Substrates compared: coco, rockwool, soil, hydro
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Substrates compared: coco, rockwool, soil, hydro

A from-zero guide to what cannabis roots grow in: how each medium holds water and air, how it handles nutrients, how forgiving it is, and how to pick one.

Beginner11 diagramsEvidence-linked · 9 sources~11 min read
In this guide
01 What this is (and who it's for)02 Accuracy, self-review, and grain-of-salt notes03 Key terms, defined once04 Water versus air: the porosity trade-off05 How each medium handles nutrients06 Re-use, cost and footprint07 How to choose: a beginner's decision path08 Setting up and running each, step by step09 Common pitfalls and how to dodge them10 Realistic expectations
Start here

What this is (and who it's for)

A substrate (also called a growing medium) is just the material a plant's roots live in. It holds the plant up and acts as the reservoir for water, air and dissolved nutrients. This guide compares the five materials cannabis growers actually use, assuming you know nothing yet.

By the end you will understand the two trade-offs that decide everything: how much water versus air a medium holds, and how much it cushions your nutrient mistakes. With those two ideas you can pick the right medium for your setup.

Where do the roots live?1In a solid mediumcoco, rockwool,peat, livingsoil hold theroots→2In water and airDWC: rootsdangle inoxygenatednutrient waterFive options, two families. The rest of this guide is about telling them apart.
Figure 1. The whole field splits into media that physically hold the roots and water culture where the roots hang in the solution itself.
Who this is for

Total beginners choosing a first medium, and anyone who wants to know why their setup behaves the way it does. It pairs with the coco crop-steering paper, which goes deep on driving one specific medium hard once you have picked it.

How sure is this?

Accuracy, self-review, and grain-of-salt notes

How sure is this paper?

We've gone to great lengths to keep these guides honest. One of the main ways we do that is self-review: we actively look for claims that are subjective, only lightly backed by literature, or based on grower practice rather than a controlled study — and we call those out instead of dressing them up as settled science.

Often there simply is no paper for the decision you're making. In those cases we're drawing on what other growers report and what has worked in our own rooms. That can still be useful — but it is not a lab proof. Do what works for your plants, your room, and your meters. If a table disagrees with your crop, believe the crop and log the difference.

Solid
Well supported by plant science, standards, or broad multi-source consensus
  • Air-filled porosity and water-holding differ by media class
  • Soil buffers pH/nutrients; inert media do not
Operational
What many growers and rooms actually run — start here, then tune
  • Beginner medium pick heuristics (forgiveness vs control)
Grain of salt
Subjective, thin literature, single studies, or “this works for us” practice
  • Living soil pH self-correcting 'within minutes for any water'
  • Single AFP % as universal for all rockwool SKUs

See something glaringly wrong? Tell us and we'll fix it. Please open a GitHub issue with the paper name and what looks off (include a source if you have one): Report an accuracy issue. Local law, labels, and licences always override any recipe here. Inline notes labelled grain of salt flag the highest-risk over-trust points in the text.

Vocabulary

Key terms, defined once

Two numbers describe how any medium behaves when wet. The rest are about nutrients. You do not need to memorise these, just get the gist.

Water-holding capacityThe share of the pore space that still holds water after the medium drains. Higher means it stays wetter for longer.
Air-filled porosityThe share of the pore space holding air at that same drained moment. Water and air share one fixed total, so more water means less air.
EC (electrical conductivity)How much dissolved nutrient salt is in the water, in milliSiemens per centimetre (mS/cm). Higher EC is a stronger, saltier feed.
pHAcidity on a 0-14 scale. The cannabis root-zone sweet spot is about 5.5-6.5, tightening to 5.8-6.2 for precise soilless control.[4]
CEC (cation exchange capacity)A medium's ability to grab and hold nutrient ions in reserve, measured in meq/100g.[2] High CEC means the medium buffers (cushions) your feeding mistakes.
BufferingThe medium absorbing your errors. A buffered medium forgives a bad mix. An inert one passes it straight to the roots.
DrybackHow far a medium dries between waterings. It is the lever crop steering uses, covered in the coco paper.
One fixed pore space, split between water and airSaturate the medium and the slider shoves toward water, starving roots of oxygen.Water-holdingAir0%50%100%
Figure 2. Total pore space is a fixed bar. Wetting the medium pushes the split toward water and squeezes out the air the roots need to breathe.
A daily water-content cycleSaturate to field capacity, hold it, allow a controlled dryback, repeat. The size of the dryback is the steering lever.working bandtoo dry0255075100offP3 minonP1FCP2offwater content %
Diagram. Every medium runs a daily wet-to-dry cycle; the shape differs by how much water it holds.
pH and nutrient availability (soilless)most nutrients open up around pH 5.5-6.5lockedokbestoklockedpH 4pH 5pH 6pH 7pH 8Drift out of the green band and nutrients precipitate or stop being taken up, even though they are in the tank: lockout.
Diagram. pH behaviour and buffering differ by medium and set nutrient availability.
Core concept 1

