Some crops can be grown specifically for the purpose of improving soil to allow the gardener to grow other, more desirable plants. This practice is also called green manuring. In addition to adding nutrients, green manuring can provide food to sustain microorganisms and aerate soil, and can even till the soil with deep root systems. While you can simply till the plants into the soil, you will need to wait at least 45 days before planting other crops. A more efficient way to use these plants is to compost them and use the compost from the previous year as fertilizer.
Rye straw grows very tall--up to 7 feet--so the plant produces lots of organic matter. The roots also loosen compacted soil, providing natural aeration without any labor.
Rye should be planted in the fall for harvest in the spring. You can harvest the rye before using the straw for compost. Expect between 4 and 26 lb. of grain per 100 square feet.
Mustard has a very short growing maturation, so if you need soil enrichment quickly it may be a good choice. It will reach maturity in as little as six weeks. It is also very productive. Expect up to 270 lb. of green material for every 100 square feet you plant. Mustard is often used in orchards. It has excellent abilities to return health to tired soil. It also attracts honeybees. Harvest just after flowering.
If you have a plot of soil that has been overworked but you would like to begin planting again, try alfalfa. Plant alfalfa yearly to build up soil over several years. It is an excellent source of nitrogen since it is a legume. It also sends deep roots down into the subsoil, allowing nutrients that are buried deep to be pulled to the surface.
Depending on the climate, alfalfa will be ready to cut between three and 12 weeks after planting. You will be able to harvest up to 360 lb. of green material over the course of six cuttings. When you are ready to grow other plants in the plot, you will need to dig up all the alfalfa roots.
Banner Fava Beans
Another excellent choice for adding nitrogen to the soil is banner fava beans. Be careful harvesting these for food; though they are very popular in Egypt, they can be fatal to those who are allergic. This plant will bring up excess moisture and yield up to 360 lb. of organic material, as well as provide up to .16 lb. of nitrogen for every 100 square feet planted. Between 5 and 18 lb. of beans can be harvested every year.