Living Soil vs Coco vs Rockwool: Which Is Best for Cannabis?

Header photo: Plantlady223, Wikimedia Commons, CC BY-SA 4.0.

Ask cannabis growers about the best growing medium and you quickly enter three different worlds. One grower wants a living soil full of compost, fungi and bacteria. Another prefers coco because it combines fast growth with a forgiving root environment. A third wants rockwool because almost everything entering the root zone can be measured and controlled.

All three systems can grow excellent cannabis. The important difference is not whether one medium is inherently superior, but where the control of the root environment sits. In rockwool, much of that control sits with the grower and the irrigation solution. In living soil, considerably more is delegated to the substrate and its biology.

Coco falls somewhere between those extremes: it can be fertigated almost like a hydroponic substrate, yet it has its own nutrient-exchange properties and can support a substantial microbial community. That makes substrate choice much more than a question of what physically holds the roots.

First: what exactly is “living soil”?

There is no universally accepted scientific definition of living soil in cannabis cultivation. Commercial products range from peat- or coco-based mixes inoculated with microbes to complex mixtures containing compost, worm castings, organic fertilizers, biochar, minerals and established microbial communities. A review of cannabis growing media notes specifically that no formal standard defines what can be called living soil.

For this article, I use living soil to mean a biologically active growing medium in which a substantial part of nutrient availability is created or buffered by organic matter and microbial processes rather than supplied entirely as dissolved mineral fertilizer.

That does not mean the plant absorbs “organic nitrogen” directly from a piece of compost. Microorganisms break organic materials down and nutrients eventually become mineral ions that roots can absorb. The difference is that part of the nutrient cycling happens inside an ecosystem rather than only inside a fertilizer tank.

A well-functioning living substrate therefore needs more than nutrients. It needs suitable moisture, oxygen, organic material and enough physical structure for roots and microorganisms to function together.

Rockwool starts at almost the opposite end

Rockwool is manufactured by melting mineral material and spinning it into fibres. New horticultural rockwool contains essentially no organic matter and begins as a highly uniform, biologically inactive material. This is one reason it became so successful in high-tech greenhouse horticulture: the grower starts with a root environment whose water content, EC and nutrient composition can be manipulated very directly. That does not mean rockwool stays sterile.

Roots immediately release carbon compounds into their surroundings. Irrigation water, plants, equipment and the greenhouse environment introduce microorganisms, and bacterial populations can establish surprisingly quickly in planted rockwool. Research on hydroponic rockwool describes a genuine root microbiome, with microbial activity strongly influenced by irrigation, pH, EC and root exudates.

So the real distinction is not that living soil has microbes and rockwool has none. It is that the two systems start from very different biological and chemical conditions.

Time matters more than growers sometimes realise

I have seen this distinction clearly in professional horticulture. Crops such as hydrangea and gerbera can be grown in pots containing rockwool flakes. Those crops may remain in the same substrate for several years. Although that substrate started with almost no biological ecosystem, it certainly does not remain biologically empty. Given roots, moisture, nutrients and enough time, microbial communities establish themselves.

A tomato or pepper crop is different. Its commercial production cycle is roughly a year, including the propagation period. Cannabis is usually much shorter again. That matters: a cannabis plant grown for only a few months does not have years to build a mature root-zone ecosystem from a nearly biological blank slate. In a short crop, the biology and organic matter you start with can therefore have disproportionate importance.

This is one reason I think deliberate microbial introduction is particularly interesting in short-cycle crops. You are not waiting for the ecosystem to assemble entirely by chance.

Coco occupies an unusual middle position

Coco is organic material, but that does not make it living soil. Clean, buffered coco can be used almost like a hydroponic substrate. A grower can supply a complete mineral nutrient solution at relatively frequent intervals and manipulate water content and EC quite precisely.

Unlike rockwool, however, coco has meaningful cation exchange capacity and interacts chemically with calcium, magnesium, potassium and other ions. It also provides an organic physical matrix in which microbial communities can develop.

A recent review specifically focused on cannabis growing media describes coco as a semi-inert medium rather than a completely inert one. Substrate choice can substantially affect rooting, water availability, nutrient uptake and flower biomass even when nutrient programs are adjusted for each medium. That combination of control and biology is one reason I find coco so interesting.

