Soil, Coco, and What Roots Want

What You Need to Know

Here’s the line that took me three dead plants to believe: roots need air more than they need water.

Not instead of water. More than. The order of priority for a root is air, then water, then food. Most beginners picture soil as a sponge that holds water and nutrients — and that’s only half right. Your growing medium is really an oxygen delivery system that also happens to hold water and food. The air pockets between the particles are where root tips grow, where beneficial microbes live, and where gas exchange happens. Fill those pockets with water and the whole system shuts down — the roots can’t breathe, and a root that can’t breathe can’t feed either, even with nutrients sitting right next to it.

So when we compare growing media, we’re really comparing how each one balances two things that pull against each other: holding water, and holding air. Get that balance right for your skill level and most of the problems in the rest of this level never happen.

The Media, Compared

There’s a proper academic review of this — Nemati and colleagues (2021) went through the growing media used across North American cannabis production, because the legal history meant almost nobody had published the comparison before. Their categories map neatly onto what’s on the shelf at a grow shop.

• Peat-based soil. Decomposed sphagnum moss as the base. High water retention (around 60% water-holding capacity in the review), naturally acidic, good for container growing. The traditional default, but peat harvesting carries real sustainability concerns — relevant in Ireland, where the bogs are protected carbon sinks and the industry is shifting away.
• Coco coir. Shredded coconut husk. Holds water and air at a ratio roots love (about 50% water-holding capacity, with good air space), pH-neutral, and reusable for two to three cycles. The review flags its strong cation exchange capacity — more on that in a second — and notes it’s the environmentally preferred alternative to peat. It’s a renewable byproduct that would otherwise be waste.
• Rockwool. Spun mineral fibre. Sterile, chemically inert, pH-neutral, around 95% air space and very low water retention (~10%). Excellent for hydroponic systems and cloning, where you want total control and no buffering from the medium.
• Living soil. A built ecosystem of microbes, amendments and organic matter that feeds the plant through biology rather than bottles. Powerful and sustainable, but more complex and more expensive to get right — a later-grow ambition, not a first move.

Seb’s Corner — cation exchange, lightly. You’ll see “CEC” on the technical sheets and it sounds like a wall to climb. It isn’t. Cation exchange capacity is a measure of how many positively charged nutrient ions — calcium, magnesium, potassium, ammonium — a medium can hold onto and release back to the roots. Think of it as the medium’s pantry: a high-CEC medium stocks nutrients on its surfaces and hands them over gradually, which buffers the plant against your feeding mistakes. The Nemati review puts coir’s CEC at roughly 15–30 meq/100g, comparable to peat — which is part of why both are forgiving. Rockwool, by contrast, holds almost nothing; it’s inert, so every nutrient the plant gets has to come from you, every feed. That single property — does the medium hold a pantry, or not — explains most of the difference in how forgiving each one is. High CEC buffers you. Inert media demand precision.

The One Catch With Coco

Coco looks like the perfect middle ground, and it nearly is — but it plays by hydro rules, not soil rules, and that catches people. Two differences matter. First, coco is inert: it holds no nutrients of its own, so everything comes from your feed, every watering, to runoff. Second, coco naturally binds calcium and magnesium, so you add CalMag to every feed or the plant shows a deficiency even when your mix looks correct on paper. Water coco like soil — every few days, no runoff, no CalMag — and salts build, pH drifts, and nutrients lock out by week three.

For a first or early grow, soil is the forgiving choice. Its water-holding and cation exchange give you a buffer while you learn to read the plant. Coco is the logical next step once feeding and pH are second nature.

Containers — The Pot Is Part of the Medium

The medium and the pot work as one system. A breathable fabric pot air-prunes the roots: when a root tip hits the fabric wall, the dry air outside stops it growing, so the plant branches new roots further back and builds a dense, fibrous root mass instead of a circling spiral. Fabric pots also drain better and keep roots cooler. A smooth plastic pot, by contrast, sends roots circling the wall until they choke the plant’s own root system — and if it has no drainage holes, it turns the medium into the swamp we’re trying to avoid. For an indoor grow, fabric pots are the cheap upgrade that quietly prevents the most problems.

