General Considerations 199 
and that all species thrive best on one kind of soil (see page 
24), but some can be grown under less favorable conditions; 
enduring, however, not preferring, as some would have it, 
drier or wetter situations and shallow soils. 
While, for the start of the tree, the soil in the plant hole can 
be improved, it must not be forgotten that, in most cases, 
the tree must eventually grow into the native soil, in which 
it must be adapted to help itself. Especially is this true in 
regard to the depth. ‘There exist, however, wrong notions 
as to the depth of soil needed: a soil of four to six feet is 
deep for most species, and if fissured rock underlies the sur- 
face soil at from two to three feet, the conditions are favor- 
able enough for adaptive species, even with dceep-going roots, 
the roots penetrating into the fissures which form good 
drainage channels. Only when impenetrable layers of rock 
or ground-water lie within two or three feet from the surface 
will species with tap-root or heart-root fail to thrive, eventually 
showing the effect in rapidly tapering and spindling form. 
In streets, the natural deficiencies of the soil are further 
accentuated by such disturbances of water-supply as the 
impediment of pavements, and the underground drain pipes, 
etc., under which conditions hardiness and adaptiveness as 
regards root development are naturally most essential. 
We should here again lay stress upon the fact that there 
is a more or less close interrelation between soil and climate, 
and that the one can in part compensate for the other; 
that is to say, a poor soil (as regards water-supply) will 
accentuate the deficiencies of climate, while a deep, well- 
watered, well-drained soil will make it possible for species 
to endure climatic ills to which they would succumb under 
less satisfactory soil conditions. Hence hardiness is, at 
least in part, dependent on soil. 
Similarly, there may be provided some natural or artificial 
