The Do’s and Don’ts of Using Sawdust for Composting, Understanding Risks and Environmental Impact

The amazing thing about composting is that we can take waste materials that were once living and recycle them to enrich the soil and provide nutrients for plant growth. Among the diverse range of materials that can be composted, sawdust often finds its way into the mix as a readily available material. Using sawdust in composting provides a sustainable solution for managing wood waste generated from various activities such as woodworking, carpentry, and tree trimming.

However, not all sawdust is created equal, and there are important considerations to bear in mind when sourcing sawdust for use as a composting ingredient. Depending on the source of the sawdust, it may potentially contain toxic chemical contaminants that could compromise the quality of your compost and pose risks to both the environment and human health.

In this article, we’ll look at the nuances of using sawdust for composting, learn which types of sawdust to avoid, and why the contaminants they contain are harmful to the environment and to living organisms.

But before we do that, let’s quickly look at the basics of composting to understand where sawdust fits into the composting equation.

Composting Basics, Using the Right Mix of Ingredients

To make compost, we need to use a blend of materials that achieve the ideal carbon to nitrogen ratio (C:N) typically ranging from 25:1 to 30:1, which is based on the nutritional needs of the soil microorganisms responsible for breaking down organic matter in the compost pile. This ratio provides an optimal environment for microbial activity and efficient decomposition, leading to the production of high-quality compost.

• Materials that are high in carbon are referred to as ‘browns’. These typically are dry, ‘brown’ materials, and include compost materials such as sawdust, cardboard, dried leaves, straw, branches and other woody or fibrous materials, which all rot down very slowly.
• Materials that are high in nitrogen are referred to as ‘greens’. These typically are moist, green materials, and include compost materials such as lawn clippings, leafy green garden waste, fruit and vegetable scraps, as well animal manures, which all rot down very quickly.

The easiest way to achieve this ideal C:N ratio of 25-30:1 is by combining one bucket of greens leafy materials or animal manures with two buckets of brown, dry materials such as wood chips, straw, cardboard, or sawdust for example.

For more information on composting materials, see the following articles:

• Carbon-Nitrogen (C:N) Ratios of Common Composting Materials
• What Materials Can You Put Into Your Compost Bin And What Not To Compost
• The Definitive Guide On How To Compost Everything From The Garden And Kitchen

Understanding Sawdust in Composting

Sawdust, a byproduct of woodworking, can be an excellent addition to compost piles due to its high carbon content. Generally, sawdust has a C:N ratio ranging from about 200:1 to 500:1 or even higher. This high carbon content makes sawdust an excellent source of browns in composting, helping to balance the nitrogen-rich greens such as kitchen scraps or grass clippings.

As it breaks down, sawdust releases nutrients into the soil, improving its structure and fertility. Sawdust is a valuable addition to compost because it’s produced from woody materials, and is finely broken up which enhances the rate of decomposition.

How Do Composted Woody Materials Such as Sawdust Improve Soils?

Woody plants differ from non-woody (herbaceous) plants in that they contain woody tissue for structural support, as their name suggests.

All woody plant tissue contains the compound lignin, a complex organic polymer found in the cell walls of many plants, especially trees. It provides rigidity and strength to the plant’s tissues, helping them remain upright and resistant to bending or collapsing under their own weight.

While lignin is abundant in the cell walls of woody plants such as trees and shrubs, it is either absent or present in minimal amounts in non-woody plants such as grasses, herbs, and flowering plants. These non-woody plants rely on alternative structural components, such as cellulose and hemicellulose, to provide support to their cell walls.

What is interesting about lignin is that it’s very resistant to rapid degradation, and this property plays a crucial role in the formation of stable humus, the dark, organic component of soil that remains after the decomposition of plant and animal materials.

As lignin-rich materials decompose, they contribute to the formation of humus by providing long-lasting organic compounds that resist further decay and breakdown. Once it’s part of the humus, lignin acts like a binding agent or glue, holding together and stabilising the organic matter in the soil, promoting soil aggregation. This stable humus improves soil structure, water retention, nutrient availability, and overall soil fertility. Additionally, humus plays a crucial role in carbon sequestration, helping to mitigate climate change by storing carbon in the soil for long periods.

As a composting material, sawdust decomposes very slowly due to its high C:N ratio (high carbon content), and needs to be combined with nitrogen-rich materials, such as kitchen scraps, grass clippings, coffee grounds, blood & bone fertiliser or manure to break down.

A handy tip when composting sawdust is to ensure proper aeration and moisture levels in the compost pile to facilitate effective decomposition.

Risks of Contaminated Sawdust

While sawdust is generally regarded as a beneficial composting material, caution must be exercised when sourcing it. Contaminated sawdust, such as that infused with dried epoxy resin glue containing Bisphenol A (BPA), or derived from synthetic woods like particleboard, medium-density fiberboard (MDF) or interior-grade plywood, poses potential hazards to both the composting process and the environment.

What is Particleboard?

