The secret of a good garden is good soil. For a better vegetable or flower garden next year, plant a cover crop this fall.

It's called "green manuring": When the beans, tomatoes, potatoes, petunias, zinnias and marigolds are through for the season, plant a cover crop of annual rye. It helps improve almost all soils, and particularly those that are predominantly clay or sand. The ideal garden soil is fertile, well-drained and high in organic matter.

Heavy clay soils dry out slowly and are difficult to cultivate and work properly. Extremely sandy soils dry out too fast between applications of water.

A cover crop can be grown whenever the land's not in use. You turn the annual rye under long before it reaches maturity, yet even when immature the tops and roots combined improved the soil. In addition, the cover crop reduces the amount of soil lost during winter by splash erosion and surface runoff.

Splash erosion is due to the impact of the raindrop on the soil surface. Each drop, falling as a tiny projectile, breaks a small particle from the soil mass.These particles may then be removed by surface runoff during heavy rainfall. During a particularly bad winter the loss can be much heavier than one would suppose.

Grass plants are the best natural soil-conditioners known. They produce an extensive system of small roots that penetrate the soil. When the crop is turned under, soil microorganisms convert them into humus.

The organic-matter content influences the amount of water available to plants that can be held in the soil. While moisture-holding capacity is important, too much water will crowd the air out of the soil, so good drainage is necessary. Without air, plants can't grow, take up nutrients or even absorb water, with air in the soil, the carbon dioxide produced by soil bacteria and by the respiration of plant roots can be diffused into the atmosphere.

If water moves freely through the soil, air can fill the empty pore spaces. When water moves too fast, the result is a droughty soil.