Water versus air: the porosity trade-off

Roots need oxygen as much as water. A medium that stays saturated drowns the roots and invites rot, while one that holds too little water dries out and stresses the plant. The five media sit at different points on that see-saw.

Rockwool is roughly 96% total porosity but only about 11% air-filled, with around 91% water-holding, so it holds a lot of water and releases it freely[7]. Peat sits near 90-95% total porosity with a higher 18-25% air fraction[9]. Coco coir is the standout: even saturated it can keep around 22% air, beating rockwool's ~10%, which is why coco resists drowning[2]. DWC is the extreme case, where roots dangle in nutrient water and get oxygen only from a pump.

Air-filled porosity when fully wetAir at the roots when saturated. Higher means harder to overwater.0815223022%Coco coir21%Peat11%Rockwool
Figure 3. Coco keeps the most air when wet, which is why it is hard to overwater. Peat is naturally airier than rockwool.[2][7]
After draining, irrigated rockwool holds roughly 80% solution, 15% air and 5% fibre by volume.[7]
MediumTotal porosityAir-filledWater-holdingAir when saturated
Rockwool~96%~11%~91%~10%
Coco coir~94-96%highhigh~22%
Peat~90-95%18-25%highmoderate
The whole game in one line

More water means less air. The medium that keeps air around the roots even when wet is the one that forgives a heavy hand on the watering.

What a substrate is made ofsolids hold structure; pores hold water and airwater 35%air 53%Cocowater 30%air 66%Rockwoolsolid 18%water 45%air 37%Peat mixAir-filled porosity is root oxygen; water-holding capacity is the buffer. The ratio is what makes a medium wet or airy.
Diagram. A substrate is solids, water and air; the ratio of water-holding to air-filled porosity is what makes it wet or airy.
Core concept 2

How each medium handles nutrients

An inert medium like rockwool holds almost no nutrients of its own. Its native EC is negligible and its pH is alkaline, around 8, so whatever you feed is exactly what the roots get[7]. That is precise, but unforgiving of a bad mix.

Coco is the opposite. Its high CEC, roughly 40-100 meq/100g, buffers EC swings, but raw coco's exchange sites come pre-loaded with potassium and sodium and will strip calcium and magnesium out of your feed[2]. That causes a cal-mag deficiency unless the coco is buffered (pre-soaked in cal-mag) or bought pre-buffered[4].

Living soil is the most forgiving of all. Microbes, organic matter and minerals hold the root zone near pH 5.2-6.5 over hours to days when biology is healthy. Many organic growers still do not acidify routine waterings, but avoid extreme alkaline water and monitor if problems appear. In DWC there is no buffer at all: the reservoir is the only thing standing between your plants and a mistake.

The forgiveness scale: how much each medium cushionsLeft absorbs nothing. Right corrects your pH for you. Coco buffers EC but needs pre-buffering for Ca/Mg.Inert: DWC, rockwoolBuffered: cocoSelf-regulating: living soil050100
Figure 4. From no cushion (DWC, rockwool) through coco's EC buffering to living soil, which self-regulates pH after every watering.
Match the cushion to your patience

If you cannot promise a perfect feed every time, pick a medium that forgives you. The more control a medium gives, the less it protects you from yourself.

Core concept 3

Re-use, cost and footprint

Media differ enormously in how many runs you get. Rockwool can be re-used for up to about three years with proper sanitation, but it is energy-intensive to make and banned from landfill in some countries, which is why it has declined in parts of Europe and Japan[8]. Coco can be rinsed, re-buffered and re-used for a few cycles, then composted into garden soil, though most is shipped from overseas.

Living soil is the re-use champion. A recycled organic living soil (ROLS) bed is amended between runs and grows indefinitely, getting better with age. DWC has no medium to dispose of, but you replace the nutrient reservoir constantly and depend on continuous electricity for the pumps. Peat is cheap and effective but is a slowly renewing resource with a real carbon cost, which pushes many growers toward coco[9].

