Here at Nuts & Bolts, nearly all of our production beds are formed by placing a thick layer of wood chips directly on top of the grass (and poison ivy), sometimes up to a foot thick, and then capping that mound of wood chip with some compost and topsoil. One of the most common questions that we get is “WHY ON EARTH WOULD YOU DO THAT!?”

Oftentimes people think that you should never put wood chips into your garden because it will suck all the nitrogen out of the soil and leave the plants stunted and sad. While there is some truth to this, it is far from the full story. In reality, building beds like this leads to more nutrient and water retention and availability; improved soil structure, aeration, and drainage; a more balanced soil microbiome which leads to reduced disease pressure; healthier plants; and a tasty crop of edible mushrooms! 😊🌱🍄

Let’s take a look at what really happens to these beds as the wood chips decompose.

To be clear: Wood chips will soak up nitrogen as they decompose, but that’s exactly what we want!

You see, plant nutrients like nitrogen, phosphorus, and potassium are only available to plants when they are present as ions, that is to say, when they have a net electric charge. The atmosphere is roughly 70% nitrogen, but atmospheric nitrogen is electrically neutral and cannot be used by plants. While certain bacteria, like those in the Rhizobium genus, are capable of “fixing” that atmospheric nitrogen into ammonia, which has a positive charge and is usable by plants, all these ionic nutrients, nitrogen and other, need some way to be stored in the soil lest they wash away with the rain. That is where the wood chips come in.

Or more accurately, Soil Organic Carbon. As all types of plants, animals, and other organisms decompose, their bodies become soil organic carbon, and eventually, humus. Humus is the name for fully-decomposed organic carbon in soils, and it is one of the most amazing materials on Earth.
Humus is what gives forest soils that fluffy, spongy texture that promotes such good water and air flow and supports vigorous root growth.

Humus is what gives forest soils that fluffy, spongy texture that promotes such good water and air flow and supports vigorous root growth. Humus is also negatively charged and thus capable of binding to many of the positively charged nutrients that plants need, preventing them from washing away and storing them for when they are needed. But while humus is mostly carbon, it doesn’t form from carbon alone. And that’s where the “nitrogen sucking” comes in.

As organic matter, like wood chips, breaks down, it sucks up nutrients like nitrogen, phosphorus, sulfur, and others in the process.

These nutrients are critical for the formation of stable humus, but if they are scarce in the soil, then the decomposition of the organic matter can take up all the available nutrients and leave little left for anyone else trying to grow there. However, the long-term benefits of high-organic matter soils FAR outweigh this short-term cost.

Once the wood chips (or other plant matter) has decomposed, it stops being a sink of nutrients and becomes an incredible source! The negative charge holds on to a tremendous amount of nutrients that might otherwise be lost. This positively charged layer can then bind to a second layer of negatively charged ions to hold even more nutrients! All in all, humus can hold up to 30 times as much as other soil components like clay, and infinitely more than sand, which is electrically neutral!

A poorly drawn chemical diagram of humus. Note how the multiple negatively-charged hydroxyl (O-) and carboxyl (COO-) ions bind to positively-charged nutrients, some of which in turn then bind to more negatively-charged nutrients.

In addition to the nutrient holding capacity, high-organic matter soils also have significantly improved soil structure, which leads to better air and water flow and increased ability for root growth.

Soil organic carbon is also one of the best water-retention agents, helping to protect against drought. With the climate changing and precipitation coming in stronger but more sporadic events, this improved water retention coupled with increased transport and drainage is insurance against both flood and drought, which are seeming to both be more common nowadays.

High soil organic content also leads to an increase in the diversity of soil microbes, which can help reduce the pressure of diseases and improve the productivity of plants. They can also support many types of macroscopic fungi, some of which are edible for humans!

Winecap musrhooms fruiting heavily in early May. These beds were formed and inoculated less than one year ago and are already producing abundantly!

Winecap mushrooms (Stropharia rugoso-annulata), shown above, are an amazing mushroom that thrives on hardwood chips.

It is not only edible (and delicious), but also makes other minerals and nutrients more accessible in the soil. The fungus exudes digestive enzymes and acids from their hyphal filaments which breakdown inert minerals and converts them into bio-active forms. When the fungal tissues die and decompose, these nutrients become available to all the plants and other organisms in the soil food web. As the winecaps are feeding the plans and ourselves, they also break down the wood chips into incredible soil!

Okay, sure. Long-term the wood chip beds make sense.

You probably won’t grow the best tomatoes in them for the first year or two, but eventually they will be highly productive. But what do you do in the meantime? I mean, we are trying to run a business here after all, we can’t just wait around for two years. That’s true. And that’s where our process gets clever.

Once we shaped up the beds with wood chips, we dumped a large amount of very high-nitrogen compost on top, seeding the beds with lots of microorganisms and a lot of nitrogen to feed the decomposition. Then we added a casing layer of topsoil, and seeded it out to a ton of nitrogen fixing plants like Clover, Buckwheat, Beans, Elaeagnus, and Alder. These are plants that form strong associations with bacteria like the Rhizobium mentioned above who do the actual nitrogen fixing. The resulting plant matter is very high in nitrogen, and then we chop-n-drop the plant matter onto the beds so the roots die back and pulse more nitrogen into the soil and the rest breaks down on top to leach its way in. This strategy of planting nitrogen fixers helps ensure that there is enough nitrogen to fuel decomposition with some to spare for shallow-rooted or low-feeding plants as well, whose roots will stay in the capping layer and not even notice the decomposing wood chips beneath, or who just don’t need too much nitrogen to begin with.

While some of those nitrogen fixers, like the Beans and the Elaeagnus provide an edible yield directly, others simply accelerate the decomposition and fuel the beds to be highly productive later. The Winecap mushrooms in the soil also help to shuttle around nutrients to the plants when they need it.

We shaped our beds in the summer of 2020, and by now many of them are well on their way to being beautiful soil.

While the structure of the wood chips is still very much apparent, the color is dark and rich and the soil smells Earthy and alive. And all our seedlings and transplants are looking very happy!

As the years pass and the wood chips break down further, it will only accelerate the incredible growth that we are already seeing.

As an ongoing maintenance project, we will continue to mulch the pathways with fresh wood chips to help keep the grass and other weed pressure down. Then as those wood chips decompose, we can scrape them up from the isles and onto the beds, along with some fresh compost, and put fresh chip into the pathways to keep feeding the winecaps!

Hopefully now you understand a bit more about why we use wood chips so extensively in the garden, and why they are so truly incredible.

They improve nutrient holding capacity and soil structure, moderate air and water transport to protect against flood and drought, increase microbe biodiversity to reduce disease pressure, and provide a delicious, symbiotic treat.

What are your thoughts on using wood chips in the garden? Has your opinion changed since reading this article? Let us know in the comments below! Find something inaccurate or missing from this post? Let us know that too!

If you want to learn more about the science of soils or double check our claims, check out this website from the NSW Department of Primary Industries. They’ve got literally hours of lectures available for free, as well as some great info graphics and more!

Why Wood Chips?