So, you had a tree removed. The remaining stump was ground into bits, and now you have a big pile of debris occupying the location where your tree once stood. What are you going to do with all of that material? The decision on what to do with the space long-term can be difficult, and herein this article is some information for you and advice that may help guide your process.

To start, that mound of material may seem lifeless, but there’s a whole lot of unseen activity occurring within that debris. For simplicity’s sake, we’re going to focus on what’s happening with carbon (C), nitrogen (N), and how it affects plants.

The soil is full of nutrients, and some of them are ‘limiting nutrients’, because certain organisms require them more than any other nutrients. Nitrogen is the limiting nutrient for plants. Plants are dependent on microorganisms to convert nitrogen into a form they can take up. Soil microbes decompose organic nitrogen stored in soil by converting it into ammonium. Next, a bacteria, Nitrosomonas, converts ammonium into nitrites, and then Nitrobacter bacteria convert nitrite into nitrate. Nitrate is the form of nitrogen that plants can readily take up. However, the dependency that plants have on this microbial process means that these decomposing microorganisms have first priority over any available nitrogen in the soil.

Though nitrogen is used by these microorganisms, carbon is the limiting nutrient for soil microbes, which they use at a 10:1 ratio with nitrogen. As long as the soil has a C:N ratio less than 30:1, there is sharing of nitrogen by the microbes with the plants. However, if the C:N ratio is greater than 30:1, then the plants will not get nitrogen because the microbes will not share it.

When we grind stumps a lot of woody material is introduced, which means A LOT of carbon is added to the soil. Pieces of wood and bark have C:N ratios from 100:1 to 200:1, while sawdust has a wide range of C:N ratios from 250:1 to 750:1. This addition of carbon results in an explosion of microbial activity with a coinciding crash of nitrate levels because the microbes are consuming the nitrate in the presence of high amounts of carbon. Eventually, the microbes consume all the nitrate that is not stored, and they start to die because they can’t maintain a 30:1 ratio. When microbes die the nitrate levels return to (and can even increase) their levels prior to the influx of carbon.

So, what should you plan to do after you’ve had a stump grinding service performed? Keep in mind that one of the reasons it seems like so much material, is because the stump grinding process introduced air into two dense things: the stump, and the soil. The stump grinder mixed these things together and fluffed it up with a bunch of air. So, depending on what you decide to do, you’ll likely want to allow everything to settle back in over a few months’ time. If you’re planning to have the space occupied with new plant material (turfgrass, herbaceous plants, shrubs, or another tree), then it will be necessary to provide nitrogen to the plants in the form of supplemental fertilizer. There are many fertilization options available and, for our clients, I’d recommend speaking with your Arborist Representative about which of our specialized fertilization plans is right for you.

Now, if you would allow me to impose myself into your decision-making process, please take this opportunity to install an ecological planting. Once the material has settled into the area, you can amend it with compost for some extra organic matter. If you have a high clay content, then you could amend it with a compost/topsoil mix. The fine wood debris of the leftover material will help moisture retention, while the larger wood pieces will aid in drainage of excess water. After you’ve incorporated all the material to your satisfaction, you’re ready to install some plants. You could do a landscape bed or berm of pollinator-friendly perennials, or a grouping of flowering shrubs with different flower colors, bloom times, and foliar textures. Also, of course, you can start anew with another tree for the space. Perhaps, you could do a mixed planting that incorporates all three types of plants, supports biodiversity and, when done well, increases your property value.

If you have an interest in following any of these avenues after you’ve had a stump grinding service, then please contact us to set up a meeting with one of our Arborist Representatives and we can start a conversation about how to transform your space.

- Isaac Hurley, Plant Health Care Technician