Soil Profiles – Not Just Dirt!

Soil provides vital and life-sustaining ecosystem services but is often underappreciated as a natural resource. It is essential for the functioning of urban ecosystems and the trees we plant in them! Trees require an adequate supply of uncompacted, well-aerated, and moist soil in order to thrive, not just survive. These soil conditions enable tree roots to obtain all the essential elements they require for healthy growth – nutrients, oxygen, and water. In built-up urban areas, and even in our backyards, these soil circumstances are often limited or unavailable altogether. When a tree’s need for nutrients, water, and air can no longer be met, the health of the tree suffers and the tree begins to decline and eventually die. Trees grown in these conditions hardly ever reach maturity and do not provide the many benefits that healthy trees offer.

A Profile on Soil

Soil is broken down into multiple layers called horizons. Each horizon is made up of different materials, has different properties, and plays a different role. The top layer as you may have heard is topsoil and it is composed of leaf litter and other organic debris. Continuing down we have a layer in which most biological activity occurs and where our tree’s roots live. This layer contains an accumulation of organic matter and other minerals. Even further we have a layer that starts to become depleted of organic matter and an accumulation of clay. Lastly, we have a layer of weathered rock and then bedrock.

Urban Soils

The term “Urban Soil” was first used to describe the characteristics of highly disturbed soils in urban areas. Urban soil was later defined as “a soil material having a nonagricultural, man-made surface layer that has been produced by compacting, mixing, filling, or by contamination of land surface in urban and suburban areas.” We can get a good idea of the average soil profile in the urban environment by looking at our front yards. When a new home is built, basements are dug, foundations are laid and the ground is leveled and graded. During this process, the uppermost layers of soil, the layers of soil in which most biological activities exist, are removed. Often fill material is brought in from somewhere else to finish the grading process. This disrupts the soil layers from the natural makeup of the land, changing soil composition, pH and chemical makeup as well as soil structure. These changes can prevent trees from receiving the nutrients they need, be damaging to existing trees and can cause problems when planting replacements. Instead of the nutrient-rich soils that trees prefer, we are often placing them in a combination of clay and rock; better yet, we are expecting them to thrive all on their own.

What We Can Do

The changing soilscape directly impacts the growing trees around us. Without careful consideration as to how we impact the soils, human activity can have serious negative effects on our canopy. We can not only improve the soil around us but by knowing the hardships our trees face, we can give them a helping hand and a fighting chance. Whether we are planting a new tree or caring for an existing mature tree we can add nutrients back to the soil to provide for what may be lost.