(2) Influence of soil: Soil is made up of fine particles of sand and
rock and of vegetable matter called _humus_. A tree will require a
certain soil, and unsuitable soils can be very often modified to
suit the needs of the tree. A deep, moderately loose, sandy loam,
however, which is sufficiently aerated and well supplied with
water, will support almost any tree. Too much of any one constituent
will make a soil unfit for the production of trees. If too much clay
is present the soil becomes "stiff." If too much vegetable matter is
present, the soil becomes "sour." The physical character of the soil
is also important. By physical character is meant the porosity which
results from breaking up the soil. This is accomplished by ploughing
or cultivation. In nature, worms help to do this for the soil, but
on streets an occasional digging up of the soil about the base of
the tree is essential.

Humus or the organic matter in the soil is composed of litter,
leaves and animal ingredients that have decayed under the influence
of bacteria. The more vegetable matter in the humus, the darker the
soil; and therefore a good soil such as one finds on the upper
surface of a well-tilled farm has quite a dark color. When, however,
a soil contains an unusual quantity of humus, it is known as "muck,"
and when there is still more humus present we find _peat_. Neither
of these two soils is suitable for proper tree growth.

[Illustration: FIG. 90.--A Tree in the Open. Note the full development
of the wide crown with branches starting near the ground. The tree is
the European larch.]

(3) Influence of light: Light is required by the leaves in the process