Plant These Native Nitrogen-Fixers
These plants can contribute fertility to permaculture systems while supporting the native ecosystem
Nitrogen-fixers are a hallmark of permaculture systems. They are plants that (usually) form symbiotic relationships with soil bacteria called rhizobia that convert atmospheric nitrogen into ammonia (I say "usually" because there are some fascinating exceptions.) This symbiosis is a remarkable exchange: the bacteria share ammonia with the plant—which needs it to make proteins and chlorophyll—and the plant, in turn, shares organic compounds, like sugars, with the bacteria that it produces via photosynthesis.
Bacteria can't photosynthesize and plants can't produce the enzymes needed to convert atmospheric nitrogen into ammonia. This relationship works to the benefit of both organisms, which is marvelous, but also to those around it, who don't even participate in the exchange. This is usually the primary reason nitrogen-fixers are found in permaculture systems: to add nitrogen to the neighboring plants. Put enough nitrogen-fixers in a well-designed system, and you'll never need to fertilize with nitrogen. That's a pretty huge deal.
This benefit to other plants occurs because the rhizobia form little colonies on the roots of their host plant called nodules, which look like small pale beads. These bacterial colonies produce ammonia even when their plant host does not need it. That excess ammonia is released into the surrounding soil, increasing the available nitrogen for other plants in the vicinity.
Additionally, portions of any plant's roots die back regularly, especially the very small rootlets. This happens because of shifting soil, temperature fluctuations, insect attack, and a million other reasons. When roots with nodules die, the ammonia stored in the nodules is released along with it. By these two means, nitrogen-fixers create a continuous flow of ammonia into their environment.
Nitrogen-fixers are incredibly efficient at their task. Alfalfa, a perennial legume, can add up to 500 lbs of nitrogen per acre per year. Specialty annual clover can add up to 300 lbs. That is A LOT of fertility in a very short timeframe.
Most nitrogen-fixers aren't quite that productive, though. The USDA classes nitrogen-fixers as low (1-85 lbs per acre), medium (85-160 lbs per acre), or high (160+ lbs per acre), and not many make it into the "high" category.
There is another layer to this, too. From my perspective, it is imperative to use native plants to the greatest extent possible in permaculture systems. The reason for this is that everything surrounding a plant--from soil microorganisms to caterpillars to birds to fauna--have evolved in its company, and in doing so, they have come to rely on it for various vital functions.
Non-native plants cannot performs these functions as well as natives can, and sometimes they don't perform them at all. Other times they are downright harmful, as you'll see below. So we are necessarily degrading our surrounding ecosystem if we are not planting things that offer it the services it needs. This is not say we should never plant non-natives; only that we should look to them secondarily, while ensuring that sufficient natives are planted to offer a well-rounded suite of ecosystem services.
With that in mind, I've assembled seeds of native nitrogen-fixers that I'll be planting en masse this spring, along with the relevant inoculum. These plants are native my area in the Northeast; if you're located elsewhere, you'll want to peruse some quality seed and plant sources by region to find things well-suited to your site.
So, without further ado...