Nitrogen is often the limiting nutrient in a garden, thus N-fixing plants like legumes and many others will slash the need for fertilizer, and at the same time pour more organic matter into the soil than fertilizer ever could. This is perhaps the most important class of plant to have in a young garden. In poor soils, having 25 percent N-fixing plants to begin with is not too many. They can be culled as the garden matures. Remember to use nitrogen fixing trees and shrubs too, not just perennial and annual herbs.
Plants that contribute to nitrogen fixation include the legume family – Fabaceae – with taxa such as clover, soybeans, alfalfa, lupines, peanuts, and rooibos. They contain symbiotic bacteria called Rhizobia within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the fixed nitrogen is released, making it available to other plants and this helps to fertilize the soil. The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not. In many traditional and organic farming practices, fields are rotated through various types of crops, which usually includes one consisting mainly or entirely of clover or buckwheat (family Polygonaceae), which were often referred to as “green manure.”
Although by far the majority of nitrogen-fixing plants are in the legume family Fabaceae, there are a few non-leguminous plants, such as alder, that can also fix nitrogen. These plants, referred to as “actinorhizal plants”, consist of 24 genera of woody shrubs or trees distributed among in 8 plant families. The ability to fix nitrogen is not universally present in these families. For instance, of 122 genera in the Rosaceae, only 4 genera are capable of fixing nitrogen. All these families belong to the orders Cucurbitales, Fagales, and Rosales, which together with the Fabales form a clade of eurosids.