There are a number of plants that have the ability to take nutrients from the air and store them in their roots. Chances are this nutrient is nitrogen and the plants are often legumes. There is a soil-borne bacteria that harvests the nitrogen from the air and the plants store it and bacteria make use of the carbohydrates produced by the plant. This is called a symbiotic relationship. Humans have been aware of this relationship for thousands of years and often rotate the legumes with nitrogen-hungry crops like grains. The early peoples would not have known why their crops grew better with this rotation, but the increase in production was noted and the practice spread from ancient India all across Asia, the Middle East and finally to Europe.

As we finally understood the science behind this, modern man used this green manure to improve the soil by adding organic matter. This not only adds nitrogen but improves tilth in heavy clay and acidifies the soil, decreasing the pH. The web-like roots also help hold soil in place and prevent erosion.

One of the plants most often used as a green manure is hairy vetch, Vicia villosa. This European native has been an agricultural tool for so long that it is now widespread world wide and naturalized in most temperate countries. The distinctive purple blooms are often seen in fallow fields and along roadways. Where left on its own, it forms dense mats of weedy-looking vines that tangle and scramble over the ground. Farmers plant it in the fall, since it is a tough, winter growing annual that quickly takes off as soon as the sun shines in early spring. They allow it to grow until it starts to bloom and then turn it under as compost. The field sits idle for 30 days as the plants break down and then planted with the cash crop as usual. This practice allows both the nitrogen fixation and the green manure benefits from one planting. Hairy vetch is also mixed with other grasses and used as forage and hay for livestock. The nutrition isn't as good as alfalfa, but can still contribute significantly as animal feed.

vetch

To utilize hairy vetch in smaller gardens or raised beds, the plants are seeded in the fall as usual, however, once they have reached maturity, they are cut and composted in separate bins or beds. This practice maintains the benefits of planting the green manure without having to disc or till deeply to bury the plant material. Once the vetch has broken down, gardeners spread it in their smaller beds like any other compost, while the decomposing roots release their nitrogen directly in the bed.

Since Vicia villosa is a non-native for most of the world, it is also considered invasive in some areas, so be sure to check with your local agricultural authorities before planting any. I know that the states in the northern tier of the U.S. like Minnesota and Wisconsin have it listed as invasive. Here in Kentucky, it is not. My father planted it as green manure in the 1960's and I'm sure that farmers all over the continent did so then as well. Hairy vetch has escaped cultivation everywhere, however it has only proved to be a pest in some areas. If you need to eradicate it on your property, mow it before it blooms. Doing this for several seasons will take care of the dormant seeds that germinate each year and eventually the plants will fail to return.