486 Report OF THE HORTICULTURAL DEPARTMENT OF THE 
when the temperature falls below 41° F.; that its action is 
only appreciable at 54° F., while it becomes most vigorous at 
98° F., but that at 113° F. its activity drops back again to what 
it was at 59° F. Here, again, is another and very urgent need 
for the right soil temperature.” The augmentation of any one 
of the processes named above is of importance, and all taken 
together make certain that any condition which raises the soil 
temperature must very greatly accelerate vegetative activity 
and thus become of practical importance. 
The aeration is better in the tilled soil—tThe fact stated in 
the heading of this paragraph is so obvious as to hardly need 
discussion. To till is to stir and loosen the soil. When soil 
granules are massed loosely, as in a tilled field, unoccupied 
space exists between the granules and this space permits the 
free movement of air. If the granules are packed together 
tightly, as in sod land where the soil cannot be loosened, there 
is comparatively little unoccupied space between granules for 
the movement of air. AIll sod land, as compared with that 
under tillage, is poorly aerated, and in most cases the older 
the sod, the less well ventilated, for as time passes the soil 
granules are more closely packed by the elements and the roots 
of the grass. The ideal soil is often compared to a sponge, 
not only because of its capacity to hold nutritive solutions but 
because of permeability for air. Hilgard’™ says in this con- 
nection “ there can be no question that the higher productive- 
ness of well-cultivated soils is largely due to the greater and 
readier access of air to the roots.” 
The functions of air in the soil are discussed in all text books 
on soils and need be only briefly mentioned here. Air insures 
the presence of both oxygen and carbonic acid in the soil. 
Oxygen is the “ vital air” of plants as of animals — absolutely 
necessary to life; it is necessary, too, in all decays, fermenta- 
tions, putrefactions, and nitrifications and without it there 
could be no beneficial soil micro-flora. Carbonic acid plays an 
“Soils. By E. W. Hilgard. New York: 1906, p. 279. 
