Challenges and Opportunities of Cedar Shavings (2010)

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Extractives in cedar

It is true that cedar is higher in extractives like phenolics (e.g. Venner et al. 2009) than some other woods, and these can be toxic to plants, seedlings (and aquatic life if woodwaste leachate enters waters). On the other hand, the phytotoxicity can help to keep down weeds, which is why wood and bark chips can make such good mulch, and these extractives will go away.

C/N ratio

The high carbon-to-nitrogen (C/N) ratio of any carbon-rich woodwaste means that woodwaste is slow to decompose. Cedar has a C/N ratio of about 600:1; C/N ratios of 30:1 can be considered a level above which soil amendments should be managed not as N fertilizers but soil conditioners. The concept is the same behind that of backyard composting. The extractives (tropolone) in cedar may make cedar particularly resistant to decomposition (Debell et al. 1997) but again, they do break down and for practical purposes, cedar decomposes at about the same rate as other softwoods (e.g. spruce, fir) in BC and slower than hardwoods.

Management strategies

Composting with a nitrogen source (before land application). This will address concerns about phytotoxic extractives of plant wastes and the C/N ratio (Kostov et al. 1996). Speaking very generally here, use a nitrogen-rich material like poultry manure. The process is most effective with smaller wood particles and needs to be sufficiently intense. Refer to the Agricultural Composting Handbook for more information on composting http://www.agf.gov.bc.ca/resmgmt/publist/300Series/382500-0.pdf

Add nitrogen, mix the cedar-based material deeper into soil, or simply wait or use a combination of these approaches (if too much cedar was already applied). If adding nitrogen to accelerate the decrease in the C/N ratio, consider how much nitrogen will be applied. Although the woody material will initially tie up (immobilize) soil N into organic forms, that N will eventually be released (mineralized) to plant-available forms and the goal is for that release to be timed with when crops will take up nitrogen. Tilling into mineral soil will also help to break apart any clumps of material that might result in poor aeration.

References

DeBell, JD, Morrell, JJ, and Gartner, BL. 1997. Tropolone content of increment cores as an indicator of decay resistance in western redcedar. Wood and Fiber Science 29: 364-369.

Kostov, O, Tzvetkov, Y, Petkova, G, and Lynch, JM. 1996. Aerobic composting of plant wastes and their effect on the yield of ryegrass and tomatoes. Biol. Fertil. Soils 23: 20-25.

Venner, KH, Prescott, CE and Preston, CM. 2009. Leaching of nitrogen and phenolics from wood waste and co-composts used for road rehabilitation. J. Environ. Qual. 38: 281-290.

Personal communication with Caroline M Preston*, Natural Resources Canada.

*Neither the author nor Dr. Preston claims expertise in this subject area.

David Poon, PAg
Soil and Nutrient Management Specialist
BC Ministry of Agriculture and Lands
1767 Angus Campbell Road
Abbotsford, BC V3G 2M3

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