The Web Beneath the Roots — What Actually Happens in Living Soil

Every grower who's committed to living soil already believes something like this: the soil isn't an inert bag of nutrients, it's a community, and the health of your plant depends on the health of that community. It's a good instinct, and it's the whole premise of this series. But "community" is a word that can quietly do a lot of hand-waving. It's easy to nod along with "feed the soil, not the plant" without ever seeing what that community is actually doing to itself, in real numbers, while your roots sit in the middle of it.

That's what makes an old, unglamorous study out of Colorado State University worth pulling back out. It isn't about cannabis, and it isn't recent — it was published in 1984, in a grassland soil, on a grass species most growers have never heard of. But it did something most soil-biology writing doesn't: it isolated one single relationship inside the soil food web — a mycorrhizal fungus and the bacteria living in its immediate neighborhood — and simply measured what happened. No hand-waving. Counted colonies, on plates, compared with statistics. That kind of unglamorous, methodical measurement is exactly what makes the more general "soil food web" story feel less like philosophy and more like biology you can build a growing method around.

A Quick Refresher: Who's Actually Down There

Before going deeper, it's worth restating the cast, because the rest of this piece leans on all of them. The soil food web is built on primary decomposers — bacteria and archaea — that break down organic material and release the nutrients locked inside it. Bacteria in particular drive nitrogen fixation, most famously through the Rhizobia species that trade fixed nitrogen for sugar inside legume root nodules, and they compete aggressively with pathogens for the same space and food. Fungi split into two working groups: mycorrhizal fungi, which form direct symbiotic partnerships with roots and extend a plant's effective reach for water and phosphorus far beyond what the root itself could access, and saprophytic fungi, which decompose dead organic matter and physically bind soil particles together with their mycelium. Above them, protists graze on bacteria and release the nitrogen they contain as plant-available ammonium, while nematodes feed on bacteria, fungi, other nematodes, or roots depending on the species, and in doing so regulate the size of every population beneath them.

On paper, this reads like a food chain. In practice, it behaves more like an economy — thousands of overlapping transactions happening in the thin films of water around every soil particle, all of it ultimately funded by whatever the plant's roots are putting into the ground. And that's the part this week's study actually gets inside of.

The Study: What Happens When a Fungus Moves Into the Root Zone

Researchers R. N. Ames, C. P. P. Reid, and E. R. Ingham — yes, that Ingham, the same microbiologist whose work popularized the term "soil food web" in the first place — grew blue grama grass in sandy, nutrient-poor soil collected from a Colorado grassland. Half the plants were inoculated with Glomus mosseae, a vesicular-arbuscular mycorrhizal (VAM) fungus, at transplant. The other half were grown exactly the same way, without the fungus. Both groups had the same five species of naturally occurring rhizosphere bacteria inoculated into their soil from the start, so every plant began with an identical microbial starting lineup.

Sixty days later, the researchers harvested everything and counted. They measured bacterial populations in the soil that fell away from the roots easily — the "non-rhizosphere" soil — and separately in the soil that clung tightly to the root surface, the true rhizosphere zone. They also measured how much of each root system the mycorrhizal fungus had actually colonized, and cross-checked all of it against root length, root branching, and shoot growth.

The distinction matters more than it sounds like it should. The rhizosphere is a genuinely different biological neighborhood from the soil a few millimetres further out — a definite zone, as the researchers put it, existing as little as two to four millimetres from the root surface. If a mycorrhizal fungus changes anything about bacterial life, this is where you'd expect to see it first.

The Fungus Barely Colonized the Root — And Still Changed the Neighborhood

Here's the detail that should reframe how growers think about mycorrhizal inoculation: colonization of the root system by G. mosseae topped out at only 5.5% of the lateral root length measured. This wasn't a case of the fungus taking over the root system and obviously dominating its biology. It was a light, partial colonization — and it was still enough to produce measurable, statistically significant shifts in the surrounding bacterial community.

