It’s a pretty neat trick: beans, peas, clovers – any of the legume family of plants – pull inert nitrogen out of the air and convert it into ammonia, an essential component of protein and DNA. These plants enrich the soil as they grow. For most of human history this was more or less the only way that atmospheric nitrogen could be added to soils.*
They do this with the help of diazotrophs , bacteria that live in the roots of most legumes. The bacteria are attracted by the presence of carbohydrates on the root hairs and root surfaces, which provide energy to grow and multiply. Invading the roots, the diazotrophs form small lumps called nodules and begin producing ammonia. The ammonia gives the plant a boost, helping it to thrive without drawing nitrogen from the soil; when the plant dies the nutrient is released into the surrounding earth.
I meditated on this mutually beneficial alliance of plant and bacteria last week while I cleared some empty planting beds of weeds. The garlic has been harvested, lettuce has bolted, and radishes have grown stalky and bitter. A cover crop sown in these beds will add organic matter and control erosion; why not add some legumes to increase the nutrients? Many legumes grow better in spring and summer but it’s not too late to plant field peas, clovers or vetch along with some winter rye for a thick, weed suppressing carpet. Guess I’ll have to stop by the supply store.* The invention in the early 20th century of a method to create ammonia through non-biological means and using it to make synthetic fertilizer was key in creating the modern agricultural system. Before that, nutrients were recycled through manures and compost, fallowed fields, and growing legumes.