Fixing Nitrogen

Beans are some of the most rewarding crops to grow, nourishing the soil as they nourish our bodies. Beneath the garden soil, tiny nodules on roots of the Pea family harbor specific Rhizobium bacteria, which live in symbiosis with the plant. These bacteria colonize the roots of the legume, taking nitrogen from the air and converting it into ammonia, which is a form of nitrogen that plants can use. This process is called “fixing” nitrogen. In return for this valuable transformation, the bacteria receive their nodule dwellings, as well as a steady supply of plant sugars from their legume hosts.

This process is well-documented, and was practiced by indigenous farmers well before the exact mechanism was understood, simply by observing with keen eyes, the relationships between plants and crops.

These Rhizobium bacteria are host-specific, however, meaning that they only live or occur naturally in areas that their preferred host species are found. Since the Rhizobium bacteria in our area have evolved to live in association with wilder, woodier legumes (lupines, vetch, deer plant, even the invasive Scotch Broom), the right bacteria might not exist naturally in our garden soil. Furthermore, while it is true that these nitrogen fixing plants feed the soil, it is worthwhile to look at the cycle of that nitrogen fixation, in order to understand it better. The bacteria, if present in the soil, live on the roots, and make nitrogen available to their hosts. The host plant readily accepts this rare gift, and stores it, for a time, in the very root nodules in which it was created. When the plant begins to make seeds, such as peas or beans, it draws upon the nitrogen in these stores, and transfers that nutrition into the fruits and seeds, leaving little nitrogen behind in the soil. This is why cover crops like fava beans or vetch are intended to be dug into the ground before they set seed, before all the nitrogen is bound up in the beans. If the requisite bacteria are not present in the soil, however, the beans are just as hungry for nitrogen as any other food crop, and should be fertilized accordingly.

So how to know if you have the right bacteria in your soil, to help your bean plants along? One good way to determine this is to pull up a mature plant, roots and all. Take a look at the root structure; if the white roots are covered with lumpy white bumps, like tiny root warts, you are in good shape. Those bumps are the nodules, that indicate the presence of Rhizobium bacteria. If your bean plants do not have these nodules, however, the chances are that they are not fixing nitrogen for you. After all, many of the legume crops that we grow were not evolved to live in our particular environment, and as such the proper Rhizobium bacteria species may not occur naturally in the soil. Luckily, a simple solution for this potential deficit exists. Many garden centers, including our own, sell a powdered inoculant that can be used to coat the seeds at planting time. Once established in the soil, the Rhizobium bacteria can live happily in the ground for years at a time. So as long as you periodically grow beans or peas in the same place every few years, you should have a thriving population of beneficial bacteria. That is, unless you are using synthetic nitrogen fertilizers.

Synthetic nitrogen fertilizers were developed in the early 1900’s, and their introduction dramatically increased the production of agriculture worldwide. However, as with any new technology, their long-term effects were poorly understood. Over time, the application of large amounts of nitrogen to the soil leads to a build up of salts, which can be seen as a visible white crust on the bare dirt of many large-scale agricultural operations. This salt, and the excess of unused nitrogen in the soil, leads to many environmental problems, both visible and invisible. The excess nitrogen finds its way into streams and waterways, leading to algae blooms that can smother the native habitats and deprive them of oxygen when the algae dies and decomposes. In addition, the soil that has had synthetic fertilizers applied to it will become inhospitable to the very bacteria that create nitrogen naturally. A soil that has synthetic nitrogen applied to it on a regular basis may continue to produce healthy(ish) crops as long as fertilizer continues to be applied. But when the artificial nitrogen supply is removed, the soil is left without nutrients, and without the bacteria that fix nitrogen, a dead soil. Nitrogen is essential to plant growth; it’s the first number on every bag of fertilizer sold; without it, crops grow spindly, weak, and small. Much like the mysterious, invisible world of the intestinal microbiome, a healthy soil is populated with a diverse assortment of beneficial bacteria. When these bacteria are not present, the soil is stripped of life which contributes to the overall health of the garden. Bacteria and fungi that occur naturally in the soil perform many vital functions, everything from breaking down organic material and minerals to facilitating the flow of oxygen and water and nutrients through the soil, and creating the very structure of the soil itself. The soil fertility that bacteria make possible correlates directly to the nutrients present in the food grown in that ground. So even though a bell pepper or a bean might be red and ripe when grown with synthetic nitrogen, the vitamin and mineral content is noticeably less when compared to plants grown in healthy, organic soil.

So of course we advocate for organic agriculture, in the home garden as well as in the larger agricultural world. The fact remains, though, that beans will not fix nitrogen unless the proper bacteria are present, so to make sure that your beans are getting (and giving) the most benefit to the garden ecosystem, by checking for those nodules! If you don’t find many, use an inoculant the next time you plant a legume crop; inoculants are cheap and easy to use, and can contribute greatly to the health of the soil. When your bean plants are done for the year, there will be residual nitrogen stores in their vines, leaves, and roots. We like to leave the roots in the soil, so that plenty of the bacteria that dwell in the nodules remain in the soil, but the vines can be susceptible to spreading diseases and pests, so we prefer to compost the spent plants rather than dig them into the soil.

After such a deep dive into the world of beans and bacteria, it’s time to come up for air, a little. This week's featured recipe is a joy of the summer garden, fresh and vibrant and redolent of herbs and sunshine and spice. Pick a basket of beans from the generous vines, and send a little gratitude down into the ground as well, for all the invisible life that is hidden within the soil, that binds and bonds the world together in interconnected, exquisite relationship.