Coco Is Not Soil: Why Cannabis Roots Behave Differently in Coir

Coco coir is one of the most popular growing media for indoor Cannabis, but it is often treated as if it were simply a lighter version of potting soil. That misunderstanding causes a surprising number of problems.

Coco is an organic material made from coconut husks, but that does not make it soil. At the same time, calling it completely inert is not correct either. Coco holds water and air, exchanges nutrients with the root solution and can support a substantial microbial community. In practice, it sits somewhere between the biological complexity of soil and the tight root-zone control of hydroponics. Understanding that difference changes the way you irrigate, fertilize and manage the root zone.

What actually makes coco different?

A good growing medium has to do several jobs at once. It must hold enough water for the plant, retain enough air for the roots and provide a physical environment in which new roots can expand. Coco does all three reasonably well, which is one reason it has become so successful in horticulture.

Coco also has a relatively high cation exchange capacity, or CEC. In simple terms, negatively charged sites in the material can temporarily hold positively charged nutrients such as potassium, calcium and magnesium. Rockwool has almost no meaningful CEC, so the nutrient solution around the roots is much more directly determined by what the grower supplies. Coco actively interacts with that solution.

That does not mean coco feeds the plant like a fertilized soil. Its natural nutrient composition is not balanced for crop production. Coconut material can contain relatively high concentrations of potassium, sodium and chloride while containing comparatively little calcium and magnesium. The exact composition depends strongly on origin and processing. This is why the quality and preparation of the coco matters.

Why coco is washed and buffered

Fresh coconut material may contain salts that you do not want in the root zone. Washing removes much of the soluble sodium, chloride and excess potassium. Buffering is a different process: calcium, often together with magnesium, is used to occupy exchange sites that would otherwise interact strongly with the first nutrient solution supplied to the crop.

This is also where much of the Cannabis discussion around CalMag originates. Coco does not mysteriously eat calcium for the entire crop cycle. The important issue is the exchange chemistry of inadequately prepared coco, particularly during establishment.

Good horticultural coco is therefore often already washed and buffered before sale. If you buy a professional-grade product that has already been prepared, repeatedly flushing and heavily loading it with calcium and magnesium may solve a problem that is no longer there. With cheap compressed coco bricks or material of uncertain origin, checking the starting EC and whether the product has been washed and buffered becomes much more important. This is one of the reasons two growers can have completely different experiences while both claim to be growing in “coco”.

Coco should not be watered like soil

The physical properties are just as important as the chemistry. Coco can retain a large amount of water while still maintaining considerable pore space for oxygen. That combination allows roots to remain active at water contents that might be uncomfortable in a dense mineral soil.

For Cannabis growers this has an important consequence: the traditional soil idea of waiting for a container to become very dry before watering again does not automatically translate well to coco. In a well-rooted coco system, irrigation can be used to keep water content, nutrient concentration and oxygen availability within a relatively narrow range.

As the plant removes water, salts remain behind and root-zone EC increases. If the medium is allowed to dry too far, the plant is therefore not only experiencing lower water availability; it may simultaneously experience greater osmotic pressure from the concentrated nutrient solution.

That does not mean coco should always be saturated. Dryback can be used deliberately as a crop-steering signal, just as it is in professional greenhouse horticulture. The difference is between a controlled change in root-zone water content and simply allowing coco to become severely dry because it is being treated like potting soil.

The root zone can be steered

This is where coco starts to become particularly interesting. With a relatively small substrate volume and controlled fertigation, a grower can influence the water content and EC around the roots much more precisely than in a large volume of traditional soil. More frequent irrigation generally creates smaller changes in water content and EC, while a larger dryback creates a stronger physiological signal. These are not simply watering techniques; they influence the plant.

The same crop-steering principles used commercially with tomato and pepper can also be applied conceptually to Cannabis: root-zone conditions can push a plant slightly toward vegetative or generative behaviour. The objective is not to make the roots suffer but to keep conditions within a productive range while deliberately influencing plant development.

This is another reason why copying an irrigation schedule from someone else's grow rarely makes much sense. Plant size, container volume, coco structure, climate, light intensity and root development all affect how quickly the root zone changes.

Coco can produce just as seriously as rockwool

Professional greenhouse horticulture gives us a useful comparison because crops such as sweet pepper have traditionally been grown very successfully on rockwool.

