Clay Soil

If your soil is very heavy (claylike) you will want to loosen it making it more porous and better drained. Clay soils are sticky-slippery when wet, and bake hard when dry. They hold water and fertility well. However, the tiny clay particles have the shape of plates and nest so tightly that they practically eliminate pore space. Roots can hardly penetrate, and water drains poorly. Such soils are difficult to cultivate except at a certain stage of moderate moistness which may occur only a few days out of a year! Usually they are so slow to dry out in spring that you cannot do early planting. Then in summer they may cake and crack unmercifully. While this may not be too bad under a lawn, it is bad in a garden.

For your garden soil, try mixing two or three inches of organic material into the top six inches of a cultivated bed, to loosen it and improve its structure. Drainage can also be improved by loosening the soil through cultivation and by mixing in soil amendments. If the soil remains soggy much of the time, it may even be desirable to lay tile lines about two feet beneath the surface, to carry off standing water to a drainage channel.

There are several common misconceptions about improving clay soil. One is that adding sand to a clay soil will loosen it up and improve it. Adding sand to a clay soil will probably make it harder and more like cement. Another is that adding gypsum (used to correct acidic soils) to a clay soil will improve it through a process called "flocculation"; this is true but only if you have a very sodic soil (high in sodium), a quite rare soil type not found on the Northern California Coast.

Inorganic Materials as Helpful Additions

It is not always economical or convenient to use organic materials to improve your soil, and there are are other things that work well. Garden stores sell various products useful for loosening heavy soils or retaining moisture and nutrients in lighter ones. Among them are vermiculite, which is made up of particles of puffed mica, and perlite, which is pulverized volcanic stone. These materials are often used in mixing greenhouse potting soils or for making golf greens. They may be helpful for containers or limited portions of the garden, but treatment of extensive areas becomes rather expensive because you may need as much as 50% in poorly-structured soil.

Other notes

Clay soils are usually rich in plant food, especially in potash. Plants once established in them, particularly deep-rooting plants, are carried ahead vigorously. The farm crops that succeed most generally on clay soils are the cereals, grasses and some tree fruits, notably the apple, pear and plum. Clay land is especially valuable for hay. The treatment of a clay soil should be that which will remedy its chief defect — heaviness. Under-drainage will do much to accomplish this result. Under-drainage removes the surplus water in a dry time and promotes aeration and warmth in these soils, many of which are sadly deficient in these respects. The fine particles of clay may be separated from each other and the soil loosened and lightened by mixing them with particles of humus or sand. Barnyard manure or a green manure crop will lighten a heavy clay soil, as well as give body to a light sandy soil. Manures applied to clay soils in the fall lose but little of their plant food by leaching. It is rarely practicable to haul sand upon a clay soil and plow it under, because of the expense, but if this can be done expediently the result will be gratifying. [Not recommended to use sand.]

Extreme caution should be used in plowing and tilling clay soils. If plowed when too wet they become cloddy. There is a certain point between wetness and dryness when a clay soil crumbles quite readily; it should be tilled only at this time, so far as is possible. The texture of a clay soil may be ruined for several years by one injudicious plowing, when it was too wet. Unless the soil is very tenacious, and "runs together" or "puddles" if left bare over winter, clay land may be fall-plowed to advantage, leaving it rough and exposed to the mellowing action of freezing and thawing. The crust that forms so easily over the surface of clay soil in summer should be prevented by frequent shallow tillage. Something may also be done to improve the texture of clay soils, in certain cases, by liming them. This causes many of the fine grains to stick together, forming larger grains, thereby making the soil looser and more porous.

Organic amendments

The best amendment for soil of any texture is organic matter, the decaying remains of plants and animals. As it decomposes, organic matter releases nutrients that are absorbed by soil-dwelling microorganisms and bacteria. The combination of these creatures' waste products and their remains, called humus, binds with soil particles. In clay, it forces the tightly packed particles apart; drainage is improved, and the soil is easier for plant roots to penetrate. In sand, it lodges in the large pore spaces and acts as a sponge, slowing drainage so the soil stays moist longer.

Among available organic amendments are compost, well-rotted manure, and soil conditioners (composed of several ingredients); these and others are sold in bags at many full-service nurseries, or in bulk (by the cubic yard) at supply centers. Byproducts of local industries, such as rice hulls, cocoa bean hulls, or mushroom compost, may also be available.

Finely ground tree trimmings (wood chips) and sawdust are also used, but because they are "fresh" ("green") amendments, they'll use nitrogen as they decompose, taking it from the soil. To make sure your plants aren't deprived of the nitrogen they need, add a fast-acting nitrogen source such as ammonium sulfate along with the amendment (use about 1 pound for each 1-inch layer of wood chips or sawdust spread over 100 square feet of ground).

Though the particular organic amendment you use is often decided simply by what's available at the best price, many experts favor compost over all other choices. Vegetable gardeners in particular prefer compost, and they often also add plenty of well-rotted manure to their planting beds.

References/Notes

• How to improve your soil
• Clay soils
• Improving soil structure
• Got Clay Soil? Gravel to the Rescue. Use of clean crushed, sharp-edged, not rounded gravel. The jagged texture allows air pockets in the clay, making it more porous so plants root better and take up nutrients more easily. Best of all, the gravel stays where it’s put, not breaking down like organic amendments.
• Expanded Shale - a new possibility for amending clay soils. "expanded shale will open up and aerate heavy, sticky clay soils faster than any material that I have ever tested." See TXI's TruGro. TruGro is a basically inert material that is a by-product of TXI concrete manufacturing. It is marketed as a product to aerate the soil and hold water. It costs more than lava sand which works better in all ways. For the price, any of the following amendments would be superior: lava sand, basalt, zeolite, Texas greensand, decomposed granite and compost.
• Trees that grow well in clay soil: balsam fir, boxelder, blue beech, hawthorn, ginkgo biloba, butternut, apple, crabapple, pear, willow, oak, aspen, cottonwood, elm.