46 BULLETIN 140, U. S. DEPARTMENT OF AGRICULTURE. 
of the sap and, therefore, ultimately on the rapidity of transpiration 
from the leaves. And again : 
A warm summer is always accompanied by a high temperature of the ground 
or by a rise of its temperature. The increase is the more decided the more the 
excess in the temperature of the air is accompanied by a large quantity of rain 
or has been immediately preceded by it. In warm and comparatively dry sum- 
mers the rise of the earth's temperature does not perceptibly exceed the 
normal. * * * The dampness of the soil is sufficient to allow the variations 
in the temperature of the air in winter and spring to exercise a decided influence 
upon those of the soil, whereas, in summer an excess of rain would be necessary 
to accomplish this, and that, too, to a greater degree if the soil be covered with 
vegetation. 
Quetelet, as far back as 1849, in his " Climate of Belgium," ex- 
pressed regret that he had been unable in his crop-climate studies to 
consider the influence of the temperature of the soil, although " it 
is absolutely necessary so to do in order to treat the phenomena of 
vegetation in a complete manner." 
Mr. Knight 1 has observed that "varieties of the same species of 
fruit tree do not succeed equally in the same soil, or with the same 
manure," and further, that this applies to the peach, pear, and apple, 
" as defects of opposite kinds occur in the varieties of every species 
of fruit, those qualities of soil which are beneficial in some cases will 
be found injurious in others. In those districts where the apple and 
pear are cultivated for cider and perry, much of the success of the 
planter is found to depend on his skill or good fortune in adapting 
his fruit to his soil. 2 
McClatchie and Coit, 3 in discussing varieties, state that — 
The same variety reacts very differently to the various stimuli produced by 
different environments. Hence, we arrive at the commonly held and correct 
idea that each climatological area has its own peculiar set of varieties which 
succeed best under its own climate and soil conditions. 
Hence it follows that the supply of soil moisture available to plants 
and the temperature of the soil to depth equaling or exceeding that 
of the root zone of plants and trees, seem to account in part at least 
for the phenomena of the soil-varietal adaptations. These two fac- 
tors constitute the soil climate and in subsoils they are governed 
indirectly but chiefly by the texture and structure as related to the 
moisture supply. In the surface soil these have been or may be modi- 
field to some extent by the addition of humus, but the latter influence 
is entirely insufficient to control the matter of inherent adaptation 
of soil types to crops, or to different varieties of the same crop. 
It is evident, then, that many of our crops bear testimony, both from 
experimentation and from well-established agricultural practice, to 
1 Lindley's Theory of Horticulture. 1841. Chap. 20. 
2 Bui. 61, Arizona Agr. Expt. Sta. 
3 Trans. Royal Hort. Soc, I, 6. 
