Fall is the preferred time to prepare garden soil for next spring’s vegetable garden. Spring is often wet making it difficult to work soil without forming clods that remain the rest of the season. Fall usually is drier allowing more time to work the soil when it is at the correct soil moisture content. Even if you work soil wet in the fall and form clods, the freezing and thawing that takes place in the winter will break them down, leaving a mellow soil the following spring.
Insects often hide in garden debris. If that debris is worked into the soil, insects will be less likely to survive the winter. Diseases are also less likely to overwinter if old plants are worked under. Also, the garden debris will increase the organic matter content of the soil. Working the debris into the soil is often easier if you mow the old vegetable plants several times to reduce the size of the debris.
Fall is an excellent time to add organic matter. Not only are organic materials usually more available in the fall (leaves, rotten hay or silage, grass clippings) but fresher materials can be added in the fall than in the spring because there is more time for them to break down before planting. As a general rule, add 2 inches of organic material to the surface of the soil and till it in. Be careful not to overtill. You should end up with particles like grape nuts or larger. If you work your garden into the consistency of dust, you have destroyed the soil structure. (Ward Upham)