Growing Mediums: Physical Properties and Practical Mastery

Understanding Your Growing Medium’s Physics

What You Need to Know

The medium is not just a container for roots. It’s the interface between your nutrient solution and the plant’s root zone, and its physical properties — porosity, water retention, drainage rate, hydraulic conductivity — determine irrigation frequency, nutrient availability, and oxygen supply to the roots. Nemati’s review of five main growing medium categories found no universal winner. But the reasons each medium succeeds or fails are completely different, and understanding that physics matters more than chasing a “best” option. Your job is picking the medium that matches your irrigation habits and infrastructure, then mastering it.

The Science

The review evaluated five medium categories: coir-based, rockwool, phenolic foam, peat-based, and living soil. For each, they documented physical properties (porosity, air-filled porosity, water holding capacity, hydraulic conductivity), chemical properties (CEC, pH buffering, nutrient retention), and practical management considerations. The key physical data at a standardised 14 cm substrate height tells the real story.

Coir-based: Total porosity 88–94%. Air-filled porosity 24–34%. Container capacity 60–69%. Easily available water 22–30%. Coir sits in the middle ground between rockwool and peat — good aeration, decent water retention, relatively forgiving. The hydrophilic nature of coir (it rewets easily) reduces the risk of dry pockets. However, raw coir is loaded with sodium, potassium, and chloride from the coconut husk, which can cause calcium and magnesium lockout if the coir isn’t properly washed and buffered. The variability in coir quality between manufacturers is the biggest hidden risk. You can buy two bags of “the same product” and get different physical properties because particle size distribution varies.

Rockwool: Total porosity 91–95%. Air-filled porosity 20–24%. Container capacity 65–71%. Easily available water 61–68%. Rockwool holds more easily available water than any other medium — it’s a sponge. But its moisture retention curve drops sharply, meaning it goes from “saturated” to “dry” over a narrow range of tension. This makes it incredibly responsive to irrigation strategy (great for crop steering), but also unforgiving if you miss a feed or overwater. Rockwool has zero CEC — it doesn’t hold nutrients. Every ion you feed either gets absorbed by the plant or drains out. And when rockwool dries out fully, it becomes hydrophobic and nearly impossible to rewet evenly with drip irrigation alone.

Phenolic foam: Total porosity 92–96%. Air-filled porosity 28–42%. The highest air-to-water ratio of any medium tested. Phenolic foam maintains its air-filled porosity even after heavy watering because of its rigid cell structure. It’s sterile, pathogen-free, lightweight, and inert (zero CEC, zero buffering). The downsides: it’s restricted to heights under 10 cm because water retention drops dramatically in taller profiles, and it’s not biodegradable. Environmentally, it’s rockwool’s problem in a different shape.

Peat-based: Total porosity 84–90%. Air-filled porosity 10–20%. Container capacity 65–74%. Peat has the highest buffering capacity for water, pH, and nutrients of any medium in the review. It’s forgiving — it holds onto things longer and releases them more slowly. But it’s also the most variable (peat moss quality differs between harvests and sources), it’s acidic (requires liming), it’s hydrophobic when it dries out, it compacts over time, and its lower air-filled porosity means overwatering is easy. Most peat mixes add perlite (10–35%) or wood fibre (20–40%) to improve aeration. The table of common peat mix components shows just how many additives are needed to make peat behave: perlite for drainage, vermiculite for CEC, coir for wettability, bark for structure.

Living soil: No standardised data because living soil has no standardised definition. It’s the most variable category by far. The concept is sound — a biologically active medium that mineralises nutrients through microbial action, suppresses pathogens through competitive exclusion, and maintains long-term structure. The practice is challenging: cannabis grows fast (11–13 weeks total), which is barely enough time for many organic amendments to fully mineralise. Living soil growers typically overcharge with amendments to compensate, creating unpredictable nutrient release patterns. And the physical properties of living soil (water retention, aeration, drainage) depend entirely on the recipe, which varies between every grower.

Container height effect: One finding that’s overlooked by almost everyone — the physical properties of your medium change with the height of your container. In a 15 cm tall pot, peat-based mix has 49% container capacity and 29% air-filled porosity. In a 42 cm tall pot, the same mix drops to 36% container capacity and 42% air-filled porosity. The physics of perched water tables means that taller containers drain more completely, leaving more air and less water. Your 3-gallon fabric pot and your 7-gallon fabric pot with the same medium are not holding water the same way. Your irrigation frequency needs to change with container size, not just medium type.

How To Apply This

• Pick a medium and learn it properly before switching. Every medium in this review can grow excellent cannabis if you understand its water dynamics. The growers who struggle are the ones who swap mediums without adjusting their watering strategy.
• If you’re on coir, verify it’s washed and buffered. Unbuffered coir can lock out calcium and magnesium for the first several weeks until the cation exchange sites equilibrate. If you’re getting early cal-mag deficiency symptoms in coir, the medium might be the problem, not your nutrient solution. Buy from a reputable supplier and check the EC of runoff on your first watering — if it’s high, the coir wasn’t washed properly.
• If you’re on rockwool, water frequently at low volume. Rockwool’s moisture curve means it needs to stay wet. High-frequency, low-volume irrigation events (multiple per day, especially in flower) keep the easily available water zone topped up. If rockwool dries out, don’t try to force-feed it through a dripper — soak the whole block.
• Match your irrigation to your container height. A tall pot needs less frequent watering than a short pot with the same medium because it drains more completely. If you moved from 3-gallon to 5-gallon pots and your plants seem overwatered, you’re probably watering too often for the new container geometry.
• Stop looking for the “best” medium. Nemati’s review explicitly concludes that no single medium is optimal. Coir is the most balanced, rockwool is the most responsive, peat is the most forgiving, and living soil is the most complex. Pick the one that matches your watering habits, your attention level, and your budget.

