Agriculture Reference
In-Depth Information
Bark Mulch, Wood Chips and Sawdust
Bark mulch and wood chips are some of the most commonly used mulching materials in
the garden. They can be useful or harmful, depending on the source and the use. They can
be found in bags or bulk bins at the garden center. They satisfy many of our mulching
goals, but unfortunately, they have a couple of potential problems making them one of the
mulch types I don't generally use.
Bark contains oils that repel water and wood chips can become hydrophobic, preventing
water from getting through to the soil. Bark in particular is low in nutrients, so it doesn't
improve soil fertility as much as other mulches. Conifer bark such as cedar and fir can be
high in toxins, as conifers have evolved a strong arsenal of compounds that are their first
line of defense against insects and diseases. These chemicals can also cause toxicity prob-
lems in the soil.
Wood, including both chips and sawdust, is very high in carbon and very low in nitro-
gen. This means microbes have to pull much of the available nitrogen from the surrounding
area in order to break down the wood, which can end up causing a nitrogen deficiency in
your plants. If you first include this wood as the carbon source in your compost pile, this
situation can be avoided. Otherwise, you may need to bring in a source of nitrogen regu-
larly.
However, if you have shrubs and trees that like a fungal environment, wood chips will
promote that, especially if you don't overwater. Fungi still need water, but not as much as
bacteria. If they are coarse wood chips and if you put on just a couple of inches and leave
them on the surface without digging them in, the nitrogen shortage may not be a big prob-
lem.
Wood chips from the non-bark part of the tree and even sawdust can have some good nu-
trition in them, so they're a great addition to composts where they can be properly mixed
with nitrogen materials. As long as they're from non-treated wood, they can even be used
lightly in gardens, especially if mixed with leaves; but on their own, they have the same
high carbon problem. Sawdust breaks down more quickly and can decrease the amount of
air and water that enters the soil if you use too much as a mulch.
All of this being said, if you have access to a large amount of local bark mulch or wood
chips at a good price (or free), and you have room to store them, feel free to take them and
compost them or pile them and let them compost for a few years. They can become good
organic matter. I often work with what I have and try to use local materials. Also, both bark
mulch and wood chips can definitely be used in paths, where I am not trying to create a
balanced soil environment. They can do a decent job of controlling weeds there if you ap-
ply a few inches and keep this mulch topped up.
