266 
EARTH HEAT. 
Next in importance to deep drainage, therefore, is deep tillage. 
It supplements drainage by often repeated exposure of a certain 
depth of soil to the action of the air and sun, by which its oxygena- 
tion is carried on more rapidly than it can possibly be when not so 
exposed. 
Earth Heat. — The earth grows warmer as we go down. If its 
temperature were tested in winter, we should find an increasing 
warmth with each foot of depth below the frost. The more porous 
and dry the soil, the less depth it will freeze, and the more rapid 
the increase of temperature below the frost line. This explains why 
gravelly subsoils make warm soils, and suggests that deep drain- 
age is the most efficient means of providing for trees an equable 
“bottom heat.” 
In the northern States the range of earth-freezing is from one 
to three feet deep. It is not always deepest where the cold is 
greatest ; for where a considerable altitude makes the winters more 
severe, the greater snow-falls are likely to husband the earth’s 
warmth as with a feathery blanket, so that the soil may be frozen 
no deeper at Utica than at Philadelphia. But when the surface 
protection is the same, altitude and latitude tell quickly on the 
climate in its effect on trees. 
Roots at the surface of the ground are either torpid in their icy 
encasement, or alternately thawed-out or frozen-in during four or 
five months. Those a foot below the surface are ice-bound not 
much more than half this time j those two feet below, a third ; and 
those three feet below, not at all. All the roots which are just 
under the frost-line during any part of the winter, are in no colder 
soil than the winter surface-soil of the Gulf States. Whether six 
inches or three feet under the surface, where the ground is not 
frozen, the roots maintain some action. 
The younger and smaller a tree or shrub, the nearer its roots 
are to the surface, and all its fluctuations and severities of tem- 
perature j and therefore the greater need of guarding against them. 
The analogy between animals and plants is greater than most per- 
sons suppose. “ Keep your feet warm and dry, and you will not 
be likely to take cold,” is a trite piece of advice, because it is so 
