Growing Media For Cannabis
Your selection of growing media can affect yield, crop time, nutrient feed ratios, irrigation methods, and the duration of time between each irrigation.
A new grower should be aware of their growing medium options to make the most informed decision possible prior to sowing seed or planting a rooted cutting!
On most nutrient feeding charts, there will be three types of media reflected: Soil, Coco, and Hydroponics.
Soil is made up of organic matter, minerals (sand / silt / clay), living organisms, gasses, and water. Organic matter refers to plant, animal, and microbial matter.
The organic matter in the soil should be in an advanced state of decomposition to increase the bioavailability of the nutrients in the soil to the root zone.
Soil with a high level of organic matter will produce the best results.
Cannabis grown in soil will often produce slightly smaller yields and take a little longer to grow compared to other growing mediums; but, it will produce a terpene-rich, aromatic flower that some growers prefer!
Soil growing is also an easy way to grow cannabis outside.
A cannabis grower can develop their own soil over time. Growers can pre-mix a known recipe at home or buy potting soil that is pre-mixed.
A perfect cannabis soil will have a good balance of moisture absorbance and drainage capability in relation to your climate and the frequency of watering. You do not want your plants to become overwatered due to poor drainage or wilted from drought-like conditions caused by poor water retention!
Cannabis, in general, thrives in a light loam soil with excellent drainage. The term loam refers to a mixture of sand, silt, and clay particulates that indicate a soil's structure, texture, and moisture retention ability.
Clay is the most fine of the three particulates with excellent moisture retention but poor drainage capability.
Silt is considered medium-fine coarse particulate and has a good balance of moisture retention to drainage.
Sand is the coarsest of the three particulates, with the least amount of moisture retention and the highest drainage capability.
A mix of 20% clay, 40% silt, and 40% sand, will provide a perfect soil texture and structure for cannabis.
A cannabis grower should aim for a soil pH between 5.8 - 6.2 though most strains will not show nutrient lockout above this range until exceeding a pH of 7.0.
There are many premixed organic potting "super" soils on the market today. They come complete with slow-release organic nutrition which allows a grower to feed only fresh water for several weeks until nutrient reserves are depleted.
These soils are amended with worm casting, bat guano, bone meal, blood meal, fish emulsion, mycorrhizae, beneficial microbes, and other organic matter. They are ready for use right out of the bag, though growers will likely have to add compost or amendments to their soil once the slow-releasing nutrient reserves in the soil become depleted.
For some growers, vegan growing may be important to them. It will be key to note that super soil, while often organic, is generally not considered to be vegan. If you are interested in vegan growing you can look into using vegan nutrients.
For growers with poor soil drainage, soil amendments like perlite, vermiculite, and expanded clay pebbles are inert natural products that increase the aeration of your soil.
COCO (Coco Coir): Coco or coco coir, is a medium made from ground up coconut husks and coconut pith. It generally comes in 3 particulate sizes: pith, fiber, and chips.
Pith, the smallest particulate of the three, holds water very well but has poor drainage.
Fiber particulate are more string like and very porous, but have poor water retention.
Chips are chunks of coco that will hold water well but allow for ample drainage as well.
Coco manufacturers often use a mix of these three particulate sizes in their product.
Coco is great for cannabis because it has fantastic water retention while remaining porous and light. Coco is also virtually inert of any plant nutrients, which means that a grower can dial-in the exact ratios of nutrients that are desired.
Coco is considered a hydroponic medium but has many of the same water retention properties as soil.
A first-time coco grower should make sure the source of their coco has been rinsed of salinity and pH buffered to compensate for excess potassium prior to purchase.
If the coco has not been treated, one can treat their coco prior to potting with cal/mag (calnesium) and a flushing/rinsing agent (Plant Flush) the product of salinity and before purchasing. A compressed brick of Coco will need to be re-saturated prior to potting.
Many hydroponic growers amend their coco with 30% perlite to increase drainage if their mix of coco is high in pith content.
Perlite is an extremely light volcanic glass that has white pebble-like appearance and is a common amendment for aeration and drainage in soil, coco and soilless mix.
Coco can be reused and is considered much more eco-friendly than peat-based potting mixes because peat takes 15-25 years to establish after harvesting.
Coco can also be rinsed and reused after harvest. It should be noted that reused media can run the risk of harbouring pathogens and should be done with caution unless the media has been properly sterilized.
