While Britney Spears may eventually come around on the topic in her song “Toxic,” most plants never say “I think I’m ready now” when it comes to toxic growing media.
Phytotoxicity is something ag engineers and researchers are combatting as they work toward finding alternatives to peat, however. Discussing dealing with toxicity in fresh organic substrates as an AmericanHort HortScholar at the most recent Cultivate event was Jack Bobo, a North Carolina State University Ph.D. student, in the Horticultural Substrates Lab with Dr. Brian Jackson.
His current research focuses on mitigating the toxicity linked to engineered wood fiber substrates and cultivating annual crops in peat-free and peat-reduced growing media within greenhouse environments.
Growers know peat is a P.E.A.T. (performance evaluation assessment tool). It is the golden standard of growing media, with about 80% total porosity, about 60% container capacity and about 20% air space. It has low bulk density, excellent water and nutrient holding capacity, is free of pests and pathogens and is non-toxic.
So why even look at other materials? Bobo explained there are European peat bans and restrictions already in place, there are supply chain issues and costs and there are environmental concerns. While technically a renewable resource, it’s one humans are using faster than it can naturally regenerate. North America has not reached these peat limitations – yet.
Other materials that show promise as peat replacements or supplements include coconut, wood, miscanthus, pine bark, date palm, banana, biochar, rice hulls, sugarcane, hemp fiber, peanut hulls, olive husks and spent mushroom substrate. The keys to their success are detoxification and manipulation.
Bobo is studying the soluble chemistries of various pine trees to see which work best. He makes “teas” of them for tomato plants to grow in (with fertilizer), looking recently at Pinus ponderosa (Ponderosa pine), P. lambertiana (sugar pine), Calocedrus decurrens (California incense cedar), Pseudotsuga menziesii (Douglas fir), Abies concolor (white/concolor fir) and a “trail mix” of everything mixed together.
He noted all these tree species are softwoods because they are easier to break down due to their lignin and hemicellulose content. Loblolly pine (P. taeda) is so prevalent in current growing media because in the Southeast, it’s very easy to access.
Bobo also looked at these woods’ volatile chemistries, noting, “If you can smell it, the roots can taste it.” For example, aromatic pine needles didn’t even allow the test tomatoes to germinate.
He preconditions every substrate by composting, aging, oven-drying, steaming, cold-washing, hot-washing and then adding in activated carbon. Activated carbon works well in wood fiber for volatile organic compound recovery, but work is ongoing with other media.
Once the substrates were screened for toxicity, organic materials could be manipulated to mimic peat’s physical properties via processing techniques, particle size and additional additives (clay, sand, hydrogels, etc.).
In the research presented at Cultivate, the Ponderosa pine and California incense cedar “teas” performed the best, but still not nearly as well as the experiment’s control, which used peat, deionized water and fertilizer.
“Peat is here to stay – however, more growers and substrate producers are looking at using less,” Bobo concluded. “For non-peat substrates, the greatest barrier to entry is toxicity. There are a few promising strategies for toxicity mitigation, though. Once we move past that, we can look at manipulation of physical properties.”
by Courtney Llewellyn