Professional pepper cultivation shows the difference

Dutch pepper cultivation provides a useful real-world comparison because rockwool has been a standard substrate for decades.

My experience with coco changed how I looked at substrates

In professional sweet pepper production, I was involved with crops grown on organic, coco-containing substrates alongside the much more familiar rockwool systems. Once the grower had learned how the substrate responded to irrigation, coco was not necessarily more difficult to manage. It was mainly different. That distinction matters.

Changing from rockwool to coco while keeping exactly the same irrigation strategy can cause problems because water distribution, drainage and dryback are different. Dutch commercial experience describes the same issue: some coco structures drain rapidly and need shorter, more frequent irrigation events, while other organic substrate mixtures hold water much more strongly.

When the grower understands those properties, production can come surprisingly close to rockwool. A Wageningen University & Research pepper comparison produced 28.1 kg/m² on rockwool and 26.5 kg/m² on coco, with the researchers attributing much of the difference to less effective generative steering of the coco crop early in the season rather than an inherent limitation of the substrate. More recent professional trials have likewise shown comparable total pepper production from organic and rockwool substrates when management is adapted appropriately.

Cannabis research does not identify one universal winner either

A direct cannabis experiment comparing rockwool, peat-perlite and coco found that growing medium affected water availability, nitrogen uptake, biomass accumulation and the way biomass was partitioned through the plant. Cannabinoid concentration, however, was affected much less strongly by substrate than growth itself.

Another experiment published in 2025 compared coco with a peat-based potting substrate and also tested a five-strain Bacillus inoculant. The peat-based mix produced greater overall biomass, while coco promoted stronger root development and in that specific experiment produced higher cannabinoid and terpene concentrations per gram at maturity. Adding the microbial treatment increased flower yield in both substrates.

Those results are interesting, but they should not be turned into rules such as “coco makes more terpenes” or “soil yields more”. Different cultivars, irrigation strategies, nutrient programs, substrate volumes and environments can all change the result. The more useful conclusion is that the substrate alters the root environment enough to change how the plant grows.

Living soil gives the root zone more buffering

The attraction of living soil is that not everything changes immediately when the grower makes a small mistake. A larger volume containing organic matter, exchange sites and active biology can buffer changes in nutrient availability and moisture. That can make living soil relatively forgiving, particularly when the system is mature.

The trade-off is that corrections can also be slower. If a grower decides that the crop needs substantially more nitrogen tomorrow, a biologically mediated nutrient system cannot always respond as quickly as a mineral fertigation system. Microbial mineralization depends on moisture, oxygen, temperature and the quality of the organic material.

Living soil therefore exchanges some direct control for biological buffering. For many home growers, that is not necessarily a disadvantage. A system that remains reasonably stable without six irrigation events per day may actually fit the available equipment and time much better.

Rockwool offers control, but you have to provide the control

Rockwool works extraordinarily well in professional horticulture because growers can measure and manipulate almost everything around it. Water content, drain percentage, EC, pH, irrigation timing and nutrient composition can be monitored continuously. At home, that advantage can become a responsibility.

A small rockwool block contains relatively little chemical or biological buffering. When irrigation or nutrient concentration moves in the wrong direction, the root zone can change rapidly. Professional growers solve this with accurate dosing equipment, sensors, automated irrigation and experience.

A home grower does not necessarily need all that equipment, but the less automation available, the less attractive extreme precision may become. A substrate should suit the grower as well as the plant.

Coco can be a practical compromise

This is where coco can make a lot of sense for home cannabis. It still responds rapidly enough that a grower can adjust nutrition and irrigation. It can hold substantial amounts of water while maintaining good aeration when its physical structure is appropriate. It also offers more chemical interaction and biological potential than a fresh slab of rockwool.

That does not mean it is maintenance-free. Poor-quality coco can contain excessive sodium, potassium or chloride, and badly prepared material may need washing or buffering. Irrigation practices also need to match the size and structure of the coco rather than copying a soil schedule. But once a grower understands that, coco can offer a useful middle ground between biological buffering and hydroponic control.

