Why Does Topsoil Contain Wood Chips?

We use soil for many different purposes, including fuel and plant growth. Topsoil is the most crucial layer for plant growth because it is rich in organic matter and nutrients. However, there are times when you purchase topsoil that contains some odd things like wood chips.

Topsoil has wood chips in it because it may have been collected from land development sites where wood chips from cut trees were mixed in with the soil. Alternatively, the topsoil may have been mulched or enriched with unfinished compost with wood chip residues.

In the rest of this article, I’ll discuss what topsoil is typically made of and why you sometimes find wood chips in it. I’ll also explain how wood chips affect soil health, their use in landscaping and gardening, and tips on how to apply them on topsoil.

Why Wood Chips End Up in Topsoil

As the name implies, topsoil is the topmost soil layer comprising about 2-12 inches (5-30 cm) of humus- and nutrient-rich material. This quality makes it conducive for roots to grow into and support healthy plant growth.

The humus or organic matter and nutrients come from decayed plants and animals. It may be odd to find wood chips in your newly purchased bag of topsoil. However, it’s rather common in nature. The probability increases if there’s human intervention involved.

Here are some common reasons your topsoil has wood chips in it:

Collection from Land Development Sites

Many gardening stores source their topsoil from farms that are developed into residential or commercial areas or forest clear-cutting operations. As a result, the topsoil is likely to contain some wood chips, shavings, or sawdust.

Although these woody materials will gradually decompose and add organic matter to the topsoil, it’s still important to consult your supplier where the material came from. Topsoil harvested from farms that used pesticides or forests where trees harbored diseases can be detrimental to your home garden.

Amendment or Mulching Residues

Small wood chips are effective soil amendments because they can relieve soil compaction. They may also be added to a compost pile but take longer than other organic materials to decompose. When mixed with clay soils, wood chips become a veritable superpower.

Many gardeners have attested to this, suggesting that they promote microbial growth within clay soils and encourage worms and other helpful creatures to visit. If your garden soil is quite heavy with clay, you can’t go wrong with a few wood chips in your topsoil.

Wood chips are also often used as mulching materials for perennials because they help control weeds, retain soil moisture, and regulate temperature.

They decompose slowly, allowing the release of essential nutrients into the soil over a more extended period than other soil amendments, such as compost, which needs to be replenished every so often.

This slow disintegration allows the soil to slowly absorb moisture as needed without any detriment to the plant. However, this slow decomposition rate also contributes to why you sometimes find wood chips on your topsoil.

Effects on Soil Health

The first thing you must know about wood chips is that they’re carbon-rich materials. In order to break them down into organic matter, the decomposing bacteria require a significant amount of nitrogen.

Unfortunately, this nitrogen will become unavailable to plants and may be lost to the atmosphere, resulting in nutrient deficiency. That’s why many gardeners apply extra nitrogen fertilizers into the soil when using carbon-rich amendments or mulch like sawdust, straw, and wood chips.

Before deciding to use topsoil with a few wood chips in it, it’s crucial to understand how this will affect the soil health and, ultimately, your plants.

Below, I’ll go through some of the pros and cons so that you can make an informed decision:

Pros

When used correctly, wood chips will benefit your garden in the following ways:

Affordable Soil Amendment

Wood chips tend to be very simple and cheap to manufacture, making them relatively inexpensive for consumers. They are easily found in nature, so the process of harvesting them doesn’t have any major negative impacts on the environment. 

It’s always best to choose smaller pieces of wood chips (about 1/4 inch or 0.6 cm) as soil amendments. When applied evenly, they can loosen compacted soil and improve aeration, moisture retention, and drainage.

Improved Moisture Retention

Garden soil is generally very good at retaining moisture, especially if it contains lots of natural organic matter. However, a garden with sandy soil can benefit from wood chips because they take in moisture, which they slowly release for the plant roots.

As mulch, wood chips also keep moisture within the soil by preventing rapid loss from evaporation. This means you don’t have to water the area quite as frequently.

Pro tip: Choose wood chips from softwood like cedar and pine for better moisture retention because softwood holds more water than hardwood.

Root Protection and Insulation

If you’re trying to protect your soil from the harshness of a cold winter, sprinkling 4 inches (10 cm) of wood mulch over the soil will help to keep some of the heat in. If you live in an area that often experiences very sudden frosts, wood chips provide a natural blanket for your soil. 

However, wood chips within the topsoil don’t have a similar insulating effect.

Aesthetic Advantage

If you want your soil to thrive while also adding a bit of textural aestheticism to your garden, wood chips will do the job well. They come in different colors depending on the source material and can make your garden look very organized and put together. You can also purchase organically dyed wood chips.

Cons

While wood chips can be an excellent additive to the soil, it also has a few disadvantages, including the following:

High Nitrogen Consumption

Since the process of wood chip decomposition uses lots of nitrogen, this nitrogen will be removed from the soil. Unless you put a bit of fertilizer with added nitrogen on the soil, you may find your plants suffer from the lack of this essential macronutrient.

Some symptoms include stunted growth, lack of foliage or green growth, or yellowing leaves. 

Possible Soil Suffocation

If you apply a thick layer of wood chip mulch, you could end up suffocating your soil. Excess moisture in dense soil won’t be lost through evaporation, and your plant roots won’t survive because your soil isn’t aerated. The roots will remain in soggy conditions and eventually rot from fungal infection.

