There are several crops that can be planted as "green manure" for you to turn under for soil improvement. These crops, like Annual Rye Grass, are planted in the fall or winter and then tilled back into your garden in the spring.
Legumes, like alfalfa, are the very highest in nutrients, but they don't grow as fast as some of the grasses. My favorite grass to grow is Annual Rye Grass since I can grow an abundant quantity of organic material in a fairly short amount of time. Besides the abundant top growth, it produces incredible quantities of small hair roots which break up the soil, decompose quickly and add humus to the soil. It will grow about knee high. It also makes for a super fun place for kids to play in. Just till it back in to the soil in the spring.
Another benefit of planting Annual Rye Grass is the humus it adds to the soil.
Benefits of adding humus with your soil:
1. Improves aeration, drainage, water penetration and water retention and ease of root penetration
6. Gives the soil a spongy quality which keeps it from compacting
7. Gives a good, soft, crumbly texture which makes it easy to work with
8. Makes nutrients more accessible for healthy plant growth
9. Makes the plants more resistant to disease