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How To Apply This

1. Pick your medium by your experience, not by hype. Soil for a forgiving early grow; coco once you’re comfortable feeding to runoff and managing pH; rockwool or living soil later, for specific systems.
2. Buy a cannabis-specific bag from a grow shop — not multipurpose compost from a garden centre. Match the mix to the plant’s age: a light mix for seedlings and young plants, a richer mix for the final pot.
3. If you go coco, commit to its rules: buffer or buy pre-buffered, mix roughly 70/30 with perlite, add CalMag to every feed, and water daily to 10–20% runoff.
4. Use fabric pots, sized to the plant’s stage, so the roots air-prune and the medium drains and breathes.
5. Water to 10–20% runoff and then let the medium dry back (in soil) so the air pockets reopen between waterings. The lift test from Level 1 still rules: heavy pot, leave it; light pot, water it.

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Watch Out For

The pot is where confidence outruns knowledge, so go gently.

Treating coco like soil. This is the classic crossover mistake — two soil grows down, you hear coco is better, buy a brick, and water it every three days with no CalMag. By week three the lower leaves yellow and spot, growth stalls, and it looks like a deficiency. It isn’t bad luck. It’s soil logic applied to a hydro-style medium. Different rules, learned up front.

The home-made super soil. One good grow in and the temptation is to mix three bags, add worm castings, bat guano and a handful of blood meal because more amendments must mean more better. A seedling planted into that nuclear mix burns within days. The people who formulated the bag already got the ratios right so you don’t have to.

Perlite as a topping. Perlite makes air channels — inside the medium, mixed through. Sprinkled on top as mulch it just floats off during watering. Mix it in before you fill the pot.

Reusing soil on a first grow. Used soil is depleted, compacted, and may carry old roots, salt residue or fungal spores. Re-amending spent soil is a fine sustainable move once you’ve a few grows behind you — but on your first, fresh soil is cheap insurance.

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Quiz

1. Put a root’s three priorities in order, and explain why air comes first.
2. In one line, what does cation exchange capacity (CEC) tell you about a growing medium?
3. Why is rockwool described as “inert,” and what does that demand of the grower?
4. Name the two things coco growers must do differently from soil growers, and the symptom you’ll see if they skip the second one.
5. How does a fabric pot’s “air pruning” build a healthier root system than a smooth plastic pot?

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Sources

Nemati, R., Fortin, L., Craig, B., & Donald, L. (2021). Growing mediums for medical cannabis production in North America. Agronomy, 11(7), 1366. https://doi.org/10.3390/agronomy11071366. CC-BY 4.0. — medium categories, water-holding capacities, CEC values for coir, and sustainability framing.

Chapter 9, The Grower’s Guide (book draft) — root oxygen priority, coco’s CalMag requirement, container choice and air pruning, and the Irish peat context.

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Answer Key

1. Air, then water, then food. Air comes first because root tips and the microbes around them need oxygen to function — a root sitting in waterlogged, airless medium can’t take up nutrients even when they’re present.
2. It measures how many positively charged nutrient ions the medium can hold and release to the roots — in effect, the size of its nutrient pantry, which buffers against feeding mistakes.
3. Rockwool holds almost no nutrients of its own; being inert, everything the plant receives must come from the grower’s feed, so it demands precise, consistent feeding.
4. Feed to runoff every watering (it holds no nutrients of its own), and add CalMag to every feed (coco binds calcium and magnesium). Skip the CalMag and the plant shows a calcium/magnesium deficiency — yellowing, spotting lower leaves — even with a correct-looking mix.
5. When a root hits the breathable fabric wall, the dry air outside stops that tip and the plant branches new roots further back, producing a dense fibrous mass — rather than roots circling a smooth plastic wall and eventually choking the plant.

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Next lesson: pH — The Bouncer at the Nutrient Door — now that the roots are in the right stuff, we deal with the one invisible setting that decides whether they can actually eat what you feed them.