Like most words beginning with "al," alfalfa derives from an Arabic word, the name given to a legume of ancient Persia. It translates roughly as the "best fodder," and now, thousands of years later, it still is. Grown for livestock, the plant is a prime source of protein as well as calcium, boron and other essential elements.
It's also highly nutritious for a garden's soil. Like peas, beans and clovers, alfalfa roots grow in association with certain bacteria, forming nodules that enable the plant to collect nitrogen from the air. A plot where alfalfa has grown, leaving behind decomposing roots and nodules, gives a boost to the next crop planted there. In addition, these roots are mighty foragers, heading straight down 12 feet or more to mine nutrients from the subsoil. The old roots then feed both the plant and the soil in which they grew.
Alfalfa makes a good drought-resistant perennial cover crop for areas not in use and for parts of the garden that are vacant between early and late plantings. Tilled as a green manure, alfalfa adds fertility, stimulates microbial activity and aerates the soil. There are even "summer" varieties that die over the winter and can be overplanted without tilling.