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The Dirt on Trees

Stephanie Wagner

We love our trees!  We love their shade, their graceful patterns against the sky, and how they store carbon to combat climate change. But where would trees be without their roots? The roots we cannot see but know anchor the beauty towering over us. Where does that stability come from? The soil into which the trees reach. What do we know about that soil? Do we just take it for granted?  It’s always there!

That soil is as important to the health of our trees as the air is to the trees making food. The air supplies the carbon for tree leaves to make the food and structural building blocks to keep the tree alive and growing. An important part of that process (known as photosynthesis) also requires water that is extracted from the soil by the tree roots. But that soil does so much more than anchor the tree and provide water – it also supplies all of the minerals and other nutrients (particularly nitrogen) trees need to live.

Credit: Kelsey Amelia (

For many years soil scientists thought of soil as an inanimate object composed of ground-up rocks- but that is only half of the story! The other half is soil is air, water, and organic matter which includes a multitude of living organisms. The living, biological component includes microbes, mycorrhizal fungi, insects, and worms, most of which are too small to see with the naked eye. This living component of soil is the key to healthy, fertile soil. This unseen living system breaks down the soil to supply minerals, capture nitrogen from the air, and create spaces for water and air. Without this living component, soil just doesn’t work properly. Tree roots are an important part of this system since plants actually take some of the food they make in the form of sugars and pump it into the soil to feed these organisms. This is an interactive system where the biotic components provide nutrients and plant roots provide food for these organisms.

When we use pesticides and artificial fertilizers we kill off these living organisms and destroy this interactive system. Artificial fertilizers actually make the tree roots lazy and they stop expanding and feeding the soil. I recently noticed some trees planted in the middle of the lawn at an apartment complex along Boones Ferry Road that had fallen over in the ice storm- this is likely because their roots just couldn’t support them with the extra ice. There is a good chance that the lawn receives artificial fertilizers and the roots just stopped growing. We all benefit from understanding natural systems and supporting natural processes.