Here’s a bit of geeky plant science. On this image of the roots of white clover, you can plainly see bumps along the roots that are called nodules. Over millions of years, the plant has evolved a symbiotic relationship with certain species of soil-dwelling bacteria called Rhizobia. This group of bacteria has the ability to take nitrogen from the atmosphere and “fix” it by metabolizing it into ammonium, which is a nitrogen compound that the plants can make use of. The plants benefit by using this extra nitrogen to compete with their neighbours by growing stronger and faster. Without the nodules along the plants’ roots, the bacteria could not exist, let alone function. It’s a win-win situation.
Clover is a member of the family Fabaceae (all are called Legumes), and many plants within this family share this quirky talent to host nitrogen-fixing Rhizobia. Some of the best known of these plants are soy, peanuts, beans, peas, lupins, sweet peas, chickpeas, licorice, carob, alfalfa, and vetch. Each plant has a relationship with its own species of Rhizobia, although there is some cross-over. When the plants are harvested or die back, the nitrogen (in ammonium form) is left in the soil, making it more fertile for the next crop that is planted. Nitrogen, after all, is the most difficult of the major plant nutrients to maintain in soil.
It makes sense, then, that organic farmers would grow a crop of nitrogen fixing plants and then till them under before planting a marketable crop. Tilling the plants under takes advantage of their organic matter as well as the nitrogen in their root nodules. In conventional farming, the grower might simply apply hundreds of pounds of ammonium nitrate to her field, simply spreading the raw chemical before planting. In organic farming, a simple cover crop of nitrogen-fixing legumes is planted and grown for around three months before the main crop goes in.
Seed inoculants are simply a powdery form of Rhizobia. Seeds are dampened and then coated with this powder prior to planting. This introduces an abundant population of nitrogen-fixing Rhizobia into the soil at the time of planting, and kick-starts the whole process. If legume seeds are not inoculated, they will still develop root nodules and become hosts to Rhizobia, but more slowly.
Taking advantage of nitrogen-fixing cover crops seems very similar, in my view, to planting flowers that attract beneficial insects like predatory wasps. The grower is able to encourage natural processes to take place that will benefit the crop plants. No chemicals are used. Nothing unnatural takes place. The whole system is sustainable and environmentally sound.