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. Source: Sunset.com
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. Miramar Landfill offers compost at $10-$12 per cubic yard, that's pretty cheap!
From my research I've found the list of organic soil amendments to be endless. Compost seems to be the easiest and cheapest route. If you are interested in looking at other options such as wood chips, peat moss, newspaper, different types of manure, etc... Cornell University has a comprehensive list of the pros and cons of the different soil amendments.
