Healthy garden soil contains about 5 percent organic matter. This is the decomposed remains of plants from sources that include compost and crops that are grown specifically to enrich the soil, which are called cover crops.
Organic matter in the soil benefits plants in several ways. It improves soil structure so that plant roots can easily spread, and it retains moisture and nutrients that are released as the plants need them.
One of the greatest advantages a garden can have is a generous supply of mature compost. In early spring and late fall the garden should be covered with about 2 inches of compost. This should be lightly scratched into the top 3 or 4 inches of soil.
A common mistake is to rototill the garden after spreading compost. Tilling is OK if it is really shallow, but deep tilling puts the compost below the area where plant roots would be able to reach it. Deep rototilling also harms the biological life that is so essential for healthy soil.
Compost added to a garden in the spring should be mature---that is, it should be fully composted with no more microbial activity---and it should be screened so that large lumps are removed. Compost that is not mature will steal nitrogen from the soil, robbing your plants of this essential nutrient. Lumps will prevent you from creating a smooth seedbed and may obstruct plant roots.
In the fall, compost quality is not quite so critical. It will mature over the winter without affecting young plants, and any lumps will probably break down by spring.
How cover crops work
Soil is enriched and protected by cover crops in several ways. The cover crops chosen should be based upon the needs of your garden soil. Sometimes called green manures, these crops hold the soil in place with their roots, preventing erosion from wind and rain. Some, such as vetch and peas, fix nitrogen in the soil that benefits whatever grows there next. If planted densely enough, most cover crops will out-compete weeds.
All cover crops benefit the soil when their life cycle is complete and they are incorporated into the soil to add vegetative (organic) matter to the soil.
Choosing a cover crop
Some food crops---sweet corn, for example---are called heavy feeders, because they gobble up so many nutrients from the soil. Following corn with a nitrogen-fixing legume such as cowpeas or hairy vetch will replace those nutrients.
If weeds are a problem, a dense cover crop of buckwheat or oats will help, especially if they are sown mixed with a legume such as purple vetch. This cover crop will shade out weeds and inhibit them from germinating. Rye is a common winter cover crop.
Poor drainage can be improved by growing red clover or lupines, which have deep root systems that penetrate compacted soil layers.
Incorporating cover crops
At the end of their useful life cycle, cover crops can be incorporated into the soil by shallow rototilling or hand digging. Top growth can be mowed and allowed to wilt to make incorporation easier.