Myth Buster: Do Arborist Wood Chips Deplete Soil of Nitrogen and Other Nutrients?

Myth Buster: Do Arborist Wood Chips Deplete Soil of Nitrogen and Other Nutrients?

In areas where there are a lot of trees, many arborists and public utility companies will deliver a load of wood chips to homeowners for free. It is a common misconception that wood chips tie up soil nitrogen as they decompose. Many studies have shown that wood chip mulch increases soil nitrogen levels.  If fresh wood chips are tilled into the soil, it is possible nitrogen could be tied up during decomposition. Therefore, wood chips should be placed only at the surface around trees and shrubs (staying away from the trunk).  Despite many garden myths about arborist wood chips, they are a better mulch choice compared to other mulches. Composed of fresh bark, wood, and leaves chipped into different sizes and laid on top of the soil, wood chips are an excellent soil amendment.  Wood chips also are excellent for walkways, between raised beds and in the transition area between forest and turf.

Studies have shown that arborist wood chips are one of the best performers in terms of weed control, temperature moderation and moisture retention.  Wood chips resist compaction and vary in the size of pieces and decomposition rate, making them an excellent choice to help reduce soil compaction. They do not limit soil water infiltration and retention, reducing the need for irrigation while helping prevent runoff.  Wood chips lower soil temperature in the summer, reducing plant stress and root rot.  Arborist wood chips contain a diverse population of beneficial microbes and insects, making the soil more biodiverse and resistant to environmental disturbances and acting as an insect repellent.  Wood chips are slow decomposers and are high in lignin, suberin and tannins, which may have some role in weed suppression at the soil surface. Wood chips have been found to provide excellent weed control, likely because of light exclusion, but can also show allelopathy (depending on the wood) inhibiting seed germination. Reduced nitrogen level at the soil/mulch interface are believed to play a role in the suppressed germination of weed seeds. Arborist wood chips will not acidify the soil and rarely infect the area with pathogens. If diseased wood is used, place the wood chips away from tree roots and do not work into the soil.

Follow these tips when using arborist wood chips for best results:

• Apply wood chips before annual weeds establish
• Bare soil should be mulched as soon as possible in the spring and fall
• If weeds have germinated, mow as close to ground as possible and mulch right away.
• It is recommended to apply a thin layer of compost on the soil before applying the wood chips for trees and shrubs
• Use fresh chips with leaves and/or needles when possible to add nitrogen
• The deeper the wood chips (4-6 “), the less weeds
• Keep mulches away from tree trunks and building foundations.

References:

http://pubs.cahnrs.wsu.edu/publications/wp-content/uploads/sites/2/publications/FS160E.pdf

https://research.libraries.wsu.edu/xmlui/bitstream/handle/2376/5262/FS160E.pdf?sequence=2

https://catalog.extension.oregonstate.edu/ec1629/html

https://s3.wp.wsu.edu/uploads/sites/403/2015/03/wood-chips.pdf

https://s3.wp.wsu.edu/uploads/sites/403/2015/03/aleopathic-wood-chips.pdf

https://s3.wp.wsu.edu/uploads/sites/403/2015/03/wood-chip-pathogens.pdf

https://www.arboroperations.com.au/wp-content/uploads/2014/08/Arborist-Woodchip-as-Mulch_final-doc-.pdf