In soilless cultivation like hydroponics or container gardening, growers typically use a growing medium, also called the substrate, made from coconut fiber, rockwool, perlite or another porous material in which their plants can take root.[1]
Providing an “artificial” environment for the roots offers several benefits over soil: The growing medium is often infertile, so growers have complete control over nutrient levels. The risk of soilborne pests and pathogens is lower. And it can improve water and resource efficiency.
However, not all growing media are equal, and growers should consider their characteristics when choosing the right one. In this blog post, we discuss the importance of a growing medium, its properties, and other factors to consider before giving an overview of the different types of growing media available.
Importance of the growing medium
In outdoor cultivation, the soil supports plants. In addition to anchoring the roots, it holds water, oxygen and nutrients for the roots to access.[2] An effective growing medium must be able to replicate these functions while providing the right balance between water retention and aeration.
Roots require a consistent mixture of water and oxygen. Oxygen is required for respiration, enabling roots to absorb water and essential nutrients. In containerized production, the shallow depth increases the risk of oversaturation during irrigation. Conversely, the container’s capacity can restrict the amount of water stored between irrigation cycles. An ideal growing medium should retain enough water between irrigations while providing ample oxygen to the roots.
Properties of growing medium
A growing medium’s characteristics determine how well it provides the support, water, oxygen, and nutrients that plants need. Specifically, growers should consider the substrate’s physical, chemical and biological properties.
Physical properties
The physical properties of a growing medium include the size, shape, texture and physical arrangement of the substrate particles.
The latter are very important for replicating the soil’s ability to deliver the right balance of oxygen and water. A growing medium should have both high water-holding capacity[3] and high saturated hydraulic conductivity (Ksat), which measures the substrate’s ability to move water when fully saturated. This means the substrate can retain a moderate amount of water between irrigations while having enough macropores to prevent waterlogging and allow excess water to drain. This helps reduce the risk of root pathogens like Pythium.
Cannabis prefers a light, well-aerated substrate, which facilitates development of more lateral branches and stronger roots. A highly porous medium can also increase dry floral weight and cannabinoid concentration.[4] Durability is also important; soft aggregates which disintegrate easily should be avoided.
Chemical properties
The chemical properties of a growing medium include its pH, cation exchange capacity (CEC) and electrical conductivity (EC).
The former two directly impact nutrient availability. Most nutrients are available for absorption within the 5.5–6.5 pH range,[5] so a growing medium with a similar pH range helps to ensure all essential plant nutrients remain available.
CEC measures the substrate’s nutrient-holding capacity. The higher the CEC, the more nutrients the growing medium can hold. That’s because many nutrients are positively charged cations, such as ammonium nitrogen (NH4+), potassium (K+), calcium (Ca+2) and magnesium (Mg+2). These cations are attracted to negatively charged sites on the growing medium. The CEC indicates how well the substrate can adsorb these cations.[6]
Given that most growing media are naturally infertile, CEC is an important factor to consider. It also indicates how often fertilization is needed. Because nutrient leaching occurs during irrigation, a substrate with a medium to very high CEC reduces the need for frequent fertilization.
Biological properties
The biological properties of a growing media are also important. If not sterilized, organic material can be contaminated with pathogens or weeds. Its biology may also cause nutrient immobilization or “draw-down” and impact its stability. Organic materials have various rates of decomposition, which can cause undesirable physical changes in the growing medium.
Other factors to consider
Besides the physical, chemical and biological characteristics of the growing medium, other factors to consider include the following:
- Economics: The performance of the growing medium should be weighed against its price. Growing-medium costs vary depending on the market value of the material per unit of volume, the cost of transportation and expenses related to secondary processing.
- Environmental factors: With more consumers preferring locally sourced materials, biodegradable containers and sustainable production methods, growers may want to consider the environmental impact of each growing medium. Life cycle analysis (LCA) is a comprehensive classification system that rates growing media by their environmental impact. Factors which LCA takes into account include raw material extraction, processing, manufacturing, distribution, use and disposal.
- Sterility: Selecting a growing medium that has been sterilized reduces the risk of pests and pathogens. Some growing media can be re-sterilized by heating at 180°F (~82°C) for 30 minutes or by immersing it in a 10% bleach soak for 20 minutes, followed by multiple tap water rinses.
Types of growing medium
Here is an overview of some of the available substrates, their cost, lifespan and pH (Table 1).[7]
Growing medium
Cost
Lifespan
pH
Coconut fiber
Low/medium
Short
Neutral
Expanded clay
High
Reusable
Neutral
Expanded shale
Low/medium
Reusable
Neutral
Gravel
Low
Reusable
Alkaline
Grow stones
Medium
Reusable
Alkaline
Lava rock
Low
Reusable
Neutral
Mineral wool
Medium
Renewable
Alkaline
Oasis cubes
Low
Short
Neutral
Peat
Medium
Short
Acidic
Perlite
Low
Reusable
Neutral
Pine bark
Low
Short
Acidic
Polyurethane foam
Low
Short
Neutral
Pumice
High
Reusable
Neutral
Rice hulls
Low
Short
Neutral/acidic
Rockwool
Medium
Durable
Neutral
Sand
Low
Reusable
Neutral
Sawdust
Low
Short
Acidic
Vermiculite
Medium
Reusable
Alkaline
Check back soon for our next blog post, which dives into the different types of growing mediums, their pros and cons, and their effect on yield and cannabinoid content.
Emerald Harvest Team
[1] The exception is water-culture hydroponics—such as deep-water culture (DWC), nutrient-film technique (NFT), aeroponics and so forth—where the growing medium is water.
[2] Berg Stack, Lois. 2011. “Soil and Plant Nutrition: A Gardener’s Perspective.” Last revised 2016. https://extension.umaine.edu/gardening/manual/soils/soil-and-plant-nutrition/
[3] Water-holding capacity is measured by the percentage of total pore space that is still filled with water after gravity drainage.
[4] Nemati, Reza, Jean-Pierre Fortin, Joseph Craig, and Shaye Donald. 2021. “Growing Mediums for Medical Cannabis Production in North America.” Agronomy 11(7): 1366. https://doi.org/10.3390/agronomy11071366.
[5] Even better is a pH range of 5.8 to 6.3.
[6] Sahoo, Upasana, Dinkar Gaikwad, Mahua Banerjee, G.C. Malik, and Sagar Maitra. 2023. In Advances in Agricultural Technology, 29–45. Canada: Griffon.
[7] Thakulla, Dharti, Bruce Dunn, and Bizhen Hu. 2021. “Soilless Growing Mediums.” https://extension.okstate.edu/fact-sheets/soilless-growing-mediums.html.
