Legumes
Legumes can be thought of as the original fertilizer. Columella writes in Rome in the first century, “Where no kind of manure is to be had, I think the cultivation of lupines [a legume] will be found the readiest and best substitute. If they are sown about the middle of September in a poor soil, and then plowed in, they will answer as well as the best manure.”
In addition to lupins, the legume family includes beans, peas, clovers, and alfalfa, among many others. Whether or not you plan to use commercial fertilizers, legumes remain an economically and ecologically responsible way to introduce nutrients into the soil.
Agricultural researcher and USDA advisor Elliot Coleman writes of legumes:
Leguminous green manures are a most economical and inexpensive source of nitrogen. The nitrogen is produced right where it is needed—in the soil… The deep-rooting ability of many leguminous green-manure crops also makes them valuable as biological subsoilers. Where soil compaction exists, deep-rooting green manures can bring a startling improvement in subsequent crops, solely by penetrating and shattering the subsoil with their roots.
Employing legumes as green manure isn’t as simple as it sounds. Sustainable Agriculture Research and Education (SARE) writes:
Inoculation. If you grow a legume as a cover crop, don’t forget to inoculate seeds with the correct nitrogen-fixing bacteria. Different types of rhizobial bacteria are specific to certain crops. There are different strains for alfalfa, clovers, soybeans, beans, peas, vetch and cowpeas. Unless you’ve recently grown a legume from the same general group you are currently planting, inoculate the seeds with the appropriate commercial rhizobial inoculant before planting. The addition of water to the seed-inoculant mix, just enough to moisten the seeds, helps the bacteria stick to the seeds. Plant right away, so the bacteria don’t dry out. Inoculants are readily available only if they are commonly used in your region. It’s best to check with your seed supplier a few months before you need the inoculant, so it can be specially ordered if necessary. Keep in mind that the “garden inoculant” sold in many garden stores may not contain the specific bacteria you need. Be sure to find the right one for the crop you are growing and keep it refrigerated until used.