Clay soils, also referred to as "heavy" soils, present special problems for gardeners in many parts of Calaveras County. If they are very low in organic matter, clay soils lose their structure and become cloddy and compact if tilled, walked on, or otherwise manipulated when they are too wet. Once a clay soil is badly compacted, it may take years of careful handling to restore good structure.

Tillage is commonly the first and most important step in preparing garden soil. Tilling helps to produce and maintain a good structure for seedling growth and rooting, controlling weeds, keeping the soil permeable to water, and allowing proper oxygen diffusion. When it comes to tillage practices, you need to handle clay soils very carefully to produce good results.

When moist, clay soils should be dug with a spading fork rather than with a shovel or spade. As you turn the soil over, break up the large clods with the side of the fork. The clods that remain should be exposed to the sun and air. After they have dried and crumbled somewhat, wet them with a fine spray to soften them, then rake them when they have dried somewhat. The combination of air-drying, wetting, and raking will break up most of the clods.

Although you cannot make clay soils ideal garden soils, you can improve them by any treatment which will cause the soil particles to form small granules and crumbs. The best treatment for this purpose is to incorporate large amounts of organic matter into the soil to improve soil structure. The improved condition may remain even long after the organic matter has disappeared.

Animal manures, green plant material, compost and leaf mold are especially good for improving soil condition. If you decide to use manures, it’s a good idea to first leach the manure with water to remove excessive salts which may cause plant injury. Materials which decay very slowly, such as peat moss, straw, sawdust, rice hulls and shredded bark are somewhat less desirable because they do not aggregate the soil as well. These organic materials, when first incorporated into the soil, will compete with plants for the available nitrogen, an important plant nutrient. You should apply some extra nitrogen to the soil when using these materials, especially if you intend to plant a crop immediately after adding the organic matter.

The slowly decaying materials such as sawdust and bark do not necessarily improve soil structure immediately. They serve mainly as fillers to increase the percentage of large pores and to improve soil permeability, or drainage. Large quantities of these materials are generally required to have value as fillers; as much as one-half by volume of soil could be needed for the treatment. It may be worth your time, energy and money to add such materials to your garden soil or to use them in preparing large amounts of potting or bedding mixes. However, if you only need small amounts of good soil--such as for potted plants--you are probably better off purchasing soil mixes.

If you use an organic material that decomposes slowly, cover the soil to a depth of about 4 inches. Incorporate this organic matter into the soil to a depth of about 8 inches. Once your crops are planted, you’ll need to water carefully to avoid excessive wetness below the 8 inch depth.

Gypsum is often recommended to improve clay soils. However, this amendment is primarily useful for improving sodic (alkali) soils, which often have poor structure. Gypsum will not improve a clay soil unless the soil also happens to be sodic. Also, most clay soils in California already contain sufficient lime. Lime should not be used unless the soil is excessively acid.

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