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How do I improve my clay soil?

Good soil is the key to successful gardening. Unfortunately, most gardeners are not blessed with good soil and must work to improve it. This is especially true if you have soil with a high clay content, as is much of the soil in St. Louis.

Clay soils present several obstacles that need to be overcome. It tends to compact easily and form a hard, dense layer on the surface that prevents water, air and fertilizers from moving downward. Compaction reduces air space in the soil. This can lead to plants developing shallow root systems and makes them more prone to drought stress. Clay soils also tend to drain slowly. During wet weather or with frequent irrigation clay soils often stay wet for too long and in severe flooding plant roots may suffocate. Finally, clay soils are often difficult to cultivate because they dry out so slowly in spring and many gardeners attempt to work the soil before it is dry enough. When this is done, you can destroy the physical structure of clay soil and large hard chunks called clods usually result. These clods may be cement-like and take considerable effort or alternating freeze and thaw cycles to break them apart. Although clay soils have disadvantages, they also have the advantage of retaining water and nutrients well.

There is just one practical way to improve clay soil and that is to work in plenty of organic matter. Commonly available sources include peat moss, rotted animal manure, and compost made from leaves, plant refuse or grass clippings. Another good source of organic material is green manure. Green manure is a crop such as annual rye grass that is grown and then turned under when it is one-third to one-half grown when it is still green and lush.

To improve your soil, dig organic matter into the soil each year either in early spring or fall. Spread anywhere from a one to four inch layer of organic matter on the soil and work it into the top 6 to 10 inches of soil. Remember, organic matter breaks down over time so you need to replace it regularly; yearly is best.

In some cases such as with perennial flower or bulb gardens, you cannot work organic matter in every year. For these gardens work organic matter in when you plant new bulbs or flowers and mulch in the fall. When you plant trees and shrubs, adding organic matter to the planting hole is usually not recommended, but you can work organic matter into the circle of soil skirting the plant. Be sure and go out two to five feet in all directions from the trunk to improve the soil into which the new roots will be growing.