by Sally Colby
Dr. Brian Jackson, associate professor of horticulture at North Carolina State University, would answer that question “Yes.” Jackson focuses his research on the development of traditional and new substrates for potting soils for horticultural crop production.
Jackson wanted to understand, both as a scientist and as someone active in the horticulture field, where and when wood fiber was used in horticulture system. “We know a lot about the history of peats and other substrates,” he said, “but not wood – who invented it?”
Trees have been used for a variety of purposes throughout human history, most recently for mulch, insulation, erosion control and insulation. Jackson said the technology used in related industries to make those materials has led to development of wood for horticulture purposes.
Although wood fiber is new technology, Jackson said “Wood is not wood.”
“Just like peat is not peat,” he said. “There are lots of variances within these products, and one of the most exciting things about wood is that it isn’t just wood – it can be multiple things.”
Jackson said 26 tree species have been tested for substrate suitability, and he has tested 14. “Conifers tend to be best,” he said. “That relates to wood chemistry – the chemicals in the wood and strength and how quickly it breaks down. What eliminates many hardwoods is that substrate from them may have good properties initially, but break down quickly.”
Pinus taeda, or loblolly pine, has proven to be the premier wood fiber species in North America. “Through multiple trials, it has proven itself to be the best tree for grinding and processing to make substrate materials,” said Jackson. “It’s the least toxic, more malleable in processing and it’s cheap.” Jackson added that it’s a native species and there are more than 30 million acres of managed loblolly pine in the southeastern U.S.
One question Jackson hears frequently is “Isn’t this sawdust?” “Sawdust is a byproduct of various industries,” he said. “Not just forestry, but sawmills and timber facilities. None of the wood materials currently on the market for substrate are byproducts – they’re specifically made, highly-engineered materials.”
Jackson said sawdust isn’t suitable because its particles are very small and often toxic because the source is usually fresh material. He added that none of the wood fiber materials currently on the market are of a particle size anywhere close to that of sawdust, and those who have tried using sawdust have had poor results.
The three primary methods for manufacturing wood substrate are extrusion, disk refining and hammer mill. Jackson said hammer milling is the basis of the wood fiber revolution in the U.S., which began around 2003 with university research. “Hammer mills are so cheap compared to other manufacturing processes,” he said. “There’s potential for hammer mill products because these systems already exist. All bark producers and many mulch and soil producers have hammer mills on site.”
However, hammer mills have numerous variables that can influence the end product. Moisture content of feedstock running through a hammer mill matters, but isn’t an issue for extrusion or disk refining. Jackson listed wood chip size, screen size, RPMs, tip speed of the hammers and air handling systems as factors in creating consistent, reproducible products with a hammer mill. Since the hammer mill process doesn’t generate heat, there’s potential for toxicity and stability of the end product.
While Jackson doesn’t view wood as a peat competitor, there is attention to the role of wood. He said, “The more I work with these materials, the more I think wood can enhance peat and make some peat products better, and using peat with wood to do things we’ve never done before.”
Jackson explained some of the results of mixes such as wood with peat and bark. “If you take peat/perlite and mix it, the particles are still separate,” he said. “Perlite never integrates itself with peat because the two particle sizes don’t allow that. In using peat with wood fiber, they lock together. You can turn the irrigation on or water with a hand wand and nothing splashes out of the pot.”
The matrix formed by a peat/wood mix promotes good drainage and root development. “The best way to describe it is spongy,” said Jackson. “One of the things we’re trying to do is better understand how these wood fiber mixes interact with peat and other materials in pots, and more importantly, how water and plant relations change.” The bottom line in using wood relates to cost, and Jackson said research is proving that wood products can serve the same function as perlite at lower cost.
Nothing new comes without negatives, and nitrogen immobilization is a grower concern. “Wood fiber materials manufactured with high heat and a sterilization-like process tend to have less nitrogen draw down than fresh pine tree substrate made from hammer mill products,” said Jackson. “There is nitrogen tie-up or nitrogen loss that occurs with higher wood percent in a mix because these are organic materials. It’s carbonaceous material and we can’t stop it from breaking down – we can’t stop microbes from taking their share.”
Another potential issue is confusion for workers who irrigate based on sight because the surface of wood substrate looks different than traditional substrate. Wood substrate alone has a higher pH than peat or bark, so a higher wood percentage requires pH testing to determine whether lime additions should be modified.
Greenwood toxicity is a concern, and more prevalent with hammer-milled green wood materials than any of the refined wood fibers. Jackson noted that wood naturally contains phytotoxic chemicals (oils and tannins), but that’s what makes wood resilient and resistant to microbial breakdown. “This is more of an issue with greenhouse crops than with nursery crops,” he said. “Herbaceous seedlings and plugs are more sensitive than a woody liner, for example. The degree of severity of toxicity depends on how much wood is used, and the machine process.”
Coloration of wood substrate has been addressed because consumers want to see a root ball with familiar soil color, not blond. Another concern is potential competition for wood resources, primarily the pellet industry, but Jackson doesn’t see any current competition for wood resources. Wood sources, greenwood toxicity, nutrition, irrigation management and other cultural parameters are areas that will be addressed with future research.
One of Jackson’s goals this year is to learn from growers who are using wood substrate products. Projects are focused on water (hydrology of wood fiber mixes), pH, wood stability, long term systems and large containers such as the ones being adopted by the small fruit sector.
“Wood is a blank canvas,” said Jackson. “There’s still a lot that’s unknown and a lot of variability. But you make what you want – an aggregate, fine fiber, coarse fiber, more draining, more air. There’s more flexibility than compost, coconut coir or bark. It’s local, abundant and sustainable. It’ll change the role of perlite, primarily related to cost.”