There is no clean winner. The lowest-waste option (living soil) needs the most up-front work.
MediumTypical re-useEnd-of-lifeCost tierEnvironmental note
Living soilIndefinite (ROLS, amend)Stays in serviceLow after setupLowest waste once established
Coco coirA few re-buffered cyclesComposts into soilLow-mediumHigh shipping footprint
PeatSingle use typicalCompostableLowSlow to renew, carbon cost
Rockwool~3 years with sanitationLandfill-restricted in placesMediumEnergy-heavy to make
DWCNo mediumNothing to discardMedium (gear + power)Constant reservoir + 24/7 power
Decide

How to choose: a beginner's decision path

Match the medium to your tolerance for fiddling, not to what wins a yield contest. The right answer is the one whose daily demands you will actually keep up with.

Pick by how hands-on you want to be1Water-only, low-techliving soil orqualitypeat-based soilmix→2Feed daily, watch EC/pHpre-bufferedcoco, also theon-ramp to cropsteering→3Parameters already dialedrockwool, afterirrigation isconsistent→4Constant monitoring OKDWC, highestreward andlowest safetymarginStart at the top. Drop down a step only when you are ready for more daily tech.
Figure 5. A simple ladder from most forgiving to most demanding. Most beginners should start at the top two rungs.
Beginner forgiveness scoreHow much daily slack the medium gives a beginner. Higher is safer to learn on.0258109/10Living soil8/10Peat / soil mix6/10Coco (pre-buffered)4/10Rockwool2/10DWC
Figure 6. Living soil and quality soil mixes forgive the most. DWC forgives the least, so it is not a first grow.
Skip DWC for your first run

DWC can give explosive growth, but a warm reservoir or a dead air pump can kill a plant in a day[6]. Learn the basics in a more forgiving medium first.

Do it

Setting up and running each, step by step

Every medium has one non-negotiable prep step. Get that right and the rest is routine.

  1. 1
    Coco
    Rinse and pre-buffer: soak 8-24h in a cal-mag solution unless bought pre-buffered. Then feed every watering at pH 5.8-6.2 and low EC, watering little and often.
  2. 2
    Rockwool
    Pre-soak the cubes or slabs at pH ~5.5 before transplanting, because dry rockwool sits near pH 8. After that, never let it go bone-dry.
  3. 3
    Living soil
    Build or buy the bed, let it cycle a few weeks, then water-only. Leave pH alone unless symptoms appear.
  4. 4
    DWC
    Hold the reservoir at 18-20C (65-68F), dissolved oxygen at 7-9 mg/L, pH 5.5-6.0, and run the air pump 24/7.
The single thing most beginners skip is the prep column. Each one is the difference between a smooth start and an early problem.[4]
MediumPrep stepFeed / water routinepH targetDo not skip
CocoRinse + cal-mag bufferFeed every watering, low EC5.8-6.2The pre-buffer soak
RockwoolPre-soak at pH ~5.5Frequent, never dry out5.5-6.0Conditioning before planting
Living soilCycle a few weeksWater-onlyLeave aloneNot pH-ing the water
DWCSet up pump + chillerRecirculating reservoir5.5-6.0Air pump on 24/7
DWC reservoir: the kill-zone made visiblePythium root rot climbs sharply above ~23C reservoir temperature. Keep dissolved oxygen 7-9 mg/L.cooltarget 18-20Ccautionroot-rot risk >23C14C21C28C
Figure 7. Keep the reservoir in the green band. Above about 23C the dissolved oxygen drops and Pythium root rot takes hold.[5][6]
Avoid

Common pitfalls and how to dodge them

The classic beginner mistakes are medium-specific, and most look like something they are not.

The coco and rockwool rows are misread as feeding problems. They are really prep problems.[4][5]
MediumClassic mistakeWhat you seeThe fix
CocoSkipping the buffer stepCal-mag deficiency: rusty spots, yellowingPre-buffer the coco, add cal-mag to early feeds
RockwoolPlanting into dry cubesPale, locked-out plant (pH-8 lockout)Pre-soak at pH ~5.5 before transplant
RockwoolOverwatering low-air matrixSlow, soggy, suffocating rootsFewer shots, let it breathe between
Living soilpH-ing water or adding saltsStalled growth, dying microbesWater-only, leave the pH alone
DWCWarm water or dead air pumpRoot health can collapse quicklyChill below 23C, keep the pump running
The number one DWC killer

A warm reservoir above 23C or a failed air pump crashes dissolved oxygen and brings on root rot can progress quickly[6]. If you run DWC, a chiller and a backup air pump are not optional.

