Wood Chip Backyard Biology

Wood Chip Carbon & Nitrogen

The micro-organisms that decompose wood chips require nitrogen in amounts greater than are available in the wood chips alone. If additional nitrogen is not mixed with the wood chips, the micro-organisms will get the nitrogen they need from the soil, competing with plant roots for the nitrogen available in the soil. This is why bark mulch works to keep weeds down.

Wood Chips in Compost Piles

Most of the material you place in your backyard compost pile is high in nitrogen. Food waste and lawn/garden waste are the two most common ingredients of backyard compost piles, and both are high in nitrogen. Wood chips can be added to a compost pile to provide a better carbon:nitrogen ratio. Wood chips, with their rigid structure, also enhance the flow of air through the compost since they are less prone to compact. This is a good thing.

Wood Chips as a Soil Amendment

You can use wood chips to add organic material to soil. The process will take four or more years. You will need to add nitrogen along with the wood chips to facilitate decomposition of the wood chips without depleting the available nitrogen in the soil.

For each 10-by-10 area:
  • Year One: Add one pound of ammonium sulfate for each one inch layer of wood chips.
  • Year Two: Add one half pound of ammonium sulfate for each one inch layer of wood chips.
  • Year Three: Add one quarter pound of ammonium sulfate for each one inch layer of wood chips.
  • Year Four: Add one eighth pound of ammonium sulfate for each one inch layer of wood chips.
In other words, start with one pound ammonium sulfate for each 100 square feet the first year, and decrease that amount by half in each of the following years, until the wood chips are fully decomposed.

Wood Chips & Soil pH

Wood chips will lower soil pH, making it more acid. That is a good thing for acid loving plants like evergreen trees and shrubs, but might be bad for other plant species. In areas where soil is already neutral or acid, the addition of wood chips can result in excessively acid soil.

Wood Chips Miscellaneous

A layer of wood chips will help hold in moisture once it reaches the soil, but in low moisture areas it can prevent dry site adapted plants with extensive surface roots from getting moisture through dew and light rains.

Depending on the type of trees, wood chips can also release tannins that may be toxic to some of the soil organisms.


For the reasons stated above, you should not spread wood chips directly on, or till them into, a vegetable garden or other areas where you want seeds to germinate. Wood chips are great around established shrubs where denitrification of the surface soil will inhibit germination of weed and grass seeds.


Next time you drive by our office take a look at the shrub bed along Grant Avenue. It is mulched with wood chips from our local brush site. Note also the tax savings from reduced maintenance. No need to weed for the last two years.

Compost, Wood Chips & Carbon:Nitrogen Ratios

Wood chips have an average C:N ratio around 600:1, but only the outer surface of the wood chip is really available to react with the microbes in the compost pile. In practice only about 1/3 of the wood chip will decompose in a 3 - 6 month composting period. So, when determining a compost mix, only count 1/3 of the stated C:N ratio. Wood chips, with an average C:N ratio of 600:1 would be mixed in as though they had a C:N ratio of 200:1.

Later, when you screen out the partially decomposed wood chips they can be added to the next batch of compost. In fact, adding the partially decomposed wood chips to a new mix will help inoculate the new mix with the microbes necessary to activate the compost pile.