Alfalfa Medicago sativa:
Notably one of the most powerful nitrogen-fixing and maintenance free plants there is. The roots of the alfalfa plant have been known to go down into the soil as far as 50 feet bringing up many minerals and nutrients to the surface of the soil. The average depth for alfalfa roots is 20-30 feet.
The great thing about alfalfa is that once it gets established, you will never have to do anything with it because it’s basically maintenance-free, and also that you can usually get 3 or more cuttings per season from one alfalfa plant. Like legumes, alfalfa also has the ability to fix nitrogen in the soil where other plants can utilize it, so cut it back 3-4 times per season and put the tops in the compost pile or till them into the garden and especially the greenhouse which always needs to have the nitrogen in the soil replenished. Alfalfa makes a great compost activator.
Alfalfa is very drought tolerant which is why so many farmers use it for hay. Just keep in mind those long roots make alfalfa impossible to eradicate if you want to get rid of it in the future. So be sure to plant it someplace where you want it to be forever. Alfalfa is a large, bushy plant usually becoming 3-4 feet tall and wide and so it offers shade to nearby plants and give shelter often keeping other plants alive longer during droughts.