Trees need sufficient soil volume for the root system to grow in. This volume varies a lot with soil type and other site considerations. Compacted soils, soils under pavement, soils under partially permeable covers, and non-irrigated soils will not add much to the tree’s resource base. The Figure below shows the average soil volume needs for trees of various canopy sizes. Because roots grow relatively shallow, soil depth below 3 feet is usually not considered useful to the tree.
The soil and water available to a tree should be considered as two sides of the same coin. Dry soil doesn’t help trees; moist soil of insufficient volume doesn’t help trees. Trees need enough soil, and they need that soil to be moist enough to allow for root function. Plants feed themselves, making sugars from sunlight, water and carbon dioxide. In order to do that, though, they need certain chemicals, which we call plant nutrients. Most of those come from the soil, and the tree takes them in with the soil water it is taking in.
Trees use those chemical nutrients to build all sorts of other chemical compounds, and those in turn are used to build cells and all of their contents, including enzymes, hormones and other complex molecules. For instance, both iron and nitrogen are critical to building the chlorophyll molecules that convert the sunlight, water and carbon dioxide into simple sugars.
A healthy soil has millions, even billions, of fungi, bacteria and other microorganism living in it, as well as larger things like mites, nematodes and earthworms. The majority of these life forms contribute in a positive way to the health of the soil, and together they are called the “soil food web”. If the web is intact and strong, plants tend to grow well (assuming no other problems exist...). Many urban soils do not have a vibrant soil food web, for a variety of reasons:
We don’t let fallen organic matter (leaves, fruit, branches) lie on the soil surface and decompose. If they did, they would supply new fuel to the soil food web, but for aesthetic reasons we clean up that debris and haul it away.
Urban soils are often compacted, which makes the soil a less livable environment, even for microscopic organisms.
Urban soils may have other issues, like water availability, drainage, contamination, etc.