Normally, the cover crops include some sort of legume. Legumes are plants that bear their seeds in pods. They are different from non-legume crops, such as tomatoes, squash, etc. because much of the nitrogen they require is produced through fixation of atmospheric nitrogen by bacteria in nodules on their roots. Common legumes you are probably familiar with are crops such as beans, lentils and peas.
Last fall, I decided to plant Austrian Winter Peas in one of my raised beds. I didn't get them planted until late in the fall and I was surprised they even sprouted before cold weather arrived. They were small most of the winter, but as the weather warmed this spring they grew to be about a foot tall. A couple of weeks ago, Tom tilled them under so they could decompose and enrich the soil before we planted tomatoes in that bed.
I rescued this one. It's sort of sad looking because I didn't get to it for several days after he tilled.
If you look closely, you will see little nodules on the roots that contain the nitrogen-fixing bacteria referred to above. These, along with the plant itself, are what add nitrogen to the soil when the plant is turned under.
Using nitrogen-fixing cover crops, along with compost, is one of the ways we enrich our soil without using chemical fertilizers.
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