Someone’s Trash is Someone’s Treasure

Lettuce plants grown in peat mixed with cattail (Typha spp.) fibers (10-40% volume/volume) at the Assiniboine Community College’s sustainable greenhouse complex. Photo courtesy: Poonam Singh
Someone’s Trash is Someone’s Treasure:  Using industrial waste/by-products as sustainable growing media for horticultural crops

By Poonam Singh.
Faculty, Horticulture Production
School of Agriculture and Environment
Assiniboine Community College, Brandon, Manitoba

Currently, in Canada, greenhouse growing of potted ornamental plants is carried out using peat, a soilless media, that is extracted from peatlands, sensitive ecosystems with the unique ability to sequester considerable amounts of carbon, and store excess precipitation. Peat extraction results in the release of high carbon dioxide emissions into the atmosphere, thus contributing to the greenhouse gas effect and resulting in climate change. For environmental conservation, peat-based growing media should be phased out, and peat-free alternatives should be developed, preferably from local biomass ingredients. Alternatives for peat such as coir are available, but, there are issues on sustainability such as releasing salt into the environment in the country of origin when washing out the substrate. Therefore, there is a need for searching potential media substitutes for peat with a lower environmental impact.

Although the peat-perlite mix is most commonly available and preferred growing media by growers. However, nowadays several other new media blends are also available for horticultural crop production. Finding the right media formulation necessitates an understanding of the composition and function of each component of the media blend. A favorable growing medium generally consists of two or more ingredients. Growers must be familiar with the positive and negative characteristics of each ingredient and their effect on the growth and health of plants.

Growing media components are either organic or inorganic. Organic components include, but are not limited to: peat moss, wood chips, sawdust, coconut coir, rice hulls, wood fiber, etc. Inorganic components include, but are not limited to: rockwool, perlite, pumice, vermiculite, expanded clay, etc.

While formulating new media blends, a sound knowledge of physical (air-filled porosity, easily available water, bulk density, total porosity, shrinkage, water retention and movement, hydrophobicity), chemical (pH, organic matter content, chemical composition, carbon:nitrogen ratio,CaCO3 content), and biological (phytotoxicity, nitrogen immobilization, rate of decomposition) properties are of utmost importance.

At Assiniboine Community College’s sustainable greenhouse complex, we are exploring wood fibers, hemp fibers (by-products of timber and agricultural industry, respectively), cattail fibers (wetland plant), and ash (from thermal conversion of plant materials) as constituents of sustainable growing media that could serve as a viable option for potted plant cultivation. Wood (pine and other trees), hemp and cattails are locally available materials in Manitoba. Numerous greenhouses burn several tonnes of flax shives (agricultural waste) as boiler fuel to heat their greenhouse, producing ash as a by-product. The project, currently in progress, is determining the bio-physio-chemical properties of these new up-cycled and locally available soilless substrates, exploring the ability to replace conventional peat-based media, and investigating its suitability for the crop cultivation in greenhouses/nurseries. The use of such local and sustainable materials as a growing substrate would have a dual benefit for the environment and support long-term sustainability goals.

So far, we have conducted several grow trials by replacing peat with hemp, wood, and cattail fibers at rates of 10 to 40 percent, and studied their effect on plant growth and health. Our preliminary results suggest that blending these new media components with peat (especially at 10-20%) created a unified mix with good physical properties. This was contrary to the standard peat and perlite blends, which did not create a unified mix and individual components remained separate even upon mixing. These new media blends provided a good physical environment for developing strong and healthy roots that were high in overall porosity leading to good drainage properties, good water retention and movement, and no splashing or separating, or floating during irrigations (common in peat-perlite mixes). These new blends were also less hydrophobic than the standard peat-perlite mix used by the horticultural industry. However, there were also challenges using these new ingredients in the mixes like the presence of weeds, problems with obtaining uniform consistency of each batch, and decomposition of the media. In addition, the nutrient (primarily nitrogen) immobilization that occurred in these substrates is a complex issue; we are in the process of analyzing and quantifying this parameter.

Most of the peat-based media used for horticultural crop production is discarded after a single use. However, there are many uses of spent growing media that are being explored by scientists and growers worldwide; some examples are its direct use as a soil improver (for gardening projects), as a bulking agent for composting, as a feedstock for biochar production, and its reuse as direct growing media for crop production. Care should be taken towards assessing salt build-up, and phytosanitary risks when up-cycling the media, and required strategies to combat these problems should be developed. Recycling and reusing peat media will result in a clear reduction in CO2 emissions and a lower impact on the climate.

If some or all of the strategies of developing peat-reduced or peat-free media become successful, the economic benefits would accrue to Canadian companies processing agricultural waste into growing media products, thus allowing them to expand into new markets. Horticultural plant industries will benefit from access to a valuable by-product/waste product from the timber/agricultural industry. Environmental benefits will result from combating global climate change by sequestering carbon and advancing local horticulture production into a sustainable growing system.

Sweet alyssum plants grown in peat mixed with hemp hurd (10-40% volume/volume) at the Assiniboine Community College’s sustainable greenhouse complex. Photo courtesy: Poonam Singh

About the author:

Dr. Poonam Singh works as a faculty researcher in the Horticultural Production and Sustainable Food Systems programs of the Assiniboine Community College, Brandon, MB. Dr. Singh’s research focuses on developing sustainable technologies for the greenhouse production of horticultural crops, evaluating new soilless media/substrates, hydroponic crop cultures, indoor farming systems, and horticultural crop physiology. Her current research project focuses on exploring new soilless growing media blends to partially or completely replace peat for improving the production of ornamental crops in greenhouses.

Published:  May 2022