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).
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.