A Wageningen University & Research trial compared sweet pepper grown on rockwool and coco in a closed, low-emission production system. Production reached 28.1 kg/m² on rockwool and 26.5 kg/m² on coco. Importantly, the researchers attributed much of the difference to the crop having been steered less generatively on coco early in the season, rather than concluding that coco was fundamentally incapable of matching rockwool production.

More recent commercial research shows how close that gap can become. In a 2024 trial at Proefcentrum Hoogstraten, sweet pepper grown on an organic Lensli substrate containing coco produced 21.4 kg/m² by week 38 compared with 21.6 kg/m² on rockwool. The average fruit weight was actually slightly higher on the organic substrate.

For me this matches what I have seen in practice: coco is not necessarily a lower-production system. You do, however, have to learn to grow differently in it.

What I saw in professional pepper cultivation

One of the more interesting crops I worked with was at Rainbow Growers, where a great deal of experimental work was carried out around resilient and more biologically based pepper production.

We worked with organic, coco-containing substrate systems and compared their behaviour with conventional rockwool production. Once the irrigation and nutrition strategy had been adapted to the substrate, production could be comparable. What stood out much more strongly to me was the condition of the crop. The plants on the organic substrate often looked remarkably stable and resilient.

At the time this was particularly interesting because Dutch pepper growers were experiencing serious root problems. Organic substrates, including coco-containing systems, increasingly attracted attention because crops appeared to behave differently from crops on rockwool.

That observation has since become much more widespread in Dutch horticulture. Rainbow itself now publicly describes organic substrate as an advantage for biological balance, while other commercial pepper growers have also moved from rockwool toward coco or organic substrates in response to root-health problems. But this does not mean that coco magically prevents root disease.

The Fusarium story shows why we need to be careful

Fusarium-related wilt has caused substantial problems in Dutch sweet pepper cultivation. Some growers moved from rockwool toward coco and peat-based organic substrates partly because of those problems.

Dutch horticultural journal Onder Glas reported in 2025 that growers do indeed tend to see fewer Fusarium problems in these substrates, particularly during the first crop cycle. However, Fusarium spores can still be found around roots and in drainage water, and problems have also occurred in reused coco. Coco is therefore not a disease-proof substrate.

That distinction is important. It is tempting to say “Coco prevents Fusarium”, but the evidence does not justify that. A much more interesting question is whether the root-zone ecosystem created in coco can sometimes make it more difficult for disease to dominate.

Coco can become biologically very active

This is perhaps the most overlooked difference between coco and rockwool. Rockwool is not sterile once plants are growing in it. Microorganisms colonize roots and irrigation systems even in highly controlled hydroponic greenhouses. However, rockwool is a highly uniform mineral environment with very little organic structure compared with coco.

Dutch measurements show that this difference can have a major effect on microbial development. Eurofins Agro compared pepper crops grown in mineral and organic substrates and found substantially more microbial biomass in the organic substrates. Bacterial and fungal diversity was also higher, with more organisms associated with plant health and resilience. Importantly, Eurofins itself cautions that this does not prove that every organism present is beneficial or that greater diversity automatically prevents disease.

Older Dutch measurements show a similar pattern. Coco supported more fungal biomass, protozoa and other microbial groups than rockwool, while substrate type appeared to have a remarkably strong influence on the composition of the root-zone microbiome. That makes coco biologically quite different from the empty brown sponge it is sometimes imagined to be.

Something interesting happened in one of our Bacillus trials

I saw this directly in a practical trial at Rainbow. We introduced a group of Bacillus bacteria into a coco-based pepper crop. These were the same type of Bacillus organisms that I later continued working with in microbial products. During roughly the first two months, before recirculation became part of the system, we followed the root-zone biology with a soil-life monitoring method.

The treated substrate showed around 30% more measured microbial life than the comparison. What interested me most was that we had not simply measured more Bacillus. The wider microbial community had changed as well, including fungal groups such as Trichoderma and organisms classified among the oomycetes.

This was a commercial horticultural trial, not a peer-reviewed scientific experiment, so I would not use it to claim that adding Bacillus increases the entire microbiome by 30%. There are too many possible interactions and confounding factors for that conclusion. But the observation changed the way I think about microbial products: adding a microorganism does not necessarily affect only that microorganism. It enters an ecosystem, and that ecosystem may respond.