Particleboard, also known as chipboard, is a type of engineered wood product commonly used in furniture, cabinetry, and construction. It’s made from wood particles, such as wood chips, shavings, and sawdust, that are bonded together using adhesive resins, typically urea-formaldehyde or phenol-formaldehyde, and pressed under temperature and pressure to form man-made wood panels.

What is Medium-Density Fiberboard (MDF)?

Medium-Density Fiberboard (MDF) is another type of engineered wood product used in furniture, cabinetry, molding, and interior trim. Unlike particleboard, which uses wood particles, MDF is made from fine wood fibers from various hardwood and softwood species that are produced through a mechanical pulping process, and bonded together with adhesive resins, typically urea-formaldehyde or phenol-formaldehyde, to produce the final product.

What is Plywood?

Plywood is a type of engineered wood product made from thin layers of wood veneer, called plies or layers, that are glued together with adjacent layers having their wood grain rotated up to 90 degrees to one another. This cross-graining technique gives plywood its strength and dimensional stability.

The layers are typically glued together on both sides using urea-formaldehyde or phenol-formaldehyde resin, then placed into a hydraulic press to undergo high pressure and temperature treatment. This compression process activates the adhesive, bonding the layers together into a solid panel.

What is Hardboard (Masonite)?

Masonite, also known as hardboard, is a type of engineered wood product made from wood fibers that have been compressed under high pressure and temperature. It comes in thin sheets that are dark brown in colour, making it easily identifiable.

Masonite is stronger, more durable and rigid than MDF, and is used for a variety of applications such as doors, furniture, paneling, flooring and decorative elements.

Unlike MDF (Medium-Density Fiberboard), which uses wood fibers bonded with adhesive resins, Masonite is typically made without added binders, glues or resins. Instead, it relies on the natural lignin present in the wood fibers to bind them together under pressure and heat. However, depending on the specific manufacturing process and any surface treatments applied to the hardboard, there might be trace amounts of adhesives or other chemicals present.

Sme common chemicals found in the surface treatments of hardboard may include:

• Stains, which may contain pigments, dyes, and solvents.
• Sealers which may contain acrylics, urethanes, or other polymers that form a protective film over the surface.
• Lacquers which are fast-drying coatings that provide a glossy or matte finish to hardboard surfaces and typically contain solvents, resins, and additives to improve flow and leveling.
• Varnishes which are similar to lacquers and often contain resins, oils, and solvents, but tend to have a thicker consistency and may offer enhanced durability and UV protection.
• Waxes which may be either natural waxes such as beeswax or synthetic waxes, can be applied as a to enhance their luster and provide additional protection against moisture and wear.
• Fire Retardant chemicals such as ammonium polyphosphate or borates may be used as a treatment in some applications where fire resistance is required.

What is Treated Timber?

Treated timber, such as CCA (chromated copper arsenate) treated timber, is wood that has been chemically treated to enhance its resistance to decay, insects, and other environmental factors. CCA is a preservative commonly used in the treatment of timber.

Treated timbers are made from softwood species such as pine, spruce, or fir, as these are more susceptible to decay and insect damage.

The timber is placed in a pressure vessel or cylinder filled with the preservative treatment solution, which is composed of copper, chromium, and arsenic salts dissolved in water. The air is then removed from the pressure vessel to produce a vacuum, this sucks the air from the wood fibres and forces the CCA solution into the wood under pressure.

The timber is then allowed to cure or dry, allowing the preservatives to penetrate and bond with the wood fibres, which increases the resistance to decay, fungal growth, termites, and other wood-damaging organisms, extending its service life and making it suitable for outdoor applications such as decks, fences, utility poles, and landscaping structures.

However, due to health and environmental concerns associated with arsenic, CCA-treated timber has been phased out in many countries in favour of alternative treatments such as ACQ (alkaline copper quaternary) or copper azole.

The Health and Environmental Risks of Composting Contaminated Sawdust

Now that we’ve discussed how these man-made timbers are manufactured, let’s look at the health concerns and environmental risks presented by the chemical compounds that they contain, and why we shouldn’t use sawdust from these timbers in our compost.

BPA Contamination

Bisphenol A (BPA), a chemical commonly found in epoxy resin glues for joining timber in woodworking, is a known endocrine disruptor. It’s the same chemical that created worldwide heath concerns in 2008, when it was discovered that food containers made with BPA plastics and metal drink bottles and cans lined with BPA leached the chemical into foods and beverages, posing a major health risk, which resulted in its use being banned for these purposes.

The chemical compound Bisphenol A is a synthetic estrogen (xenoestrogen), so it mimics the hormone estrogen and disrupts the endocrine (hormonal) system, causing hormonal imbalances. In animal studies, researchers have linked developmental exposure to various serious health concerns, such as reproductive harm, increased cancer susceptibility, and abnormalities in brain development and fat metabolism.

