Organic matter improves soil in several ways. According to the Oregon State University Extension Service, adding organic matter to soil improves its ability to hold water, increases the activity of beneficial soil organisms, improves texture--allowing for better aeration--and prevents polluting the environment from the runoff of fertilizers and pesticides. To the home gardener, this often translates to reduced watering needs and increased health of flowers or vegetables, as soil organisms make nutrients available to plants. The kind of organic matter applied to the garden determines how soon soil condition improves and how quickly nutrients are available to plants.
Add fresh organic matter like vegetation or animal manure several weeks before planting. Work the material into the soil with a garden tiller or hand tools. Allow the organic matter to decompose in the soil. Fresh organic matter ties up nitrogen as it decomposes. Vegetables or flowers planted in soil with fresh organic matter often suffer a nitrogen deficiency until the organic matter fully decomposes.
Layer decomposed organic matter like well-rotted manure, peat moss or compost over the soil and work into the top 3 to 4 inches of soil prior to planting. For a home gardener, a 2- to 3-inch layer is sufficient. Peat moss can lower the pH of the soil. If using peat moss, check your pH with a soil test kit before planting. Adjust the pH following the directions contained in the kit.
Amend your soil with organic matter each year to build up your soil. It may take several years to produce organic-rich soil, but your efforts will be rewarded with soil that requires less fertilizer and less water.