Tim Baland, Extension Master Gardener in Anoka County
If you have planted a heavy feeder (such as corn) in your garden, are leaving all or a part of your garden fallow this year, or are creating a new bed, you may wish to plant a green manure cover crop now or in the fall, to reduce weeds and restore nutrients to the soil. A cover crop forms a living mulch that helps to control both erosion and weeds. When that cover crop is turned into the soil, it is called “green manure” because it helps to restore nutrients, especially nitrogen, to the soil.
Suitable cover crops include legumes, such as vetch, clover, beans, soybeans, and peas. Green manure crops that are legumes work by “fixing” nitrogen in the soil. Bacteria is always present in the soil, and lives in nodules in the roots of leguminous plants. The bacteria that is present in the roots of legumes converts nitrogen gas from the air into a form that plants can use. When those plants die and begin to decompose, the residual nitrogen that is stored in the roots becomes available to other plants. Many farmers grow soybeans in rotation with other crops, such as corn, that deplete nutrients from the soil, because soybeans restore the lost nutrients.
You can plant a green manure cover crop either in the early spring or summer or in the fall. If you have created a new garden, you may wish to consider planting a cover crop for the first season, and then turning that cover crop over in the fall so that the organic matter created by the cover crop is fully incorporated into the soil by the time you plant. You should plant your cover crop early enough that it will have sufficient time to grow, die, decompose, and then be incorporated into the soil.
You should decide what green manure crops you are going to grow in your garden, when, and where. It may be helpful to draw a diagram. The location where you plant it will depend largely on the height that the crop will reach, and if that height will shade or otherwise harm the other plants that you are growing. For example, I left several sections of my garden fallow this year because I am planting vining crops that need room to expand. However, I planted clover as a green manure crop in those spaces because I know that clover will not get very high and shade the more desirable plants.
Green manure crops are not just for large-scale farming and agricultural operations. With a little planning, the home gardener can successfully use them to improve the soil, cut down on weeds, and control erosion. For more information, see: