Why do legume cover crops improve soil fertility more than many nonlegume crops?

Legume cover crops improve soil fertility more effectively than many nonlegume crops primarily because of their unique ability to perform nitrogen fixation.1

While atmospheric nitrogen makes up about 80 percent of the air, it is not directly usable by most plants. Legumes form a symbiotic relationship with Rhizobium bacteria that reside in their root nodules. Through this partnership, the bacteria convert atmospheric nitrogen into ammonia and other nitrogen compounds that the host plant can utilize for growth.12

When these legume crops are used as cover crops or green manure, the nitrogen stored in their tissues is released into the soil as the plant matter decays. This process provides a vital, organic source of nitrogen for subsequent crops, reducing the need for synthetic fertilizers. In contrast, nonlegume crops generally lack this specialized biological mechanism to capture and convert atmospheric nitrogen, making legumes superior for naturally enhancing soil nutrient levels.132