220 HOW CROPS FEED. 
times greater amount. In other words, in the cultivated 
soils taken to the depth of 14 inches, there was found as 
much carbonic acid gas as existed in the same horizontal 
area of the atmosphere through a height of 7 to 110 feet. 
The accumulation of such a percentage of carbonic acid 
gas in the interstices of the soil demonstrates the rapid 
formation of this substance, which must as rapidly diffuse 
off into the air. The roots, and, what is of more signifi- 
cance, the leaves of crops, are thus fur more copiously fed 
with this substance than were they simply bathed by the 
free atmosphere so long as the latter is unagitated. 
When the wind blows, the carbonic acid of the soil is 
of less account in feeding vegetation compared with that 
of the atmosphere. When the air moves at the rate of | 
two feet per second, the current is just plainly perceptible. 
A mass of foliage 2 feet high and 200 feet* long, situated 
in such a current, would be swept by a volume of atmos- 
phere, amounting in one minute to 48,000 cubic feet, and 
containing 12 cubic feet of carbonic acid. In one hour it 
would amount to 2,280,000 cubic feet of air, equal to 720 
cubic feet of carbonic acid, and in one day to 69,120,000 
cubic feet of air, containing no less than 17,280 cubic feet 
of carbonic acid. 
In a brisk wind, ten times the above quantities of air 
and carbonic acid would pass by or through the foliage. 
It is plain, then, that the atmosphere, which is rarely at 
rest, can supply carbonic acid abundantly to foliage with- 
out the concourse of the soil. At the same time it should 
not be forgotten that the carbonic acid of the atmosphere 
is largely derived from the soil. 
Carbonic Acid in the Water of the Soil.—Notwith- 
standing the presence of so much carbonic acid in the air 
of the soil, it appears that the capillary soil-water, or so 
* A square field containing one acre is 208 feet and a few inches on each side, 
