Up until the 20th century, it was common for farmers to use cover crops to prepare their soil for planting their production crops and to avoid the use of fertilizers and herbicides. Cover crops are legumes and grasses that are grown specifically to turn under and into the soil, to help prepare the soil for future crops. Legume crops, such as soybeans, have been shown to add much needed nitrogen into depleted soil and provide organic matter for the soil to help improve texture and produce healthy production crops. Grasses, such as buckwheat, do not add nitrogen to the soil, but grow very quickly and add potassium to the soil.
Grasses that are suitable for cover crops are barley, rye, wheat, oats and buckwheat. These all add organic matter to the soil when turned under, and they help to deter erosion when they are growing in the field. These crops are planted closer together than they would be when grown for production, and should be cut down and turned under before they go to seed. Grasses are suitable for planting as alternate crops between growing seasons.
Alfalfa is a legume, and therefore, it is among the plants that pull nitrogen out of the air and stores it within its structure. Alfalfa fixes 150 to 250 lbs. of nitrogen per acre per year. Alfalfa is especially helpful in hard soil, as it has hearty roots that dig deep and help to break up the soil. Alfalfa needs warm temperatures to germinate but is tolerant to drought. Alfalfa is planted in the late spring or summer and turned under in the fall.
Hairy Vetch can be planted in the fall and turned under in the early spring. It produces 80 to 100 lbs. of nitrogen per acre per year. Hairy Vetch is slow to germinate, but once it establishes itself, it is a very hardy plant. Be sure to turn under before it seeds, as it can become an intrusive weed if allow to reseed.
Fava beans can be planted in early spring or in late summer and harvested in early summer and fall, respectively. Fava Beans can fix 70 to 100 lbs. of nitrogen per acre in as little as six weeks. It will grow on a variety of soil types and is somewhat drought-resistant.
Peas can be used as a cover crop or as a production crop. Therefore, they work well for planting between other crops. Peas are a cold weather crop, so they can be planted early in spring for an edible crop or and planted late in the summer or early fall for a winter crop, when other vegetables are just finishing up.
Crimson Clover fixes 100 to 150 lbs. of nitrogen per acre per year, so it is very beneficial for the crops that follow it, but it is not drought-resistant and not a hardy plant. Austrian Field Peas grow rapidly and heavily and are a good source of nutrients, but may need to be mowed before turning under to break up the dense vines.