Wood Chips as a Soil Amendment

Gardening is a learning process, and most gardeners experiment with their plants and their garden spaces. This article is inspired by an experiment that we conducted at the Gardzen Test Gardens a few years ago—an experiment that didn’t work out very well for us that year. It’s about using wood chips as a soil amendment. On the surface, wood chips might look like a great addition to your soil. However, before you go adding them to your soil with the hopes of a quick improvement in soil quality and soil nutrition, there are a few things to be aware of. Let’s dig in and learn a bit more about wood chips as a soil amendment and how you should—and shouldn’t—use them in your garden.

Gardzen Executive Summary:

· For gardeners in the US and Canada, this is the time of year to think about adding soil amendments for the spring and summer plants

· Wood chips can make a good soil amendment but they have some limitations and considerations that we need to understanding

· Decaying wood can under some circumstances absorb nitrogen from the soil, robbing plants of essential nutrients and limiting their growth.

· The science behind wood decay and nitrogen is complex and there are many different opinions about how it affects using wood chips as a soil amendment.

· We once tried a combination of sawdust and horse manure in the Gardzen Test Gardens and it robbed our soil of nitrogen and kept plants from thriving until the sawdust/manure mixture fully composted in our garden beds.

· Composted or decaying wood chips can be a great soil amendment, but when in doubt we recommend adding wood chips, wood shavings, or sawdust to the composter  and letting them break down completely.

The Pros and Cons of Wood Chips as a Soil Enhancement

At first glance, wood chips might seem like they have a lot to offer as a soil amendment. They add quite a bit of organic matter in a relatively small volume. They break down slowly and feed the soil over time while also loosening compacted soil and helping to keep it loose. They help with moisture retention as the wood absorbs and re-releases rain or irrigation water. Used on the surface, wood chips are a great mulch, protecting the soil from sun, wind, and other causes of erosion. And while all of that is true, there are some other things to know about wood as a soil amendment before you start adding it to your garden.

The first thing to remember is that wood chips break down slowly. This can be a good thing when using wood chips as a soil amendment as it feeds the soil over time and allows us to make a long-term investment in our soil. Combined with compost and/or fertilizer, wood chips can be a great way to keep our soil healthy and happy over a long period. However, they are a long-term investment and should be treated as such. If you need a faster solution for a annual bed or a vegetable plot, you may want to stick with compost or fertilizer instead.

Another thing to know about wood chips as a soil amendment is that while many microorganisms and insects LOVE to eat wood chips, they’re not really worm food. For some garden applications this is an incidental consideration, for others its very important. Permaculture, hugelkultur, and some organic gardening approaches tend to rely heavily on worms as a natural tool for enriching soil. Worms can’t eat wood chips, so providing other organic material for them is best if you’d like to keep your worms healthy, happy and adding their nutrient-rich castings to your soil.

There’s one final consideration that’s so important that it deserves its own section of the blog: wood chips and their affect on nitrogen levels in the soil. There’s a lot to talk about here, both factual and fictional, and a great many misunderstandings. It’s also where and how we made our big gardening mistake a few years ago, so please keep reading and learn from our experiences so you don’t have to repeat them.

Wood Chips, Sawdust, and Nitrogen in Soil

Years and years ago, the Gardzen Test Gardens got a very generous offer from a local riding stable. They offered to deliver a load of horse manure for us to use as a soil enrichment. We gladly accepted and they delivered the manure along with the sawdust that they use on the floor of the horses’ stalls. The manure was at least a year old but the sawdust was still visible in the mix, and we added it to our vegetable beds.

Our plants started out strong that year and then stopped growing. We had the soil tested and found a nitrogen deficiency that didn’t go away no matter how much nitrogen fertilizer we added. A little more research and a long conversation with our local cooperative extension revealed that a mix of horse manure and sawdust adds a huge amount of nutrition and organic matter to soil over the long term. In the short term, the decay process of the manure/sawdust combination consumes most of the nitrogen available and renders the soil unsuitable for growing plants. The process of decay can take a year or more depending on circumstances.

This was a deeply upsetting lesson, but an important one. We learned to do more research before adding anything to our soil. We also learned that there’s a lot of debate and discussion about how wood chips and wood shavings or sawdust affect soil. Some sources claim that while sawdust will rob soils of nitrogen due to the increased surface area, wood chips are OK as there’s less surface area to interact with the soil and absorb nitrogen. Other sources claim that fresh wood chips are the problem and that partially decayed wood chips are OK. It’s a complicated issue and we’re not qualified to talk about the science of it, but we’re working to follow up with professionals who can provide better guidance.

For now, we recommend using wood chips as either mulch or in the composter rather than adding them to the soil. We strongly recommend additional precautions with sawdust or wood shavings—those go into the composter and don’t belong in soil till they’re fully broken down. And while manure can be a great addition to soil, it’s also best added after being fully composted.

Wood Chips, Sawdust, and Garden Soil Lessons

We really hope this helps you avoid some of the mistakes we’ve made. We also hope it helps you understand how wood chips, wood shavings, and sawdust interact with soil and the ways in which we should and shouldn’t use them in our gardens. There’s another lesson here, too—gardening involves some experimentation, and sometimes our experiments don’t give us the results we want. In other words: it’s OK to make mistakes! That’s how we learn and grow as both gardeners and people.

If you know more about the chemistry and biology of wood decay and how it interacts with soil, we’d appreciate your input on this one. Please get in touch either in the comments section or via email. Gardzen is all about community and we love to hear from you!