Can I use sawdust as a source of organic matter?
Do you have the opportunity to get free sawdust produced near you? Are you wondering if this source of organic matter is compatible with vegetable crops in living soil? This article explains the impact of sawdust on the nitrogen and carbon cycles as well as feedback from market gardeners in LSV.
What type of sawdust to use?
You must be careful to only take untreated sawdust, otherwise you risk seriously damaging the mycelial network present in your soil. OSB, plywood, and laminated wood are also to be avoided.
You should also pay attention to the origin of the wood. Indeed, if the wood is exotic, the soil life will not be able to digest it. If your sawdust is mainly composed of resinous species, it should be mixed with compost from green waste to avoid compaction.
Mixing sawdust to avoid nitrogen hunger?
With a C/N ratio ranging from 100 to 500, sawdust is very rich in carbon and compared to fresh green waste shreddings, there are few fungi and bacteria. Spreading sawdust will therefore cause very slow mineralization and a nitrogen hunger that will impact the growth of your plants and your yields.
Should sawdust be composted before spreading? To answer this question, a market gardener from the LSV network stored 300 m³ of pure oak and beech sawdust (no bark), in a 1 m high windrow. According to his observations, there is no composting of the windrow which ends up covered with couch grass if not maintained. Only the 15-20 cm of sawdust in contact with the soil eventually darken after some time. The market gardener then spread the windrow and observed a strong nitrogen hunger because even the fava beans were weak!
Another proposed solution to avoid nitrogen hunger would be to mix sawdust with a nitrogen input such as manure, feathers, low-straw manure, slurry, to lower the C/N ratio. The same market gardener mixed raw sawdust with shredded green waste as an initial input in autumn on a plot cultivated with onion the following spring. After a double fertilization input, the market gardener observed nitrogen hunger.
Conclusion
According to feedback, spreading sawdust, even mixed, should be avoided on beds intended to produce vegetables soon. However, it may be interesting to mulch stepping stones with sawdust as it can act as a barrier against slugs and snails.
