose.
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