Trees in urban environments need uncompacted, well aerated, and moist soil in order to thrive. Such conditions allow tree roots to obtain the essential components they require for healthy growth - nutrients, oxygen, and water. These soil conditions however, are often unavailable in city environments. 

To understand the importance that lies in soil for healthy tree growth, let's discuss a little more about what soil really is. Soil as we know it is the top layer of the earth's crust. It is the medium in which trees spread their roots and grow. Soil is comprised of finely ground rock particles and materials such as sand, silt, clay, and gravel; with voids between particles containing water and air. Sandy soils have very large particles that allow roots, air, and water to move freely. Whereas clay particles are small and packed together tightly, offering little room for nutrients and root growth. 

While carbon and oxygen are absorbed by trees through the air, other nutrients such as water are obtained from the soil and taken in via tree roots. Some nutrients in soil consist of magnesium, calcium, sulphur, and a range of other trace elements.

The three primary nutrients of soil:

• Nitrogen (for healthy stem and leaf growth)
• Phosphorus (for root growth)
• Potassium (for overall plant health - especially the immune system)

In addition to mineral composition, organic matter also plays a role in soil fertility and plant life. Organic matter also improves sandy soil by retaining water levels and alters clay soil by making it more permeable for water, air, and roots. 

Soil profiles usually have a top layer of decaying organic matter formed by fallen leaves and dead animals, as well as other debris deposited by plants. This layer is commonly referred to as the 'O horizon'. Below the organic matter is topsoil ('A horizon') which can range in depth from a few inches to several feet. This layer consists of decomposed organic matter and minerals, and is usually dark brown or red-brown in colour. This layer is where most tree roots will concentrate. 

Under the topsoil is what is called subsoil (or 'B horizon') which often lacks organic matter and therefore has poorer nutritional value. However as long as oxygen levels are sufficient and drainage is adequate, tree roots can grow into this layer. 

Below the soil layers lies the parent material known as 'C horizon' which is the main source of soil. This material can be transitional, heavy clay, or soft stone. Nutrient uptake in the soil is achieved by cation exchange. Fibrous roots pump hydrogen ions (H+) into the soil which displaces cations attached to negatively charged particles of soil, making the cations available for uptake by roots. Fibrous roots are the most important organ for the uptake of nutrients. 

Tree size and health is ultimately relative to three factors: rootable soil volume available, soil oxygen and nutrients, and the moisture holding capacity of the soil – without these soil conditions, urban trees will struggle to survive at best and cease to exist at worse.

Soil support cells that provide a structural matrix are filled with uncompacted soil to accomodate healthy root growth, while also providing a load-bearing structure for paved surfaces, are the proven method for street trees. By understanding the soil conditions that urban trees need to reach maturity, landscape architects and related professionals can take the required steps in specifying the systems and best practice procedures that will ensure the success of our urban tree canopies.