I am talking about soil — more specifically soil compaction. We have all seen the signs. Large tree roots growing along the surface of the ground. Poorly growing trees. Compaction happens when the soil particles are squeezed together. Let’s think about a healthy soil for a minute. Soils are comprised of minerals, water, organic matter and air. Healthy soils for tree roots should be composed of 50 percent open space for water and air, 45 percent minerals and 5 percent organic matter. How does compaction affect this?
When soil particles are forced together, the open space is eliminated. The open spaces in the soil are for oxygen and water. Just like us, tree roots need the oxygen. Without oxygen, tree roots are not able to convert sugars into energy for growth.
With a loss of open space, the soil loses its ability to absorb water. You see this when it rains (so you have probably seen it a lot lately). Water is not absorbed and runs off the top of compacted soils. If the water is not absorbed into the soil, the roots of the trees are not able to use it. So, in the case of compacted soils, a 2-inch rain really doesn’t mean a lot to a large tree since it’s not going to be available for the roots.
Compacted soils are also inhibitive. Tree roots have trouble growing through them. That is why we will see them growing along the surface. They have nowhere else to go. When the roots grow along the surface, they are exposed to wind, drought and lawn mower damage, and they provide less stability for the tree. When the roots grow along the surface, we see things like sidewalks and foundations buckling.
Compaction is most often seen when new houses are being built or additional features are being added to a house. Often, the heavy equipment and vehicles are driven around trees in an effort to save them for the future landscape. Contrary to popular belief, tree roots do not grow deeply beneath the tree canopy. The majority of tree roots are found within 18 inches of the surface and spread beyond the tree canopy. So think about the pressure on the soil when driving around the tree with equipment. It doesn’t have to be a house addition. This is the time of year many are thinking about adding hardscapes, arbors and other outside features to the home. Reversing compaction around an established tree is a hard thing to do. Severing tree roots can do as much damage as the compaction. The cure for compaction is thinking ahead.
If you are thinking about an addition or new hardscape feature and have trees that you would like to keep around, you must consider the critical root zone for the tree. The critical root zone (CRZ) is the minimal rooting area that should be preserved to ensure a reasonable expectation of the tree’s survival. This CRZ will depend on the age of the tree as well as the species and size. Once you have determined the CRZ of the tree, it is essential to protect these zones to keep compaction to a minimum. The Alabama Cooperative Extension System has a publication that can help. To download “A Guide to Preventing Soil Compaction During Construction,” visit www.aces.edu and search “soil compaction.”
Remember this is not just important for existing trees. New plantings of trees and shrubs in compacted soils will suffer the same fate. We have a favorite tree that everyone parks under in the summer for shade. Guess what? The roots are growing along the surface, and I have noticed some dieback in the canopy. For trees in the landscape, I suggest mulching underneath the tree, extending past the canopy. Lawns will not grow in this area anyway. By mulching underneath, the compaction of lawn mowers is eliminated. Trees are a vital resource. Let’s keep them around by keeping the soil healthy.