Typically, two-part epoxy glues are just a combination of a BPA-containing resin and a hardener. For example, the chemical composition of Selleys Araldite Super Strength Epoxy Adhesive, (sold here in Australia), sourced from the manufacturers SDS (safety data sheet), is listed below:

Part A – Resin

• Bisphenol-A epoxy resin 25068-38-6 >60 % (w/w)
• Bisphenol F epoxy resin 9003-36-5 10-30 % (w/w)

PART B – Hardener

• 1,2-Ethanediamine, N,N’-bis(2-aminoethyl)- 112-24-3 <10 % (w/w)
• 1,3-Propanediamine, N’-(3-aminopropyl)-N,N-dimethyl- 10563-29-8 <10 % (w/w)

When introduced into compost piles, BPA can leach into the soil, posing risks to soil microorganisms, plants, and ultimately, human health through potential food chain contamination. Research has linked BPA exposure to reproductive issues, developmental abnormalities, and hormonal imbalances in wildlife and humans alike.

Urea-Formaldehyde Glue in Synthetic Woods

Particleboard, MDF and plywood, commonly used in furniture manufacturing, often contain urea-formaldehyde (UF) glue as a binding agent. During composting, the breakdown of these synthetic woods can release formaldehyde into the environment. Formaldehyde is classified as a probable human carcinogen by the International Agency for Research on Cancer (IARC), and since it’s the same chemical used to preserve biological specimens in jars, and to embalm human remains to delay decomposition, its presence in compost can inhibit microbial activity, impeding the decomposition process and compromising soil health.

Treated Timbers and CCA Preservatives

Another concern arises with sawdust sourced from treated timbers, particularly those treated with chromated copper arsenate (CCA) preservatives. CCA-treated wood contains arsenic, chromium, and copper, which can leach into the soil over time, posing significant risks to both terrestrial and aquatic ecosystems. Arsenic, in particular, is a potent toxin that can accumulate in plants and contaminate groundwater, presenting a hazard to human health and biodiversity.

Environmental Impact of BPA, Urea-Formaldehyde, and CCA

The introduction of BPA, urea formaldehyde, and CCA into soil ecosystems can have far-reaching consequences. These chemicals can persist in the environment, accumulating in soils and water bodies, where they may bioaccumulate in organisms and enter the food chain. Prolonged exposure to these contaminants can disrupt ecological balance, impairing the health of plants, animals, and humans alike.

Burning these treated timbers and their sawdust is NOT a safe way of disposing of them. Combustion of synthetic timber creates a whole new range of health and environmental hazards – for an explanation of the science and detailed information about the dangers, see article: Health and Environmental Dangers of Burning CCA-Treated and Synthetic Timber

Best Practices for Safe Composting with Sawdust

To reduce the risks associated with sawdust contamination:

Avoid Treated or Composite Woods – Steer clear of sawdust derived from treated wood, particleboard, MDF, hardboard, plywood or any other composite or man-made materials. Only use untreated, natural wood sawdust from sources such as sawmills or woodworking shops.

Know the Source – Ensure that the sawdust you receive from others for composting use comes from clean, untreated wood sources free from other toxic or undesirable contaminants such as paints, stains, varnishes, finishes, fillers or plastic coatings (such as melamine). These don’t belong in the soil either!

Educate Others – Spread awareness among fellow composters about the potential risks associated with certain types of sawdust and the importance of making informed choices when selecting compost ingredients. Share this article perhaps!

I’ve seen many community gardens receiving a regular supply of sawdust from woodworking businesses or hobby woodworker friends. At the risk of repeating myself here, let them know you can only use clean sawdust produced from natural woods for your composting needs, and that it shouldn’t contain:

• contamination from glues, paints and varnishes, or timber coatings such as melamine.
• any other sawdust from chemically treated outdoor timbers such as treated pine, or synthetic timbers such as particleboard, MDF, plywood, hardboard (masonite) or any other composite timbers.

It’s important to begin educating the people providing sawdust for composting to gardeners to keep the sawdust made from clean, natural wood separate from any that contains these harmful contaminants.

Use Sawdust in Moderation – While sawdust can be a valuable addition to your compost pile, it’s essential to use it in moderation, and mix it with a range of other organic materials. Aim for a balanced mix of carbon-rich (browns) and nitrogen-rich (greens) materials to optimise composting conditions.

• Avoid putting a thick layer of sawdust into compost bins, as this can create a fairly airtight layer which can impede the air flow that’s vital to efficient aerobic composting.
• Always spread sawdust thinly when putting it into a compost pile, and for even faster breakdown, mix it in a bit.
• It’s helpful to throw in some high nitrogen materials, or a few handfuls of manure or other fertiliser after adding sawdust to the compost, to kickstart the decomposition process.

Monitor Compost Conditions – Regularly monitor the temperature, moisture levels, and overall state of your compost pile. The composting bacteria need sufficient levels of moisture to function, so aim to keep the compost damp, but not soaking wet. Some timbers can be quite waxy, so when their sawdust dries out, it may become water repellent!

If sawdust isn’t breaking down very well, or you find that adding the sawdust has cooled down the compost temperature, add more nitrogen-rich materials to heat it back up, and speed up the breakdown of the carbon-rich materials.

In summary, sawdust can be a valuable composting material, but it’s important to exercise care to avoid potential contamination from treated or composite woods, glues, paints and other finishes. By using only clean, untreated sawdust, we can improve soil structure and fertility while safeguarding the health of our compost and the environment.