You can improve your soil in a few weeks this fall with the time-honored practice of growing a cover crop. These quick crops add tons of nutrient-rich "green manure," priceless organic matter, and crumbly tilth to your soil.
What to plant? Look at what local farmers grow, and check with your county extension office for detailed advice. Legumes are perhaps the best choice because they "fix" nitrogen from the air into the soil. These crops include alfalfa, peas, beans (garden, fava, and soy), clovers, lupines, and vetches. Non-legumes, such as rye grass, oats, buckwheat, winter wheat, and pearl millet, generally grow more quickly than do legumes, however. Even garden plants such as radishes and kale can be used for a quick soil boost.
Typically cover crops are planted in the fall and tilled in a few weeks before spring planting.
But if you anticipate a long, wet spring, or a mild winter, go ahead and turn your crop in yet this fall. In general, the younger the plant you dig in, the more quickly it will decompose. The more mature the plant, the more organic matter you'll get, but the nutrient release will be slower.
Remember not to let your cover crop go to seed or you will have weeds galore.