Most gardens have soil that provides something less than the ideal environment for many garden plants. Perhaps it's rocky or scraped bare from new construction; perhaps it's too claylike or too sandy to suit the plants you want to grow. While changing a soil's basic texture is very difficult, you can improve its structure--making clay more porous, sand more water retentive--by adding 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.
Adding amendments: when and how
New beds for landscape plants should be amended before any plants go into the ground. For long-term benefits, choose an amendment that breaks down slowly. Shredded bark and peat moss hold their structure the longest, taking several years to decompose. It's a good idea to include compost in the mix as well; though it breaks down in just a few months, it bolsters the initial nutrient supply available to soil microorganisms--and these will contribute humus to the soil, improve soil aeration, and help protect your new plants from some diseases.
In beds earmarked for vegetables and annual flowers, amend the soil before each new crop is planted. Compost and well-rotted manure are preferred by most gardeners, since they dramatically improve the soil's structure, making it hospitable to the fine, tiny roots of seedlings. Unamended soil may dry into hard clods that small roots cannot penetrate, and plants may grow slowly, be stunted, or die as a result. Manure and compost break down rapidly--manure in a few weeks, compost in several months--so be sure to replenish these amendments before you plant each crop.
To add amendments to unplanted beds like those just discussed, spread the material evenly over the soil, then work it in by hand or with a rototiller to a depth of about 9 inches. If your soil is mostly clay or sand, spread 4 to 5 inches of amendment over it; once this is worked in, the top 9 inches of soil will be about half original soil, half amendment. If the soil is loamy or has been regularly amended each season, add just a 2- to 3-inch layer of amendment; you'll have a top 9-inch layer of about three-quarters original soil, one-quarter amendment.
Permanent or semipermanent plantings of trees, shrubs, or perennials benefit from soil amendment too, but you need to do the job without damaging plant roots. It's often sufficient simply to spread the amendment over the soil surface as a mulch; earthworms, microorganisms, rain, and irrigation water will all carry it downward over time, gradually improving the soil's top layer. If the plant isn't a shallow-rooted type (that is, if it doesn't have many roots concentrated near soil level), you can speed up the improvement process by working the amendment into the top inch or so of soil, using a three-pronged cultivator.
Where the climate is generally mild and winters are rainy, amend the soil in established plantings annually after fall cleanup. In cold-winter regions with spring and summer rainfall, do the job as you begin spring gardening.
Sand and peat moss: good amendments for clay soil?
Sand is often recommended to lighten clay soil. This seems a practical suggestion: after all, clay is the finest-textured soil and sand the coarsest, so mixing the two should result in just the right blend. It's not that simple, though. The problem is that you must add a great deal of sand to make a difference--at least 4 inches of coarse sand to the top 6 inches of clay soil. Improving even a moderate-size planting bed thus requires a great deal of heavy sand and heavy labor. Many gardeners compromise by simply sprinkling a little sand on top of their clay soil, but such small amounts do no good; in fact, they actually compact the soil further.
Peat moss has long been a favorite soil amendment because it breaks down in the soil more slowly than manure or compost and can thus be replaced less frequently. It is also highly absorbent; it holds water in the soil longer than many other amendments do, making it especially beneficial in sandy soils. But if your soil is naturally claylike and drains slowly, the super-absorbency of peat moss can exacerbate the drainage problem, especially if you have heavy winter rains.