This past week, as I harvested the last of my fall green beans and pulled up the vines (and put them in the compost), I saw something that made me think about the great big web of life that we rarely see and hardly think about.
There’s a huge microcosm of life underfoot, namely fungi and bacteria that have evolved over millions of years to live symbiotically with plants.
My bean roots had tiny little nodules on them, which are home to bacteria that perform a very important function. These bacteria, along with fungi, are so important to plant health that nearly all plants have an association with a bacteria or fungus.
Bumps on the bean roots
Those nodules I found on my bean roots are formed by bacterium called Rhizobium. There are many different species of this bacteria that each colonize on the roots of a different set of legume plants.
These bacteria take nitrogen from the air, which plants cannot use as a nutrient, and convert, or “fix” it, to a form that the plant can use, namely ammonia.
In exchange, the bacteria get a place to call home, some carbohydrates and proteins for energy, and oxygen. Basically, they get room and board.
These bacteria have a specific relationship with legumes, which are plants in the Fabaceae family. Most folks know about beans, peas and peanuts as legumes, but there are many, many more.
Clover and alfalfa are sown in pastures and hayfields and provide an important energy source. Even trees like locust, redbud, Kentucky coffeetree and mimosa (weedy as it is) are legumes with nitrogen-fixing bacteria on their roots.
Americans also used to sow clover in with their lawn for the nitrogen benefit before the unrealistic need to have a “weed-free” lawn took over in the 1950s.
There’s lots and lots of fungus among us
Bacterial root nodules aside, there’s lots of other interactions going on between roots and microbes right under your feet. Almost all plants on earth have a symbiotic (mutually beneficial) relationship with fungi known as mycorrhizae.
These fungi associate with the plant roots, sometimes even growing directly into the root cells.
When most people think of fungi, they think only of mushrooms. While mushrooms are the fruiting bodies of many fungi, the main fungal organism is really a large network of threadlike structures called hyphae.
When the mycorrhizae connect to the root, they basically increase the area of soil available to the plant roots for finding nutrients and water with their large mass of fungal threads. Linda Chalker-Scott, my colleague at Washington State University, describes it as a “virtual fungal freeway of nutrient and water acquisition and transfer.”
The fungus absorbs water and nutrients and provides them to the plant. In exchange, the fungus gets food in the form of sugar from the plant. It is a relationship wherein both benefit. Without this relationship, a majority of plants (about 95 percent of the plant families on earth) would be less vigorous and productive. We owe a lot to these microorganisms.
Gardening with microbes
In his book “Teaming with Microbes,” author Jeff Lowenfels talks about how gardeners can “team up” with the soil that is teeming with microbes to increase garden health — though it can be rather simple to make sure you have these microbial good guys on your team.
Mycorrhizae and bacteria can be found throughout the soil, especially soil that has a good amount of organic matter in it. The best, and easiest, thing you can do to is to amend your soil with compost and other organic matter to provide food for the microbes.
There are many products and fertilizers now on the shelves that purport to add mycorrhizae to the soil, but most soil, especially if it has organic matter, already has a good level of indigenous inoculum. You don’t really need to add them to the soil.
If you have poor soil, it is unlikely that any bacteria or fungi you add will be able to survive.
Where adding a commercial inoculum can come in handy is if you are basing your soil off of sterilized, bagged products such as potting soils, peat moss or sterilized compost. Adding an inoculum could speed up the process.
For the Rhizobium on legumes, the relationship between a specific species of plant and bacteria mean there might not be a native population of the right bacteria. Adding an inoculum (available on seed racks in the spring) can boost growth, especially if you have never grown that legume in the plot before.
Some seeds, such as clover, can come inoculated with the appropriate bacteria, though inoculation is likely not necessary if you’ve grown a legume in the soil recently.
I’ve used the inoculum before, but I don’t recall using it in the raised bed where I found it growing on the beans this past week.