© 2024 MJH Life Sciences™ , Cannabis Science and Technology . All rights reserved.
Learn about the major choices of substrate, their benefits and disadvantages, the ways watering volume and frequency are affected by substrate choice, and more.
Learn the major choices of substrate and their benefits and disadvantages, the ways watering volume and frequency are affected by substrate choice, the importance of watering rate and leach percentage and how to calculate them, and implications for water quality, recapture, and reuse.
Growing media impacts the effectiveness of watering for plants, a critical process influencing transpiration and uptake of nutrients. Since the dawn of commercially licensed cannabis cultivation, growers have begun to transition from larger substrate volumes and less efficient watering methods to smaller container sizes and greater integration of more precise irrigation techniques. With high-frequency/low volume irrigation approaches, water efficiency and productivity can be optimized. No matter how water is applied, grower methods can be optimized, and opportunities to increase efficiency of water use across the industry are considerable.
Substrate is the base on which plants grow. In commercial food production, soil is the most common substrate. For cannabis, growers often use other media, including rock wool, coconut coir, peat mixed with amendments, or water culture. When selecting a substrate, consider properties such as water retention, recommended irrigation approach, watering rate, leachate percentage, commonly used amendments, and implications for water recapture and reuse to find the right growing media and fertigation methodology.
The most common substrates used for indoor and greenhouse cultivation are described in order of prevalence of use in the legal cannabis market below. A more detailed overview is provided in Cannabis H2O: Water Use and Sustainability in Cultivation, Resource Innovation Institute’s (RII) latest collaborative report with New Frontier Data and the Berkeley Cannabis Research Center (1).
Rock wool is fibrous material made from molten rock spun into fibers and formed into plugs, blocks, and slabs of various form factors. The sterile nature of production under extreme temperatures keeps the substrate clean and free of pests and pathogens. It is inert; it does not bind any applied water and nutrition. For this reason, it must be frequently irrigated with nutrient solution. Rock wool can deliver very high plant performance, but can require closer monitoring to ensure balanced production. If rock wool irrigation systems malfunction, plants can more quickly and easily experience drought stress or even permanent wilting damage or death.
Coconut coir is compostable and can be sustainably produced. However, during the manufacturing process coir requires significant volumes of water to buffer the substrate (remove unwanted ions that adsorb to the cation exchange sites). If sodium and other chemicals are not washed from the coir they can negatively impact growth performance. The chemical properties of properly composted, washed, and buffered coir provide an optimal pH range for cannabis cultivation, while having low electrical conductivity, sodium, and potassium content.
Peat mixes are a very wide-ranging category. Peat-based mixes were historically very common in both unregulated and commercial cannabis production. Coir and rock wool have become more popular as they offer growers greater precision when managing substrate.
The impact of substrate on the frequency and volume of irrigation events is affected by the ratio of substrate volume to plant biomass. A grower might use a large volume of peat that only demands low-frequency irrigation but requires larger volumes of water per event to reach uniform saturation. A different grower may use small volumes of a different substrate that necessitate higher-frequency irrigation events at lower volumes due to the overall lower water-holding capacity.
Growers considering sustainable cultivation approaches should acknowledge the downstream implications of substrate selection on water quality and employ best practices for water management to monitor watering rates, measure leachate percentage, analyze runoff, and support effective treatment and reuse.
Substrate choice is an important factor influencing the treatment and rebalancing activities of reclamation activities. Naturally derived organic substrates such as peat or coir contain carbon and tend to produce an organic-laden leachate that contains a complex mixture of humic acids, tannins, and other polymeric organic compounds. “From a reuse and treatment perspective, these compounds contribute to an increased load on microbial control processes such as oxidant demand and UV disinfection, as well as carbon and sediment filtration processes,” notes RII member Taylor Robinson, Research & Development Manager and Chief Chemist at Silver Bullet Water Treatment. “Organic compounds from naturally derived substrates can also provide a carbon food source for microbial organisms (beneficial, benign, or pathogenic). While certainly not insurmountable, there may be increased cost and infrastructure requirements to ensure proper water quality when managing organic leachate.”
Organic leachate from rock wool substrates is not usually a concern, but because rock wool is a naturally mined product, it may contain naturally occurring metalloids and introduce chemicals into leachate. Commonly, flushing is employed by manufacturers during quality control to remove soluble metals, but soluble arsenic has been observed by Silver Bullet leaching from unused rock wool substrates. Since cannabis is a bio-accumulator, these compounds have a high potential to be stored in plant tissue throughout the cultivation cycle. Testing “hot” for heavy metals is not a nice surprise for a grower, and in some regulated regions, may deem a crop unfit for consumption or sale.
When using deep water culture (DWC) approaches, “well managed hydroponic systems can be purpose-built to reclaim and reuse water, but also run the risk of microbial or pathogen outbreaks,” noted Robinson. Many DWC and aeroponic installations have large groupings of plants with root zones directly connected via piping and the nutrient stream. If the solution is not tested and actively managed, extensive disease outbreaks may be impossible to correct before substantial crop loss occurs.
Growers considering sustainable cultivation approaches should acknowledge the downstream implications of substrate selection on water quality and employ best practices for water management to monitor watering rates, measure leachate percentage, analyze runoff, and support effective treatment and reuse.
Substrate selection is just one of the activities involved in a comprehensive water management plan that addresses fertigation approach, estimation and monitoring of watering rate and leachate percentage, recapture, water quality testing, and rebalancing for recirculation and reuse. Keep an eye out for additional tips for water management in Resource Innovation Institute’s next peer-reviewed Best Practices Guide on Automation & Controls for Cannabis Producers coming this summer. As a non-profit organization, RII brings together producers, utilities, designers, builders, and technology providers to help cultivators successfully adopt efficient techniques and ensure resilient harvests into the future.
References
About the Columnist
GRETCHEN SCHIMELPFENIG, PE, is the Technical & Operations Director of RII and facilitates the organization’s Technical Advisory Council, manages the PowerScore resource benchmarking platform, and delivers educational curriculum to the market. She authored the RII Best Practices Guides on HVAC and LED Lighting for Cannabis Cultivation and Controlled Environment Agriculture. Gretchen has a B.S. in Architectural Engineering from the University of Wyoming, an M.S. in Civil Engineering from Stanford University and is a licensed Civil Professional Engineer in California and Vermont. She has experience commissioning HVAC, lighting, and controls systems designs and installations in commercial and industrial buildings, including greenhouses and indoor growing environments used for academic research on a diverse range of cultivars. She also has implemented energy efficiency programs for commercial customers and launched equipment incentive offerings for growers in Vermont.
About the Guest Columnist
TAYLOR ROBINSON is the Director of R&D/Analytics for Silver Bullet Water Treatment and Founder of Rimrock Analytical with expertise in molecular and cell biology, general water chemistry, microbiology, and water treatment and reuse processes. Silver Bullet Water Treatment is a member of RII and active in their Technical Advisory Council’s Water Working Group. For the past 6 years, Taylor has led, conceptualized, organized, and completed numerous Silver Bullet research and development projects related to the cannabis, leafy greens, livestock, commercial cooling, aquaculture, and data center industries.
How to Cite this Article
G. Schimelpfenig and T. Robinson Cannabis Science and Technology 4(4), 13-18 (2021).