Annual ryegrass also can persist like a perennial, making it a potential weed when used as a winter cover crop. Some gardeners use annual ryegrass as a companion crop for crimson clover. The most important benefit of legumes is their ability to fix nitrogen from the atmosphere into the plant. This is different from the grasses, which can only take up nitrogen already available in the soil. Legumes fix nitrogen in association with bacteria called Rhizobia, which form nodules on legume roots.
Active nodules look pink when broken open.
When the legumes are turned under and decompose, the fixed nitrogen is released for future crops. Rhizobia are usually present in the soil, ready to inoculate legume plants and begin fixing nitrogen. Occasionally the proper Rhizobium is not present, and nitrogen fixation will not occur. In this case, root nodules will be white or green rather than pink on the inside. If you have not grown a particular legume in your garden before, you should buy the correct Rhizobium and mix it with the seed before planting.