It’s All in the Soil

The taller the tree, the deeper the roots

Many trees in our forests, yards, streetscapes, and parks live for decades if not longer. They are the backdrop to our everyday scene of life. They seem to have been there for ever and remain when we are gone. Most people have interacted with a tree growing up. Whether walking by one on the way to school, sitting under their shade on a hot summer day, climbing up into their branches or swinging from a rope hung from a branch. Unlike most birds and animals, they are easy to get close to. We can feel their bark, touch their leaves and twigs, play with their seeds like tiny helicopters.

But are we seeing the whole picture? Trees use carbon dioxide, water, chlorophyll, and energy from the sun to create sugars used by the tree to grow. In the process they release oxygen into the atmosphere, which accounts for most of the oxygen on planet earth. Without plants and algae earth would have virtually no oxygen in its atmosphere. The process of photosynthesis occurs in the leaves, but one of the main ingredients, water, comes from the soil.

What is soil, and how important is it?

Soil is composed of minerals, namely rocks broken down into sand, silt, and clay. The percentage of each determines the soil texture, which determine such things as porosity, how quickly water flows through it. Soil also contains organic matter, decomposing plant and animal residue. It contains living organisms, bacteria, fungi, microbes, and insects. They are nature’s recyclers, responsible for breaking up the complex substances in decaying plants and animals so that they can be used again by living plants. Soil also contains gas (air) and water. Air is the tiny spaces between all the soil that allows for the movement of water, roots, and all the microorganisms.

The challenge of compacted soil in urban and suburban environments

Compacted soils have reduced air space which also means reduced space for water in the soil. Compacted soil which are very common in urban areas restricts root growth, roots grow shallower and malformed, reducing their ability to uptake air, water, and nutrients. This is one of the most common problems of urban trees. Not only are the spaces where trees are planted restricted by roads, sidewalks, and structures, but the soil they are planted into has been heavily compacted. In newer subdivisions, there have been studies that show that the growth of newly planted trees is dramatically slower than trees in natural areas. Current construction methods do nothing to help our urban trees. We start by removing all the topsoil and placing it into large piles where many of the soil organisms die. We use heavy machinery to dig up subsoils and compact the soil further by moving this heavy equipment back and forth around the site. We then add gravel for road and sidewalks, dig tunnels for infrastructure, build our homes and buildings, bring in a very small amount of altered “top soil’ back to sprinkle on top and then plant trees and lawns. We are basically planting into what might be considered dirt. Very little air, very little biological life.

How to promote soil health

One of the best ways to combat sterile soils is the addition of organic matter and microorganisms. In a natural setting, there is a cycle of life and death, plants and animals die and there are other organisms that break down this matter and return the nutrients back to the soil. The organic matter is food for these organisms that live throughout the soil, and as they travel through the soil they help to de-compact and loosen the soil. This allows for more air and water and organisms to create that complex web again. Anything that can be done to increase the health of the soil has an affect on the vigor and health of the tree. A healthy tree does a better job of resisting any pests and diseases. There are many ways to improve the health of the soil around a tree. They include such actions such as removing any turf or hardscape around the base of the tree as far to or past the drip line. Removing the turf also reduces the amount of time walking back and forth across the root zone when cutting the turf which reduces compaction and reduces the chance of damaging the tree when trimming around the trunk. Adding mulch or planting under the tree and leaving some leaves and plants to die and decompose under the tree begin the cycle of adding organic matter back to the soil. When adding mulch keep it away from the base of the tree so it does not rest against the bark which can begin to rot and damage the tree base and keep it in the 1-3 inch in depth, so it doesn’t smother the roots and soil. Adding compost or a compost tea to the root zone is also beneficial. In areas with limited space trees may require additional requirements. These include such things as additional watering during dry spells, nutrient management which begins with a soil test to determine the amount or lack of nutrients and how they are available to the tree along with a plan of continuous management and strategies toward improvement. What happens underground has a major impact on what is happening above ground. Working alongside an arborist can lead to a healthy tree for years to come.