Sawdust, the fine byproduct of lumber production, is often considered for use as mulch. Mulch is any material applied to the soil surface to cover it, helping to retain moisture and suppress weeds. Whether sawdust is a good choice depends heavily on how it is prepared and where it is applied.
The Critical Drawback: Nitrogen Depletion
Raw sawdust has a high carbon-to-nitrogen (C:N) ratio, often up to 500:1, compared to the ideal microbial ratio of around 24:1. This high carbon content triggers nitrogen immobilization when the sawdust begins to decompose. Soil microorganisms require nitrogen to break down the carbon-rich material and build their bodies.
To meet their needs, these microbes consume available nitrogen in the surrounding soil that plants would otherwise use. This microbial competition effectively “locks up” the nitrogen, making it temporarily unavailable for plant roots. Plants growing in soil where raw sawdust is mixed in or heavily layered can develop signs of nitrogen deficiency, such as yellowing leaves and stunted growth. Nitrogen depletion is most pronounced when the sawdust is incorporated directly into the soil, rather than remaining on the surface.
Key Benefits of Using Sawdust
Despite the challenge of nitrogen depletion, sawdust offers several practical advantages as a mulch if managed correctly. Its fine particle size allows it to create a dense layer highly effective at reducing water loss from the soil. This barrier conserves soil moisture by reducing surface evaporation, meaning less frequent watering is necessary.
The density that aids in moisture retention also provides excellent weed suppression. A sufficient layer of sawdust blocks sunlight from reaching weed seeds, preventing their germination and growth. Sawdust is often readily available and can be acquired at a low cost, making it an economical choice for covering large areas. Over time, as decomposition progresses, the sawdust also contributes to the soil’s organic matter content.
Safe Application Methods and Pre-Treatment
Successful use of sawdust mulch hinges on mitigating the risk of nitrogen immobilization. One effective method is to pre-treat the sawdust by mixing in a supplemental nitrogen source before application. This can be achieved by blending it with high-nitrogen materials, such as fresh manure, grass clippings, or a synthetic nitrogen fertilizer. Adding a small amount of nitrogen fertilizer for every cubic yard of sawdust can kickstart decomposition and satisfy microbial needs.
Another approach is to compost the sawdust until it is fully broken down, maintaining a C:N ratio near 25:1 to 30:1 in the compost pile. This allows microbes to complete the initial decomposition phase away from the garden beds, yielding a stable, nitrogen-balanced soil amendment. When applying sawdust as a mulch, it should only be used as a surface layer and not tilled into the soil. Limiting the depth to two to four inches also helps maintain air exchange.
Toxic and Treated Wood Sources to Avoid
Regardless of preparation, sawdust from certain sources should never be used in a garden or compost pile due to chemical toxicity. Sawdust from pressure-treated lumber contains harmful chemical preservatives, such as chromated copper arsenate (CCA), which can leach toxic heavy metals into the soil. Wood that has been painted, stained, or chemically sealed should also be avoided, as these surface coatings contain compounds unsafe for garden use.
Certain natural woods, particularly Black Walnut (Juglans nigra), produce a chemical called juglone, which is toxic to many plants. Sawdust from Black Walnut and related trees like Butternut and Pecan can inhibit the growth of sensitive plants, including tomatoes and potatoes. It is safest to completely exclude any sawdust from these species from the gardening environment.