A home grower does not need greenhouse-level technology

Professional greenhouse knowledge is useful because it reveals the underlying biology, not because a home grower should recreate a Dutch glasshouse. You do not need expensive substrate sensors to learn from crop steering. You can monitor how much solution goes into a pot, how quickly the plant uses water, the weight of the container, the moisture deeper in the substrate and how the plant responds after irrigation.

You do not need automated nutrient dosing to understand EC. A basic EC meter can already tell you whether the nutrient concentration you are applying is moving up or down.

And you do not need a microbiome sequencing laboratory to benefit from a biologically active substrate. You need good root conditions: moisture without persistent saturation, oxygen, suitable temperature, organic resources when appropriate and enough time for roots and microbes to establish. The technology improves precision. The biological principle remains the same.

So which system is best?

For a grower who enjoys organic cultivation, uses reasonably large containers and wants the root zone itself to buffer and cycle nutrients, a well-designed living soil can be an excellent choice.

For someone who wants fast response, regular fertigation and considerable control without moving all the way to a highly inert system, coco is extremely attractive.

For growers who enjoy measurement, automation and tight control over irrigation and nutrition, rockwool remains one of the most effective growing media available.

None of those descriptions makes one system universally superior. They simply place responsibility in different places. Living soil relies heavily on the substrate ecosystem. Rockwool relies heavily on the grower and irrigation system. Coco shares responsibility between the two.

For short cannabis crops, I would not ignore biology

Cannabis has a relatively short production cycle. That makes me reluctant to assume that a useful microbiome will simply establish itself eventually, especially in a substrate that begins with almost no organic matter or microbial diversity.

Rockwool will develop a microbiome, but it will be shaped by whatever enters the system and by the relatively unusual environment created by mineral fibres, frequent irrigation and soluble fertilizer. Coco starts with a different ecological template. Living soil goes further and deliberately tries to establish that biological environment from the beginning.

I do not think that means every cannabis grower should use living soil. I do think it means we should stop treating the root zone as nothing more than a place to hold water and fertilizer. Roots are growing in an ecosystem even when that ecosystem starts on a slab of rock.

The real choice between living soil, coco and rockwool is therefore not simply which substrate grows the biggest plant. It is how much of the root environment do you want to manage yourself, and how much do you want the biology of the system to manage with you?

References

Malík, M. & Tlustoš, P. (2025). Soilless Growing Media for Cannabis Cultivation. Agriculture, 15, 1955.

Schober, T., Präger, A., Hartung, J., Hensmann, F. & Graeff-Hönninger, S. (2023). Growth dynamics and yield formation of Cannabis cultivated in differing growing media under semi-controlled greenhouse conditions. Industrial Crops and Products, 203, 117172.

Lyu, D., Ruan, E.D., Backer, R., Gagné-Bourque, F. & Smith, D.L. (2025). Changes in morpho-physiological traits and phytochemical composition of Cannabis sativa L. treated with microbial biostimulants across different substrates. Industrial Crops and Products, 224, 120377.

Buirs, L. & Punja, Z.K. (2025). Endophytes in Cannabis sativa: Identifying and Characterizing Microbes with Beneficial and Detrimental Effects on Plant Health. Plants, 14, 1247.

Thomas, P., Knox, O.G.G., Powell, J.R., Sindel, B. & Winter, G. (2023). The Hydroponic Rockwool Root Microbiome: Under Control or Underutilised? Microorganisms, 11, 835.

Van Os, E. et al. (2017). Vergelijking tussen emissieloze teelt op steenwol en kokos: Waterefficiënte Emissieloze Kas. Wageningen UR Greenhouse Horticulture, Report GTB-1416.

Eurofins Agro (2025). Microbiome in Organic Substrates: Differences in Microbiome May Be the Cause of Pepper Plant Wilting.

GroentenNieuws / Growficient (2024). Overstap op organische substraten in paprikateelt: inzicht cruciaal om risico’s te beperken.

Micro Matrix and root-zone biology

Micro Matrix contains Bacillus bacteria, connecting it to this article's discussion of microbial introduction and life around the roots. Its organic fertilizer component has a supporting role; the microbial component is the main focus. Adding bacteria does not by itself create a complete living-soil ecosystem or guarantee the same response in every substrate. Moisture, oxygen, nutrition and the existing microbial community remain part of the picture.