Engineering Pine Wood Chip Aggregates for Greenhouse Substrates

Since 2010, researchers in the Horticultural Substrates Laboratory at North Carolina State University have been investigating the engineering and processing variables that influence the consistency and reproducibility of traditional and alternative organic substrate components. The most researched material/component has been processed pine trees.  These pine wood/tree components have been proven to be successful when used in greenhouse and nursery mixes, mainly as a peat/bark-extender. It has also been shown that the use of wood components specifically processed into uniform wood chips could replace perlite in peat-based greenhouse substrates. To specifically engineer wood to yield a uniform and consistently shaped/sized particle requires many specific processing steps.  The engineering of pine wood chip aggregates requires different processing equipment and methodology compared to pine wood fiber (or similar) components used as peat/bark extenders.  Pine wood chip aggregates are manufactured from pine logs that are initially chipped to yield coarse wood chips (1.0 L x 0.2 W x 1.0 H –cm). These coarse wood chips are then hammer-milled through a 6.35-mm screen to yield the final aggregate material that is 0.11 L x 0.4 W x 0.2 H –cm. In addition to the specific machine types needed to produce these fiber-less uniform particles, other variables play a major role in the consistency of the end product. Moisture content of freshly harvested and chipped trees can range from 42% to 55% depending on the time of the year when the trees were harvested.  Before hammer milling, it is important to adjust (reduce) the moisture content of the wood chips to between 35% to 40%.  Moisture is the single most important factor in the manufacturing and engineering of organic materials.  Wood chips with too little moisture (below 30%) will often shatter and splitter more when milled due to the brittleness of the wood at that low level of moisture. Since different aggregate sizes are needed for different substrate mixes (propagation, plugs, bedding plant flats, one-gallon pots, etc) the screen size used to process the wood chips can be changed to produce smaller wood chips.  So, like perlite, different grades/sizes of PWC can be constructed. Despite the traditional uses (pulp, paper, timber, fuel, etc.) and more recent uses (wood pellets, biofuels, etc.) of pine trees in the United States, it is still believed that pine wood is a reliable source of sustainable greenhouse substrate components in the foreseeable future.