Ever wonder how a tree in the forest can grow so big.. without needing us to add fertilizer?
One answer is that Mother Nature has evolved an elegant system that gives plants in undisturbed soil access to all the nutrients they need. It has come to be known as the soil food web.
During their 500 million or so years on land, plants have developed a close, symbiotic relationship with microbes (bacteria, fungi, protozoans and nematodes, mostly) that live in and around their roots—in the “rhizosphere.”
Plants are amazing beings that have evolved amazing abilities. Not least is the way they’ve learned to harness sun energy to make glucose from air and water (also known as photosynthesis). Wow! But, like any one of us, a plant needs dozens of other nutrients: nitrogen, phosphorus, potassium, sulfur, sodium, calcium, magnesium, boron, zinc (to name just a tiny fraction!).
Try as they might, plants can’t pull these nutrients from the rock, sand, silt, clay and organic matter in the soil.. they don’t have the right enzymes. Happily, bacteria and fungi do.
Here’s the simple, elegant arrangement they’ve all worked out, well-pictured above in Anne Bikle and David Montgomery’s 2016 book The Hidden Half of Nature: The Microbial Roots of Life and Health. A plant “exudes” (or leaks) in some cases half or more(!) of the food they make by photosynthesis out into the rhizosphere to feed and cultivate the community of fungi and bacteria there. In return, these microbes use their enzymes to mine dozens of macro- and micronutrients from the soil matrix for their plant ally.
So, picture yourself walking through this forest. You happen upon mushrooms like these peeking up through the fallen leaves. What would you see below-ground, if you could see down there? To oversimplify it, let’s say there are two possibilities:
In one scenario, the soil is typical of land managed by humans. It’s moderately to highly disturbed and the soil microbes that have survived are nearly all bacteria, and probably not very diverse bacteria. You might then find something like the below left seedling.
In a different scenario, the plants around you have normal, undisturbed biology around their roots. Here you’ll likely see something more like the seedling on the right: a soil zone teeming with microbial life busy dissolving minerals from the soil matrix and increasing the seedling’s root capacity by a factor of thousands!
Remarkably, every soil on Earth contains that whole set of nutrients needed to grow truly healthy, resilient plants. And when normal microbial diversity is present, those nutrients can actually make it into the plants, leading to nutrient dense food for us and other plant-eaters.
But hold on– to make matters slightly more complicated, plants (with a few exceptions) can’t get nutrients straight from bacteria or fungi.. one more bit of help is needed. Enter the bacterial and fungal predators: the beneficial protozoans, nematodes and microarthropods.
As these critters hunt down and eat their prey, the concentrated nutrients from their fungal or bacterial meals build up, and build up.. and when a poop happens, that poop delivers rich plant-available nutrients right to the roots!
But the story doesn’t end there either, of course.
To stay in balanced populations, protozoan and nematode predators are kept in check by their own predators: the small arthropods and earthworms.. who are kept in balance by larger arthropods, birds and other animals.. and on it goes.
In intact, relatively undisturbed soil, this diverse web of life becomes the basis of a dynamic nutrient-cycling ecosystem that can grow, among many other things, a 120 foot tall oak tree.. easy!
It’s worth mentioning here that plants and trees in natural, undisturbed ecosystems are virtually coated with beneficial microbes. This layer of biology acts as armor, protecting the plant from any potentially disease-causing spore that happens to be floating by or vectored by an insect.
And do you want to guess where all the microbes in this above-ground plant armor come from? You got it– the soil! Soil with an intact food web, that is. They literally hitchhike upward on insect feet.
It’s another elegant outcome of half a billion years of the plant-microbe partnership: when you have a healthy soil food web, you also have plants without disease. Mother Nature, you’re amazing.
SO, FIRST THE BAD NEWS
Nearly all land managed by humans today has a damaged soil food web. And the results are sobering.
It was recently estimated that we’ve lost one-third of the world’s farmable land in the last 40 years.. the majority of it due to erosion.
Here in the US, we’re losing 1.7 billion tons of topsoil every year!
When we till the soil, or leave it bare, or put on chemical pesticides or inorganic fertilizers (think “Miracle Grow”), vital members of the soil microbial community are killed. And without them, the soil’s inherent stability and nutrient-cycling capacity are stripped away.
When this happens, plants don’t grow as well and they become more susceptible to disease. The soil releases stored carbon to the atmosphere, its porous structure collapses and it can’t hold water or let in air. Ultimately, it becomes more vulnerable to being blown, or washed, away.
Best to call this damaged soil DIRT— soil without soil life. This, sadly, is the current state of much of the Earth’s soils, and is a major driving factor in the warming of our world.. not to mention desertification on a global scale.
And if you have a bed in your garden (or certain crops on your farm) that don’t yield as well as you’d like, or get knocked out early by that blight most years, this is why.
If you’ve found your way here, it’s likely you do a lot to take care of your soil. You may be cover-cropping, spreading compost or manure, companion planting, maybe rotating crops meticulously..
But if your soil biology has been fragmented by the historical legacy of disturbances on your land, it will be very difficult to restore a full soil food web through those good management practices alone. In the majority of cases, it’s been found that even several years of certified organic management usually doesn’t readily bring back the full soil food web.
So.. what now?
WELL.. NOW THE GOOD NEWS
There’s a solution! Or a.. soil-ution?
In recent decades, soil restoration ecologists have found that the damage to our soils is totally reversible.. and fast. Dirt can be easily inoculated with the microorganisms that are missing from its soil food web, and within a single growing season be transformed back into living soil.
That, in a nutshell, is what we do.
Our first step is to assess the state of your soil’s nutrient-cycling food web using a compound shadowing microscope. In this way we can estimate the amounts of beneficial bacteria, fungi, protozoans and nematodes that are in your soil– or not, as is often the case.
This reveals which members of the soil food web we need to reintroduced in order to restore full nutrient-cycling and pest and disease resistance to your plants.
We also check for any potential disease-causers. If we find them, there are simple tests, such as for soil compaction, that can help us strategize how to resolve the effects of past or present disturbances to your soil.
The final step is to put back in to your soil what’s been lost. We do this very simply by inoculating your soil with a solid or liquid compost that has those missing organisms.
This can require one, two, or a few applications, depending on how deeply disturbed your soil ecosystem is, and whether echoes of those disturbances remain. But once we’re done, the plant-microbe partnership is restored– your plants will take over the job of feeding their microbe allies, who in turn will get to work extracting minerals and nutrients for their plants.
Without further disturbances, you have permanently restored soil!
Pretty cool, huh?