Composting wood chips is a productive method of producing organic matter to improve and maintain soil composition for vegetable and ornamental growing. Wood chip composting does present challenges due to the size of the substance, which has to be broken down; the nitrogen demands of decomposing wood; and fire hazards of large wood chip piles. Overcoming these challenges creates a beneficial compost product at the home or commercial scale.
Decomposing wood consumes a significant portion of nitrogen, which it absorbs from its environment. Insufficiently composted wood chips will remove nitrogen from the surrounding soil, depriving plants of necessary nutrients. Wood chips also decompose slowly compared to vegetable scraps, grass clippings and other small green material, due partly to the size and cellulose content of wood chips, and partly to the nitrogen demand. Both the nitrogen and timing issues may be addressed by mixing wood chips with a high-nitrogen substance such as poultry manure. Chicken, turkey, and duck manure is too "hot"---high in nitrogen---to place directly on food or ornamental crops on its own. Mixed with wood chips, both the chips and the manure break down quickly into a balanced useful compost. The Food and Fertilizer Technology Center recommends a 50-50 mix by volume of wood chips and poultry manure.
Large stacks of wood chips, such as those accumulated in commercial composting operations, present a significant fire hazard. If the heat generated by decomposition cannot escape the centre of the pile, internal smouldering begins, which may go undetected for quite some time until oxygen enters the smouldering area, such as when the pile is ploughed or dug open for use and leads to rapid, open burning. Follow local fire marshal guidelines to keep commercial wood chip piles at appropriate sizes, with adequate surface-area-to-volume ratios and sufficient moisture content and airflow to avoid spontaneous combustion.
When composting wood chips from commercial sources, ensure that none of the chips comes from pressure-treated wood products. Most commercial wood chips available for composting will be from chipped brush, such as that cleared by utility companies from along their lines, or by products from raw timber processing. However, chips that include construction debris or sawdust from finished lumber may contain pressure-treated wood, which should not be used in soil for crops intended for human consumption because of the hazardous nature of the chemicals used in pressure-treatment. Additionally, some varieties of raw wood should also be avoided in compost intended for gardening. Black walnut and its close relatives emit a substance that inhibits growth of other plants in its vicinity, which can have negative effects in a flower or vegetable bed but may be ideal along pathways, underneath swingsets, or in other locations where plant growth is undesireable.