To most of us, the idea of getting something free is appealing. The old adage, "you scratch my back and I'll scratch yours," was originally said with the idea of people doing favors for each other. In a very similar way, that is what happens in nature with plants that extract nitrogen from the air, bottle it in little containers called nodules on the plants roots, and in turn use the nitrogen for their own growth. What a deal. It's like winning the lottery without buying a ticket, putting the money in the bank, and drawing interest.
There are well over 1,000 nitrogen-fixing plants, from bacteria, algae, ferns, shrubs, and commercial crops to many species of trees. These plants serve as important components in the intricate way Nature has designed complex ecosystems. We have learned how the "fixation" process works and which plants can generate the most nitrogen. Some of the nitrogen-fixers, which are classified as legumes, include garden peas, Austrian winter peas, beans, cowpeas, vetch, clover and alfalfa, to name just a few. Many farmers have used alfalfa in their crop rotation system, for building better soils. Alfalfa penetrates deep into the soil (even heavy clay soils), extracts minerals, increases soil organic matter, and add lots of nitrogen in the process.
Our air is 78 percent nitrogen, which means we have almost 3 tons of nitrogen above each acre of land. Unfortunately, plants cannot normally use it directly from the air in the stable gaseous state. It needs to first be converted into the various nitrate compounds. In most legumes that conversion comes about because of a symbiotic relationship with Rhizobium bacteria, which form those little white (pink inside) nodules on the plant roots.
The plants provide the bacteria with carbohydrates for energy and a stable environment for growth, while the bacteria give the plants nitrogen and other nutrients. Dig up a clover or bean plant in your garden and see if the nodules are present. If not, you may need to add bacteria (inoculum) to get the "fixation" process started. Most seed companies have the specific inoculum that you will need for your crop.
The amount of nitrogen produced by nitrogen-fixers can be very significant. Depending on soil conditions and the species, you can get 75-150 or more pounds of nitrogen produced per acre. Best of all, it is in a form that the plants can use and will be slowly released for use over the season. It's not likely to be leached out of the soil, and it is not going to be toxic or burn the plants.
I see many gardens and most gardeners are not using the nitrogen-fixing plants to their advantage. They are rejecting this free offer to put money in the bank. Organic growers apply nitrogen using a great variety of slow-release fertilizer sources – compost from vegetation and manures, alfalfa meal, fish meal, feather meal, and blood meal. These are all good, non-leachable forms, which will benefit plant growth. To move in the direction of being more sustainable, it would pay to use more of the nitrogen-fixing plants.
I use Austrian winter peas, and like to have them in the ground by October 1. They will grow for 6-8 weeks in the fall, and then resume growth again in February. By May 15, they will 30 inches or more tall and have produced a lot of nitrogen. The Austrian winter pea vegetation makes excellent compost, or you can use it for mulch on the garden. It's money in the bank!