The edaphic factors—that is, the factors of the soil—of the urban environment differ radically from those of the natural environment. To accommodate the automobile, most of the areas likely to be used by automobiles have been hard surfaced. (There are some notable exceptions; St. Paul, for instance, still has 300 miles of unpaved—but oiled—streets.) Sidewalks and streets, parking lots, and shopping centers have been paved. The paving of great areas of the cities has created a runoff problem, requiring storm sewers as well as sewers for domestic and industrial wastes, and has lowered the water table of the cities. These losses and the use in many cases of plants with high water requirements have made the demands of the urban ecosystem for water much higher than the natural ecosystems of the hinterland. This condition becomes more accentuated as cities move farther into the grasslands, savannahs, and deserts.

The soil of the city often is composed primarily of graded subsoil. The original soil may have been an impoverished forest soil with shallow topsoil that was lost in grading, leaving only organically poor sand and clay on the surface. Such soils may be high in minerals but are usually low in humus and other organic matter and are of poor quality for the growth of plants, often necessitating the bringing in of new topsoil. On the other hand, more resources are usually available in the city than in the country, acre for acre, for rehabilitating soils; funds are usually available to provide fertilizer, sod, soil conditioners, and even new soil.

Since plants grow in soil and not in the paving materials of the urban ecosystem, it is the quantity and quality of that soil that determine the health and vigor of the roots of the plants, which in turn contribute greatly to the overall vigor of the plant. Plant roots can and do break the pavement and grow into sewer lines and clog them or break them. The micro-organisms of the soil can and do attack anything that is buried in the soil, even if it is plastic or lead coated. (Ceramics seem to be the only really durable material that can be buried.)

No higher plants have been successfully grown without roots. For the ecosystem of man to be populated with the species of higher plants that he seems to want, allowances must be made for the occupation of the soil by their roots. For most forest trees used for street or yard plantings the root systems are widespread (depending largely on the proximity of other trees) and shallow. The root environment of the plant is probably the most critical factor for its survival in the urban ecosystem.