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What is the best substrate for growing cannabis?

When setting up a cannabis cultivation, one of the most crucial decisions a grower has to make is the type of medium they are going to use. Generally speaking, a high-quality medium should have a proper balance of physical and chemical properties, fast-draining with high saturated hydraulic conductivity, as well as providing enough space for roots to ‘breath’ to reduce the risk of root pathogens.
The main types of mediums are peat-based, coir-based, Rockwool, and foams.

Coir-based medium
Coir-based mediums are one of the most used by North American growers. Coir usually has high aeration and drainage: thanks to its ability to absorb water, it doesn’t require additional wetting agents. Because of this, coco coir is renowned to be difficult to overwater. Additionally, thanks to its high aeration, it helps growers reducing the risk of root pathogens, to which the cannabis plant is particularly susceptible. As a consequence of that, the root zone of the crop is maximized, increasing root growth which causes enhanced water and nutrient uptake.

Rockwool is >95% basalt, the most abundant mineral on the plant. Rockwool does not biodegrade, but it can be recycled into new rockwool products; but also, it could be incorporated into composts to increase friability aeration and moisture retention, qualities that improve the habitat of microbiome responsible for the decomposition of organics, not only in the compost itself but also in the final soils it is added to as an amendment. On top of that, generally speaking, rockwool does not utilize much water, which is an extremely precious and scarce resource. 

Living soil 
One of the most interesting mediums surely is living soil. This is a type of medium where growers incorporate a microbial inoculant into the medium, and it can have different degrees of complexity. The composition of the soil should foster microbial activity and diversity as well as beneficial elements to counter certain root diseases. That’s why biodiversity is critical, as it is the key to maintain the balance between the different components. Yet, there are several challenges for cannabis growers who want to use living soil. First of all, nutrition availability: cannabis is a relatively short-cycle plant, which means that growers have a very short window of time to set up an active nutrition system in the soil. A common way to solve this is to overcharge the living soil with the necessary nutrients and additives to provide the plant with everything they need throughout the cycle; yet, usually plants growing in living soils tend to grow a little slower compared to other mediums.

Because of the lack of commonly accepted standards, there’s hardly a consensus among growers for the best growing medium to use. At the same time, it is important to point out that the specificities of an operation, as well as the grower’s methodology, often dictate the type of medium to deploy, since hardly it is a one-size-fits-all kind of situation.