Using a soil inoculant may seem kind of unnatural, so let’s start with why it might be a good idea.
The most important life forms in your garden are too small to see.
Microbes cover every soil surface and even inhabit the insides of all larger organisms.
They have a dramatic effect on plant health and nutrition, as well as our own.
In most gardens, the microbiome has been thrown out of balance by things like tilling, chemical fertilizers and pesticides.
And also the generally toxic environment we live in with pollution, antibiotics and other pharmaceuticals in our air and fresh water.
A lot of this imbalance arose because we’ve tried to control the living world by killing or removing the organisms we don’t want.
But now that we know how hard it is to create healthy ecosystems this way, we’re using a new approach instead: restoring the balance by inoculating with the microbes we DO want.
My favorite inoculants for foliar feeding are compost tea and effective microorganisms, and they’re good soil inoculants, too. I discuss them elsewhere.
But beyond these, there are other ways to repopulate our soils. Here I’m going to tell you a bit more about two very important soil inoculants: mycorrhizal fungi and legume inoculants.
I just visited home and threw a bunch of veggies into my shoulder bag to bring back to Toronto, mostly tomatoes, potatoes, zucchini, cabbages, herbs and garlic (smelled pretty good on the bus).
Almost all of the plants in the world, where the native soil is still undamaged, are connected by a vast underground fungal network.
Plants will exchange many of the carbohydrates they produce with these fungi in exchange for nutrients, water, and protection from disease.
Plants also share immune compounds, nutrients and information with each other through the fungal network.
The mystical, unified web of life depicted in the movie Avatar isn’t fantastical at all. It’s right under our feet.
The only two food plant families that don’t have known associations with mycorrhizal fungi are the Brassicaceae family (broccoli, cabbage, mustard, kale, etc.) and the Amarathaceae family (spinach, quinoa, chard, beets, etc.).
That still leaves 95% of plant species looking for a handful of very common fungal partners, which can often be absent in damaged soils.
Fortunately, it’s not hard to restore the mycorrhizal fungi.
We can get mycorrhizal soil inoculants by taking a small bucket of healthy soil from a nearby intact ecosystem, or we can be a little more sure by buying them as a soil inoculant from some garden centers or especially online.
The advantage of the latter is that you know for sure what you’re getting.
Fungal inoculants are best applied at seeding or transplanting time, though they can be watered in later in porous soils. What’s essential is that this soil inoculant makes contact with plant roots, since the root exudates bring it to life.
There’s no point putting mycorrhizal fungus in foliar sprays or compost, as it’s the plant roots that form relationships with it.
Update: 6 months after I wrote this, I decided to start selling a mycorrhizal inoculant, so if you want to learn even more about this process, you can do so here.
The vigorous beans climbing up the trellis in the front right of the photo were inoculated with both legume and mycorrhizal fungi inoculants.
The plants we think of as “nitrogen fixers” are actually just great hosts for the microbes that do most of the work.
If your peas and beans are lacking in vigor, it may be because they don’t have the right bacterial buddies in their root zone.
Most nitrogen fixers are in the Legume family. They seek out their preferred species of Rhizobium bacteria in the soil, and grow clusters of root nodules to encircle the bacteria and provide an ideal home for them.
Look for these little pinkish nodules next time you dig up a pea or bean, and see the magic at work.
If your legumes can’t find the right bacteria, they’ll try to work with others, but we can help by making sure they have exactly the partners they need to do the nitrogen-carbon dance that keeps all of life swaying to the music.
That’s where using a seed inoculant for peas or beans or clover or whatever legumes you’re using comes in.
Again, it’s often easiest to buy it online, but some garden centers carry it. I like to sprinkle a little powdered legume inoculant over my seeds before planting.
Many new microbial soil inoculants are now becoming available, including some free-living nitrogen-fixing microbes, which don’t need a legume host.
There are also non-mycorrhizal fungi such as Trichoderma that seem to protect effectively against soil diseases.
These are great if you’re ready to get into the more specialized technical side of organic gardening, but you can make a huge difference to plant health and growth right now by starting with the above tried and true ones.
Any questions about how to use a microbial soil inoculant? Or other inoculants? Or soil microbes in general? Let me know down below.