Slow Decomposition Rate

Wood chips will generally decompose over a long period of time if you’ve included them in your topsoil. How long this process of decomposition takes depends on how soft your wood chips are, the weather, and how small the pieces are.

If you’ve got very small, spongy wood chips, they will take around 12 months to decompose fully, creating approximately 2-3 inches (about 5-7.6 cm) of organic materials.

However, if your wood chips are harder or bigger, you’re looking at slightly less than 2 inches (5 cm) of organic material after 12 months. 

Pro-tip: If you want your wood chips to break down faster, it’s best to pulverize them. Doing this will spread the chips out more evenly, allowing decomposition to speed up. 

Tips for Proper Use and Application in Gardening

There are two ways by which you can use wood chips on your topsoil: as mulch or amendment.

Mulching Depth

You can apply wood chips literally on your topsoil as a mulch layer, but the recommended depth can vary depending on the purpose.

Refer to the general guidelines for mulch thickness below:

  • Weed control and moisture retention: 2-4 inches (5-10 cm)
  • Winter protection: 4 inches (10 cm)
  • Pathway: 4-6 inches (10-15 cm)

Amendment Steps

If you want wood chips as an amendment for your topsoil but they aren’t included in your mixture, you’ll need to incorporate them into the soil yourself.

Follow the steps below to add wood chips to your topsoil:

  1. Grind or dice the wood chips. As discussed, smaller wood chips will decompose faster. Pieces less than a quarter of an inch (0.6 cm) small will be unlikely to invite termites into your soil and will be easier for plant roots to grow around.
  2. Pull out all the weeds from the roots. Make sure to weed your soil thoroughly. Any weed roots left over will resprout after you’ve applied your wood chips. 
  3. Water the topsoil deeply 2-3 days before adding wood chips. This will allow some of the moisture to sink deeper, creating a perfect base for your wood chips.
  4. Lightly till the soil and spread out the wood chips evenly. Mix an inch (2.5 cm) of wood chips into the upper 3 inches (7.6 cm) of your topsoil. 
  5. Apply a nitrogen-rich fertilizer into the soil. I often use 3 lbs (1.4 kg) of ammonium nitrate for every 50 lbs (22.7 kg) of sawdust or wood chip soil amendment. This proportion is enough to ensure that your soil will have enough nitrogen for your plants and soil microbes.

It’s best to do this about 3-4 weeks before transplanting plants into the soil so that the microbes have enough time to get to work. Once this process is finished, you can continue caring for your garden as usual. You shouldn’t need to replenish the wood chips for about two years.

Every so often, you’ll need to make sure that no pesky little weeds are poking up through the topsoil. You can do this by mixing up the wood chips and topsoil occasionally. 

Environmental Considerations and Sustainability

Wood chips are, for the most part, considered environmentally friendly additives for topsoil. However, this depends primarily on the manufacturing process and whether or not you need to get them shipped to you. 

In general, it’s best to use wood chips from trees native to your area to support the local economy and avoid environmental issues with shipping procedures. It will also reduce the risk of introducing diseases or chemicals that can negatively affect plant health or growth.

For instance, some wood chips from disreputable sources may come from pressure-treated wood that contains arsenic. This substance can inhibit root growth and ultimately kill your plant. Using the same wood chips in your vegetable garden may lead to arsenic toxicity in humans or pets who consume the plant.

In addition, wood chips from black walnuts contain juglone, a natural chemical that can inhibit the growth of sensitive plants, such as the following:

  • Fruit trees: apples, blueberry
  • Vegetables: asparagus, cabbage, potato, tomato
  • Conifers: yew, loblolly pine, red pine
  • Flowers: chrysanthemum, hydrangea, columbine

You can make your own wood chips if you’re concerned about any negative environmental aspects, such as unnecessary transportation. If you’re into woodworking, you can collect the wood chips and shavings, dry them for up to a week in a sunny location with good air circulation, and bag them.

Practical Uses in Landscaping

Wood chips are widely used as an effective garden mulch. They’re also occasionally used as soil amendments to improve aeration and drainage in clayey soils. However, wood chips are also excellent as an environment-friendly and sustainable material for landscaping.

The appearance and texture of wood chips make them an excellent material for border and landscape designs. Hardwood typically comes in darker shades, whereas softwood has lighter shades. Some gardening stores also use organic dyes to help you choose wood chips that will match your landscape aesthetic.

More importantly, laying about 6 inches (15 cm) of wood chips on pathways will buffer the weight from foot traffic and prevent the soil underneath from compacting. The wood chip layer can also protect the soil from wind or water erosion.

Final Thoughts

Topsoil can sometimes contain wood chips if it is collected from land development sites like farms and forests with wood debris or garden soil that was previously mulched or amended with wood chips.

Wood chips are environmentally friendly, relatively cheap to manufacture and purchase, and aesthetically pleasing for those who love an organized garden. They also hold more moisture and add organic matter to the soil as they decompose.

If your topsoil doesn’t already contain wood chips, you can add them as a mulch layer on the surface or work them into the topsoil as an amendment. You can also lay them over pathways for better landscaping aesthetics and to protect the soil from compaction and erosion.

Dr. Moritz Picot

Dr. Moritz Picot is a horticulture enthusiast and the founder of TheGrowingLeaf.com, where he serves as the lead content writer. He established the website in 2022 as a valuable resource for both gardening aficionados and beginners, compiling all the gardening tips he has accumulated over the past 25 years. Alex has a passion for nurturing plants, transforming backyards into inviting spaces, and sharing his knowledge with the world.

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