Not More Bacteria — Different Bacteria

The most common assumption about "living soil" is that more biological activity is uniformly better — more bacteria, more fungi, more of everything, all the time. This study complicates that picture in a useful way. Regardless of which measurement basis the researchers used, one bacterial species — a fluorescent, oxidase-positive Pseudomonas-type isolate labelled CB11 — increased significantly in the rhizosphere of mycorrhizal plants compared with non-mycorrhizal ones, in three of the five comparison methods tested. Its numbers correlated with how much of the root system was mycorrhizal, and separately with how many mycorrhizal lateral roots existed.

That last point is the one worth sitting with. It isn't just that individual bacterial populations moved up or down. The relationships between different bacteria changed depending on whether a mycorrhizal fungus was present at all. Two species that behaved independently of each other in ordinary soil started behaving as a pair once the fungus arrived. That's not a change in quantity. That's a change in the structure of the community itself — which is precisely the kind of thing the term "soil food web" is trying to describe, and precisely the kind of thing that's nearly impossible to see without an experiment built to isolate it.

"The mycorrhizal fungus didn't just add another organism to the soil. It rewired which bacteria were connected to which — the kind of shift you'd never catch by counting biomass alone."

Root Exudates: The Currency Nobody Sees

The researchers' own explanation for these shifts centers on root exudation — the sugars, amino acids, and other compounds a root leaks into the soil around it, which serve as the primary food source for much of the rhizosphere's bacterial life. The working theory is that mycorrhizal colonization changes what and how much a root exudes, which in turn changes which bacteria can thrive nearby. It's a plausible mechanism, and it lines up with earlier work cited in the paper showing reduced sugar and amino acid content in root exudates from mycorrhizal plants compared with non-mycorrhizal controls of a different species.

But the authors are honest that this raises as much as it resolves. If mycorrhiza formation generally reduces root exudation, why would specific exudate-dependent bacteria like CB11 increase rather than decrease around mycorrhizal roots? Their answer, and ours: the soil food web doesn't run on a single tap that turns uniformly up or down. It's plausible the fungus is changing the exudate mixture rather than simply throttling its volume — favouring compounds that specific bacteria prefer, while starving out others. Nobody has measured that directly yet. It's a genuinely open question, which is a more honest place to land than pretending the mechanism is settled.

Why This Should Matter for Cannabis Growers Specifically

Cannabis forms its own arbuscular mycorrhizal relationships, and living-soil cultivation leans on that fact constantly — mycorrhizal inoculants are a staple ingredient in most living soil mixes and no-till beds. What this study adds isn't a claim about cannabis directly; blue grama grass and sandy Colorado rangeland soil are a long way from a cannabis root ball in a fabric pot. What it adds is a concrete, measured example of the mechanism living soil growers are actually relying on when they inoculate with mycorrhizal fungi at all: the fungus isn't just a nutrient-delivery shortcut for the plant. It's an organism that reaches into the surrounding bacterial community and reorganizes it — promoting some populations, suppressing others, and creating new relationships between bacterial species that didn't exist before it arrived.

The Honest Limitations

This is a forty-year-old study on a wild grass species, grown in a greenhouse pot for sixty days, in sandy Colorado rangeland soil with very low native nutrient levels. None of that is cannabis, none of it is a living soil bed, and none of it is a full growing season. The correlations reported — including the r = 0.53 relationship between CB11 and mycorrhizal colonization — are correlations, not proof that the fungus directly caused the bacterial shift; the researchers themselves note that the mechanism connecting the two remains genuinely unclear. Counts of one inoculated bacterial species, an Azotobacter, couldn't be reliably measured at all and were dropped from the analysis. And critically, results changed depending on how the data were expressed — per gram of rhizosphere soil, per gram of whole root, per gram of lateral root, per centimetre of lateral root, or per number of root branches all told a slightly different statistical story, which the authors themselves flag as a problem still unresolved in this field: there's no agreed-upon standard for how to measure a rhizosphere population in the first place.

None of that erases the value of what was shown. This remains one of the few studies that isolated a single mycorrhizal fungus-bacteria interaction cleanly enough to put numbers on it, rather than just observing that "biology was different" between treatments. For a series that keeps circling back to the same idea — that soil is a web of relationships, not a stack of independent ingredients — this is what that idea looks like in raw data, decades before "living soil" was a marketing term.