Peat-based soilless mix: Much like coco a soilless mix is considered an inert medium for hydroponics and does not provide plant nutrition.
Like coco, a soilless mix will provide excellent balance of water retention and drainage. An average peat-based soilless mix will be made up of sphagnum peat, perlite, and vermiculite.
Vermiculite is a natural mineral created from compressed silicates. Vermiculite is spongy and is less porous than vermiculite meaning it will retain more water over a longer duration.
Vermiculite will also slightly raise the pH of a soilless mix because it has a pH of 7.0. Peat-based soilless mixes will come pH buffered and often contain wetting agents to evenly and completely saturate the media in the pot.
Some peat-based mixes are inoculated with mycorrhizae, which is a beneficial fungi that creates a symbiotic relationship for the plant to obtain nutrients more efficiently. Manufacturers also sell mixes that provide different rates of moisture retention and drainage.
Some soilless mixes are marketed as having high porosity and are good for growers who have no issues with heat and are not concerned with using more water due to the increased frequency of irrigation that may be required.
Rockwool: Rockwool is a hydroponic substrate made from woven molten basaltic rock. Rockwool comes in cubes for seedlings and cuttings, blocks for vegetative plants and slabs for large plants and commercial growers growing multiple plants on one slab.
Rockwool cubes are designed to fit on top of other rockwool cubes, which can then be fixed onto rockwool slabs, which makes a versatile building-block style design for plants of any size. Rockwool retains 70-80% of irrigation water after feeding and fully saturated, will still provide about 15% of space for air to fill in porous gaps in the media.
This means that your roots will never be short of oxygen and CO2.
Rockwool is most often used in a top drip style hydroponic systems, as flood and drain tables can cause small plants to float and drift on a growing table.
Most rockwool cubes and blocks are free draining, while a slab may require a grower to add drainage cuts or slits in the bottom.
When hydrating a slab, it conserves water to add drainage cuts to the plastic housing after fully saturating the media just prior to transplant.
Rockwool is most effective when providing multiple short shots of irrigation that reach field capacity, meaning that the rockwool has drained excess moisture while retaining enough solution to provide nutrition with ample oxygen levels and CO2 in the root zone.
Many rockwool cubes will have pre-drilled holes in them to stick a cutting or seedling and they are designed to fit inside 72 cell propagation trays under a humidity dome. Rockwool can be reused after harvest because it does not decompose.
Research shows that though there is no inherent microbial content to sterile substrate, populations of microbes can be introduced and thrive in rockwool.
Rockwool can be used in many hydroponic applications making it extremely versatile.
Though soilless mixes and coco are virtually inert but not truly 100% inert, rockwool is.
This gives a grower the ability to make surgical like precision decisions in regards to plant nutrition.
Due to the high porosity of rockwool it can also be less forgiving if an irrigation event is missed, as it will dry out faster. If a mistake is made by the grower due to excessive EC or a pH outside the optimal range, rockwool can be more forgiving, as it drains faster and can be easily flushed to correct the issue.
Expanded Clay Pebbles: Lightweight expanded clay aggregate (LECA), are pebble-like balls made of expanded clay that has been cooked in a kiln.
The clay balls anchor your plant in place the way soil would in nature but allow water and air to flow freely in the space between them. LECA is commonly used in deep water culture, drip irrigation, ebb and flow systems.
LECA is used with net pots, which are plastic mesh baskets that hold the pebbles while allowing roots to grow through them.
Due to LECA having extremely poor water holding capacity, proper irrigation and frequency of irrigation events cannot be achieved by hand watering this media.
Though the pebbles do provide anchorage, they do not provide the soil–like water retention of coco and soilless mixes.
LECA also needs to be rinsed prior to use as the pebbles must be washed of dust that can clog irrigation lines.
Expanded clay is somewhat porous and can absorb some moisture but it is not as porous as perlite, nor nearly as absorbent as every other media.
LECA can be recycled for future use after harvest, which is easy to do because the near marble size makes them easy to extract from a root ball and most roots will be suspended below the net pot anyway.
LECA tends to be a bit pricier than other media and can also become quite heavy, but it is much cleaner to work with than soil, coco, or a soilless mix.
There is much more to be said about each medium, the world of soil alone is quite complex, but this should at least give you an idea on your options. Now that you are aware of your options, we wish you the best of luck with the choice you make for your next grow!