Wood chips: fact and opinion

17 March 2010

In The Garden With the Fluvanna Master Gardeners
By Irene C. Burke - Fluvanna Master Gardener
Fluvanna County Extension Office: 434.591.1950

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Opinions

Unsupported opinions say: Fresh wood chips from live trees leach nitrogen from the soil and must be accompanied by soil amendments when used on plantings; and fresh wood chips should be composted before being used in the garden.

Leaching

The World Book Encyclopedia defines leach as to dissolve out soluble parts from (ashes, ores, or other matter) by running water or other liquid through slowly.

Leaching Wood Chips

Wood chips, which are not liquid, cannot and do not leach anything. But water percolating through fresh wood chips from the black walnut tree (Juglans nigra L.), for example, can wash away or leach some of the soluble biochemical materials the tree makes to kill its competitors. These biochemicals (juglone) can inhibit the growth of nearby plants, especially seeds and seedlings. This is not true of cedar, pine or other wood chips.

Nitrogen Excess

Excess nitrogen in the form of nitrate (NO3-) is readily leached or washed through sandy soil, less so through clay. It comes mostly from manures and decaying organic matter. Decaying wood chip mulch on the other hand, slowly releases nitrogen into the soil.

Nitrogen Shortage

The concern is that wood chips tie-up or bind nitrogen, supposedly making it unavailable to plants and retarding growth.

Research

Research shows that wood chips used as mulch do not bind nitrogen. Linda Chalker-Scott, Ph.D., Extension Horticulturist and Associate Professor with Washington State University has, studied the relationship between wood chips and nitrogen deficiency in a replicated research experiment. The studies found that nitrogen levels are low at the point where wood chip meets soil.

Competition for nitrogen

Dr. Chalker-Scott suggests that seeds and seedlings as well as the microbes that decompose wood chips compete with each other for the wood chips slowly released nitrogen. Plant roots extending below that competition zone, however, readily access available nitrogen.

Shallow roots

Therefore, newly planted vegetable or any other annuals seeds and seedlings with shallow roots should not be planted with a wood chip mulch nor any other mulch with low-nitrogen attributes like shredded bark, fresh grass clippings and sawdust. Nor should fresh wood chips be combined with soil amendments or tilled into the soil. The nitrogen competition and resulting depletion would continue at a deeper level until microbial action is completed and the resulting nutrients released.

Composting Wood Chips

Wood chips need not be composted. As mulch, their primary function is to suppress weed seed germination, which they do quite well. Additionally, wood chips reduce compaction, moderate soil temperature, hold water, release nutrients slowly as they decompose, prevent erosion, and discourage nitrogen-seeking insects such as termites.

If youre still concerned about wood chips, use it as the top layer in a sandwich of soil with compost in the middle; this is truly unnecessary because the in-place composting process provides the same sandwich over time. Simply refresh the top layer with two or three inches of chips each year.

Facts

Wood chips do not bind or leach nitrogen from the soil. Wood chips prevent weed seed germination and growth. Wood chips are unattractive to termites. Wood chips provide a slow release of valuable nutrients including nitrogen.

Tip of the Week

Even though our population has increased by 25 percent, America's public supply of fresh water remains stable at 11percent of the total amount, according to the Dept. of the Interior's 10/29/09 report of Estimated Use of Water for the U.S. in 2005. The best time to improve water conservation is when it is plentiful. Install rain barrels; mulch bare soil and grow drought-tolerant plants.