Seb’s Corner (Level 2+)

The moisture retention curves (water potential vs. volumetric water content) presented in Figure 2 of the review are the most practically useful data for understanding irrigation strategy. Rockwool and phenolic foam show steep curves — water content drops rapidly as suction increases from 0 to -4 kPa, meaning a small change in moisture status produces a large change in water availability. This “switching” behaviour is what makes these media ideal for crop steering: moving from vegetative to generative irrigation is as simple as allowing a slightly larger dry-back between events. Peat and coir show gentler curves — water content decreases more gradually with increasing tension, providing a larger buffer zone where the plant has access to water without being waterlogged. For growers without automated irrigation and sensor-based monitoring, the flatter curve of coir or peat is more forgiving. The saturated hydraulic conductivity (Ksat) data is equally informative: rockwool and phenolic foam have Ksat values classified as “extremely high” (unmeasurable with standard methods), while coir pith measures 0.04–0.15 cm/s and peat 0.01–0.04 cm/s. This explains why peat-based mixes are prone to waterlogging — water moves through them slowly, and gravity drainage takes longer to complete. Cannabis roots are exceptionally sensitive to oxygen deprivation in the root zone, and root pathogen pressure (Pythium, Fusarium) increases dramatically in waterlogged conditions. The practical conclusion: if you can’t monitor and respond to your medium’s moisture in near-real-time, choose coir over rockwool for its wider margin of error.

Watch Out For

• Unbuffered coir causing early calcium and magnesium lockout. High EC in runoff on first watering indicates poor washing.
• Rockwool drying out and becoming hydrophobic. Once fully dry, it’s difficult to rewet evenly with drip irrigation. Soak it instead.
• Container height changing water retention profile. A 42 cm tall pot with the same medium as a 15 cm pot will have different drainage and air-filled porosity. Adjust irrigation frequency accordingly.
• Peat compaction and hydrophobicity over time. Peat-based mixes need amendment additives (perlite, bark, coir) to maintain structure and wettability.
• Living soil variability. No standardised definition means nutrient release patterns are unpredictable. Most living soil growers overcharge with amendments to compensate.

Quiz

1. Which growing medium has the highest air-filled porosity and is best for high-frequency irrigation?

   • a) Coir-based (24–34%)
   • b) Peat-based (10–20%)
   • c) Phenolic foam (28–42%) *
   • d) Rockwool (20–24%)

2. True or False: Rockwool has a high CEC and holds onto nutrients longer than coir-based mediums.

   • False * (Rockwool has zero CEC; coir has moderate CEC)

3. How does the physical properties of a growing medium change when you move from a 15 cm tall pot to a 42 cm tall pot with the same medium?

   • Container capacity decreases and air-filled porosity increases due to perched water table effects. Taller containers drain more completely.

4. If your coir-based medium is causing early calcium and magnesium lockout symptoms, what is the likely cause?

   • a) Insufficient watering
   • b) Unbuffered or unwashed coir loaded with sodium and potassium *
   • c) Light deficiency
   • d) Natural cultivar trait

5. Which medium is described as the most forgiving for growers who water by hand without real-time monitoring?

   • a) Rockwool
   • b) Phenolic foam
   • c) Coir-based or peat-based (gentler moisture curves) *
   • d) Living soil

FAQ

Is coco coir better than soil? It’s not better — it’s different. Coir is more forgiving than rockwool, faster-draining than peat, and more consistent than living soil. For most home growers who water by hand once or twice a day, coir in a fabric pot is a solid choice because it’s hard to overwater and easy to rewet. Peat-based “soil” mixes are more forgiving with nutrients (higher buffering) but less forgiving with water (slower drainage, easier to drown roots).

Rockwool seems complicated. Why do commercial growers use it? Because commercial growers have automated irrigation systems that can deliver 10–20 small feed events per day, precisely timed. In that context, rockwool’s responsiveness is an advantage — they can steer the crop vegetatively or generatively by adjusting irrigation timing. At home, without automation, rockwool is a pain. It dries out unpredictably, it’s hard to rewet, and it generates waste you can’t compost.

Does living soil really produce better-tasting bud? Some growers swear by it, and there’s a plausible mechanism — diverse microbial communities may influence terpene production through plant-microbe signalling. But there’s no controlled cannabis trial demonstrating a taste advantage for living soil over mineral-fed coir or hydro. The “living soil tastes better” claim is anecdotal, and the people making it are usually comparing their carefully managed living soil to someone else’s poorly managed hydro. Good weed can come from any medium.

My fabric pot is 5 gallons. Should I switch to 3? Not necessarily, but you should adjust your watering frequency. A 5-gallon fabric pot at 30 cm height has a different moisture profile than a 3-gallon at 20 cm — more air space, more drainage, less water retention at equilibrium. If your 5-gallon pot seems to dry out slowly, you’re probably overwatering. Water less volume more frequently, or accept a longer wet/dry cycle.

Source

Nemati R, Fortin J-P, Craig J and Donald S (2021). “Growing Mediums for Medical Cannabis Production in North America.” Agronomy 11:1366. doi: 10.3390/agronomy11071366. CC-BY 4.0.