More microbes does not automatically mean better microbes

This deserves emphasis because “more soil life” is sometimes treated as automatically beneficial, but it is not. Trichoderma includes species widely used as beneficial organisms, but even there effects are strain-dependent. Oomycetes are an even clearer example: the group contains many harmless organisms, but also notorious plant pathogens such as Pythium and Phytophthora. A microbial analysis therefore cannot simply be reduced to a score where higher is always better.

What interests me about coco is that it provides enough biological structure for these ecological interactions to develop. Recent pepper trials using the Eurofins BodemLevenMonitor and microbiome profiling similarly found a richer and more varied microbial community in coco than in rockwool. The next question is not simply how much life is present, but what that community is doing.

This matters for Cannabis too

Cannabis growers often divide growing systems into overly simple categories: soil is considered biological, hydroponics is considered sterile, and coco is placed somewhere between them without much thought.

In reality, coco gives growers an unusual combination. It can be fertigated and steered much like a hydroponic substrate while simultaneously offering an organic physical environment capable of supporting a complex microbial community. That creates opportunities, but also means you cannot blindly apply either soil rules or rockwool rules.

If you use mineral nutrition, coco can be managed as a tightly controlled soilless system. If you deliberately introduce microbes or organic inputs, its biological behaviour becomes increasingly important. Neither approach changes the fundamental fact that the chemistry, moisture and microbial community of the root zone interact with each other.

What should a home grower take from this?

The most important lesson is not that everyone should switch to coco. It is that substrate is part of the growing system. If you use coco, find out what you are actually buying. Proper horticultural coir may already be washed and buffered. Unknown or poorly processed material may contain enough potassium, sodium or chloride to change root-zone chemistry considerably.

Do not assume that every calcium problem requires more CalMag. Do not assume that coco should repeatedly become as dry as a soil-filled container. And do not assume that an organic-looking substrate automatically behaves like living soil. Most importantly, pay attention to the root zone rather than only the bottle you are pouring into it.

Coco can hold water, exchange nutrients, support oxygen around roots and develop a substantial microbial ecosystem. Those properties explain why it can be such an effective substrate, and why growers who understand it often get very different results from growers who simply treat it as brown soil.

Coco is not soil — and that is the point

Coco works because it occupies an interesting position between soil and hydroponics. It gives the grower considerable control over irrigation and nutrition, while retaining chemical and biological properties that a truly inert substrate largely lacks. Professional pepper cultivation shows that these differences can influence crop balance, root development and possibly even the way plants interact with root diseases.

That does not make coco automatically better than rockwool or soil. Each system has advantages, costs and management requirements. But it does mean that choosing a substrate is about much more than deciding what will physically hold the roots: you are choosing the environment in which those roots will live.

References

Abad, M. et al. (2002). Physico-chemical and chemical properties of some coconut coir dusts for use as a peat substitute for containerised ornamental plants. Bioresource Technology.

Soilless Growing Media for Cannabis Cultivation (2025). Review of physical and chemical characteristics of growing media including coco, rockwool and peat.

Van Os, E. et al. (2017). Vergelijking tussen emissieloze teelt op steenwol en kokos: Waterefficiënte Emissieloze Kas. Wageningen University & Research.

Proefcentrum Hoogstraten (2024). Comparison of rockwool and organic substrates in tomato and sweet pepper production.

Eurofins Agro (2025). Slapgaan en microbioom bij paprika. Microbial biomass and diversity in organic and mineral growing substrates.

Van der Knaap / GroentenNieuws (2024). Pepper trial comparing microbial development in coco and rockwool substrates.

Van Lier, A. (2025). Pas op met hergebruik kokos- en veensubstraten in paprika. Vakblad Onder Glas.

From practical experience to Micro Matrix

Micro Matrix contains Bacillus strains I also worked with in the Rainbow pepper trial described above, with one strain from that trial omitted. That practical experience informs my interest in these organisms and their interactions with the root-zone ecosystem. The trial involved a different crop and strain combination, so the observed increase in measured microbial life should not be read as a guaranteed result for Micro Matrix or Cannabis.