Reality check

Realistic expectations

No medium grows the plant for you. They differ in where the difficulty sits, not in whether there is difficulty. Living soil front-loads the work of building the bed, then runs hands-off. Coco and rockwool spread the work across daily feeding and monitoring. DWC concentrates the risk into a few parameters that must never slip.

Daily effort versus the ceiling for yield and speedLeft to right is rising daily effort and risk. Height is the ceiling you can reach when dialed in.025810Living soilPeat / soilCocoRockwoolDWCceiling
Figure 8. Effort and risk climb from left to right. The ceiling rises too, but only the disciplined grower reaches it. A tidy soil grow beats a sloppy hydro one every time.

Fed precisely, inert media like coco and rockwool can edge out soil on yield and speed in research comparisons[1], and soilless cannabis responds strongly to dialed-in feeding[3]. But that only holds once your environment and irrigation are consistent.

How to actually get good
  1. Pick the failure mode you can live with. Every medium fails differently. Choose the one whose worst case you can prevent.
  2. Run one full cycle before you judge it. You learn a medium by living a whole grow in it, not by reading about it.
  3. Change one variable next time. Do not switch everything at once, or you will never know what helped.

Get one full run under your belt, then refine. When you are ready to drive a medium hard, the coco crop-steering paper picks up where this one leaves off, and the pH guide covers the number that quietly decides whether your feed even reaches the roots.

Related papers
Coco & crop steering
Steer the plant with water
pH management
Hold pH, avoid lockout
Water quality
Source water, RO & alkalinity

References

  1. Xiong J, Tian Y, Wang J, Liu W, Chen Q. Comparison of Coconut Coir, Rockwool, and Peat Cultivations for Tomato Production: Nutrient Balance, Plant Growth and Fruit Quality. Frontiers in Plant Science. 2017;8:1327. doi:10.3389/fpls.2017.01327 https://pmc.ncbi.nlm.nih.gov/articles/PMC5539188/
  2. Abad M, Noguera P, Puchades R, Maquieira A, Noguera V. Physico-chemical and chemical properties of some coconut coir dusts for use as a peat substitute for containerised ornamental plants. Bioresource Technology. 2002;82(3):241-245. doi:10.1016/S0960-8524(01)00189-4 https://www.sciencedirect.com/science/article/abs/pii/S0960852401001894
  3. Bevan L, Jones M, Zheng Y. Optimisation of Nitrogen, Phosphorus, and Potassium for Soilless Production of Cannabis sativa in the Flowering Stage Using Response Surface Analysis. Frontiers in Plant Science. 2021;12:764103. doi:10.3389/fpls.2021.764103 https://pmc.ncbi.nlm.nih.gov/articles/PMC8635921/
  4. Cockson P, Landis H, Smith T, Hicks K, Whipker BE. Characterization of Nutrient Disorders of Cannabis sativa. Applied Sciences. 2019;9(20):4432. doi:10.3390/app9204432 https://www.mdpi.com/2076-3417/9/20/4432
  5. Le DH, et al. Etiology and epidemiology of Pythium root rot in hydroponic crops: current knowledge and perspectives. Summa Phytopathologica. (review) Available via SciELO Brazil. https://www.scielo.br/j/sp/a/cGCh3FGRRzGLVFJf577mypy/?lang=en
  6. Dissolved oxygen limitation and Pythium root rot in strawberry NFT systems: mechanisms, research gaps, and prospects for substrate-free production. Frontiers in Plant Science. 2026. doi:10.3389/fpls.2026.1829367 https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2026.1829367/full
  7. Raviv M, Lieth JH (eds.). Soilless Culture: Theory and Practice. Elsevier/Academic Press, 2008. ISBN 978-0-444-52975-6. (Chapter: Physical Characteristics of Soilless Media.) https://shop.elsevier.com/books/soilless-culture-theory-and-practice/raviv/978-0-444-52975-6
  8. Composting of recovered rock wool from hydroponics for the production of soil amendment. Environmental Science and Pollution Research. 2024. doi:10.1007/s11356-024-33041-2 https://link.springer.com/article/10.1007/s11356-024-33041-2
  9. Peat Use in Horticulture. IntechOpen (peer-reviewed book chapter), 2018. doi:10.5772/intechopen.79611 https://www.intechopen.com/chapters/62735

Citations marked in-text as [n] map to this list. Primary literature and official guidance except where noted. Cannabis tissue culture is strongly genotype-dependent, verify dilutions, hormone doses and local regulations against the primary sources before relying on them.