In order to grow, trees must have a soil of suitable texture, in proper mechanical condition, that contains sufficient available mineral elements and plenty of organic matter, and, last but not least, a constant supply of moisture and air. In addition to these there must be in active growth in the soil many forms of organic life that are in various ways preparing the material in the soil for the use of the larger plants. Not only must these things be present, but others that are deleterious must be absent, whether the substance is hurtful in itself or whether it is an excess of one that is otherwise beneficial.

Above the soil three things must be present—air, sunlight, and moisture—and, as in the soil, harmful things must be absent in order to have success. Among the deleterious substances are sulphur and other fumes and soot and other products from incomplete combustion.

Some of the more obvious things with which a city tree has to contend are: Water-tight pavements, both on the sidewalk and street, that prevent the admission of air and water; the removal of the topsoil in street grading, thus forcing the tree to exist on the good soil provided in the hole; careless digging near the tree for gas, water, and electric service, and especially for the placing of curb-stones; the saturation of the soil with illuminating or sewer gas from defective pipes (fig. 9); the pouring of salt water from ice-cream freezers into gutters, where it may find its way into the soil near tree roots; the gnawing of the trunks by horses; and the cutting of the tops by linemen and tree trimmers.

Because of the uncongenial conditions for the growth of trees on city streets comparatively few kinds are satisfactory for such use. Among those available are some that will grow under extremely trying conditions. Kinds can be found that will thrive wherever it is suitable for human beings to live. If it is impossible to grow trees on a street, as a health measure that street should be closed for human use until conditions are so improved that it will support trees.

More kinds will thrive under suburban conditions where only a small portion of the roadway is covered by an impervious coating, where the parking spaces are liberal, and where the street is lined with open lawns than under the conditions in a city, where the street is covered with a water and air proof coating and the sidewalks with an impervious material, where parking spaces are limited, and where adjoining lawn areas are small or lacking. By a careful selection of kinds, all conditions in a city can be met. In some places bad conditions could have been improved greatly by a little forethought; in others, such conditions can be bettered. These details, like many other matters connected with city planning, have been ignored, but should be considered immediately, especially by villages and small cities. Figure 6 shows how an opportunity for creating a beauty spot has been lost sight of, while figure 10 shows how a city has utilized less ground to increase the comfort and attractiveness of its business center.

A common mistake in ambitious young cities and many old ones is to pave more of the width of the street for traffic purposes than, is likely ever to be needed. By reducing the roadway and throwing the remainder into liberal parking spaces much is added to the attractiveness and comfort of a city. A contrast in the two methods of treatment is illustrated in figures 11 and 12. The recommendation that the roadway prepared for travel be made narrow is not to be interpreted as a reason for lessening the area dedicated to the public use; in fact, in most cities, especially in the northeastern quarter of the United States, too little space has been reserved from houseline to houseline (fig. 13). By reserving more room between the houses and the street for use as lawns and gardens the conditions would be made more livable, opportunity would be offered for widening the public way without prohibitive expense if traffic or business demanded it, and the growing of street trees would cease to be a serious problem.