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Unlocking the World of the Rockwool Microbiome

Is rockwool a sterile growing media? Many growers conceptualize a spectrum that puts living soil organics at one end, with its complex soil-borne ecology, and rockwool on the other with presumably little to no microbial life. But is this presumption accurate? Is rockwool as lifeless as we think? And more to the point, do we really want it to be sterile?

Why rockwool?

Rockwool has some major advantages as a growing media. First, as a manufactured product it is quite consistent with fairly little variability between or within the blocks and slabs. It is also clean, emerging completely sterile from the production process as the extreme heat involved kills off any microbes, preventing the transmission of pathogens and pests. It is also chemically inert with no nutrient exchange capacity and a relatively neutral pH, making it an ideal substrate for precision irrigation techniques like crop steering. Typically, rockwool growers utilize synthetic nutrients, and most are also using some form of disinfectant in their irrigation programs. Therefore, it is not surprising that many presume these systems to be sterile.

Although rockwool growing media arrives sterile off the production line, researchers have found that hydroponic cultivation systems utilizing rockwool develop an increasingly complex root zone microbiome over the course of the crop cycle. In fact, this process is not entirely dissimilar from what we see in soil-based agriculture, with all the plant-microbe interactions and ecology that is so well-appreciated by organic growers. We’ve all heard this story before. The root zone microbiome is the product of millions of years of coevolution between plants and microbes, and these plant-microbe interactions are fundamental to plant health. And in general, we discuss these processes in terms of natural, soil-based ecosystems or pre-fertilized, peat based growing media. It can be hard to see how these natural processes apply to rockwool and other hydroponic systems. But, as it turns out, plants and microbes coexist in rockwool systems as well.

Microbial Ubiquity and the Myth of Sterile Growing

In order to discuss the rockwool microbes, I think its helpful if we first talk about ubiquity, a concept fundamental to microbiology. To put it simply, microbes are ubiquitous, which means that they are found pretty much everywhere we think to look. They are in the air, in the water, on our skin, and in our bodies. And of course, they live on and inside plants. Take a sample from just about anywhere, add some sugars, and you can watch as microbial colonies grow and develop over time. A case study with tomato crops in rockwool showed diverse microbial growth from fertigation samples (Thomas et al., 2023). I have done this demonstration with growers who assumed their systems were sterile. If we take a sample of fertigation solution, or runoff from the growing media, we can introduce it to a sugar substrate on a petri dish and observe a surprising variety of microbes colonizing the dish over several days. So, microbes are ubiquitous and can be found everywhere, including in our cultivation facilities. That’s the first part of microbial ubiquity.

The second part involves environmental selection. This part states that microbes are everywhere, but it is the environment that selects for different microbial species. So, environmental parameters influence the composition of any given microbial community by creating conditions that favour certain microbial species over others. If we apply this to our grow facilities, we can see how growing media, moisture and irrigation frequency, temperature, pH, EC, and the availability of nutrients will all have an impact on which microbes persist in the root zone. In a rockwool garden, the ability of microbes to survive disinfectants like hypochlorous acid will typically be an important selection pressure as well. Those that cannot survive in these conditions will be eliminated, while those that can survive are left to establish colonies and dominate the system. So, horticultural systems will never be sterile, despite our best efforts. After all, with millions of microbes floating around our environment, it only takes a small fraction of these to be able to colonize the root zone for it to eventually be teeming with microscopic life. A case study demonstrated this resilience, showing Pythium oligandrum’s temporary impact on rockwool roots (Vallance et al., 2012).

Now, let’s consider the impact of plants. Specifically, let’s talk root exudates. Plants secrete sugars and other organic nutrients into the rhizosphere, providing food for microbes. These root exudates can account for up to 40% of photosynthetic activity, indicating that plants are very committed to encouraging the proliferation of microbes in the rhizosphere. So, is it any surprise that we never see sterile conditions in horticultural systems? Microbial ubiquity plus root exudates means that over time, we should expect to see an increasingly complex microbiome in the root zone. A case study observed this in rockwool, showing microbial diversity growth over a crop cycle (Thomas et al., 2023).

Indeed, this is what the research shows, though the characteristics of a typical rockwool microbiome tend to be very different from a typical soil microbiome. Rockwool tends to be home to far less microbial diversity, and tends to be heavily dominated by bacteria, due to the fact that unlike soil, it does not come into the garden loaded with a wide variety of microbes. Researchers note, however, that the rockwool microbiome tends to become more complex over time with steadily increasing microbial biomass and diversity across the crop cycle as the plants in the system work to develop microbial communities within the rhizosphere.

Cool. But why does it matter? Well, I would suggest that there are several compelling reasons to take a biological approach in rockwool cultivation by augmenting the naturally occurring microbiome with specialized microbial consortia. First, certain targeted microbes can play an important role in biocontrol by suppressing pathogenic organisms and pests. This may be especially important early in the crop cycle when the natural microbiome is only beginning to establish. Young plants are vulnerable, and planting them into growing media that starts off sterile robs them of the protection that would typically be provided by beneficial microbes. Supplementing the root zone with an effective biocontrol microbe can be great way to mitigate risk.

Microbes can also enhance root development and plant growth through biochemical signaling and the production of plant growth promoting hormones. This is a huge benefit that is often overlooked, but will typically produce easily observable results. In my experience, rockwool growers often see the most dramatic benefit from this phenomenon. Certain microbes have also been shown to activate the plant’s systemic immune system, enhancing trichome density and resin production. This induced systemic immune response also contributes to a greater accumulation of secondary metabolites like terpenes and cannabinoids. There is also research that suggests cannabis inoculated with certain targeted microbial strains may stay fresher longer and enjoy a prolonged shelf life, though more research is needed to further substantiate this claim. A case study with lettuce showed improved growth from microbial inoculation (Berkelmann et al., 1994). Together, these effects offer rockwool growers several valuable strategies to mitigate risk, improve the quality and potency of their products, and push yields beyond previous limits.

Your rockwool system is probably a lot less sterile than you thought. And given the advantages that microbes can offer, why would sterile be the goal?

References

Berkelmann, B., et al. (1994). Characterization of the Bacterial Flora in Circulating Nutrient Solutions. Acta Hortic., 361, 372-381.sterile growing media? Many growers conceptualize a spectrum that puts living soil organics at one end, with its complex soil-borne ecology, and rockwool on the other with presumably little to no microbial life. But is this presumption accurate? Is rockwool as lifeless as we think? And more to the point, do we really want it to be sterile?

Thomas, P., et al. (2023). The Hydroponic Rockwool Root Microbiome: Under Control or Underutilised? Microorganisms, 11(4), 835.

Vallance, J., et al. (2012). Fungal and Bacterial Diversity in Rockwool. Phytopathology, 102(10), 945-955.