On-Farm Conversion of Cannabis sativa Waste Biomass into an Organic Fertilizer by Microbial Digestion

Background/Question/Methods

Medicinal cannabis by-products (i.e., leftover stalks, leaves) are generally considered waste biomass although they are nutrient rich. On-farm recycling to reclaim these nutrients is possible but requires an efficient means to degrade the high lignin content in cannabis stalks. Thermophilic composting as an option poses significant logistic challenges to small-scale producers who want to manage their waste on-farm and results in an output that is often variable and inconsistent. A more convenient and cost-effective alternative is to introduce lignin-degrading microbes directly to cannabis waste in an anaerobic environment. Initially, four proprietary liquid microbial inoculants (MI) containing various formulations of ligninolytic bacteria and fungi were used to decompose hemp stalks over a 3-week period. Other treatments included were Effective MicroorganismsTM liquid culture and a water control. Formulations were sprayed onto hemp waste at a 1:1 v/m ratio, sealed in plastic containers, and allowed to ferment for up to 3 weeks. Each inoculum was evaluated weekly for enzyme activity and mass loss. Following decomposition, the digested hemp was cured in soil or coconut coir for 0, 1, 2, or 4 weeks, tested for the ability to suppress soilborne pathogens, and then used as an amendment in plant bioassays and hemp growth trials.

Results/Conclusions

Of the four initial MI formulations (named: MI1, MI2, etc.), MI4 outperformed all others at days 14 and 21 of the peroxidase and phenol oxidase assays, indicating a high activity of these enzymes when compared to the control (P< 0.05). Mass loss was similar across all treatment groups. Following digestion, fresh and cured material subsamples were assayed for their ability to suppress the soilborne pathogen Rhizoctonia solani. Fungal growth was suppressed by all treatment groups; however, cured biomass was significantly more effective in controlling pathogen growth than fresh biomass (P< 0.05). Mean germination rate of garden cress and red clover seedlings was greater in 2 and 4-week cured material, suggesting the cure phase is necessary to decrease phytotoxic effects. In addition, root:shoot ratios of hemp plants grown in MI4 treated and cured biomass were significantly greater than a synthetically fertilized control group, indicating plant growth promotion benefits of treated cannabis waste. Microbial digestion of cannabis biomass not only produces a more consistent output than composting but also requires less labor and reduces the recycling period from 8-12 months to roughly 5 weeks. Additional benefits of biocontrol and plant growth promotion are also obtainable with this recycling strategy.