208 DRY-FARMING 
forms about one fifth of the atmosphere and is the 
active principle in combustion and in the changes in 
the animal body occasioned by respiration. Oxygen 
should be present in the soil air in approximately the 
.. proportion in which it is found in the atmosphere. 
Germination is hindered by a larger or smaller pro- 
portion than is found in the atmosphere. The soil 
must be in such a condition that the air can easily 
enter or leave the upper soil layer; that is, the soil 
must be somewhat loose. In order that the seeds 
may have access to the necessary oxygen, then, sow- 
ing should not be done in wet or packed soils, nor 
should the sowing implements be such as to press the 
soil too closely around the seeds. Well-fallowed soil 
is in an ideal condition for admitting oxygen. 
If the temperature is right, germination begins by 
the forcible absorption of water by the seed from the 
surrounding soil. The force of this absorption is 
very great, ranging from four hundred to five hun- 
dred pounds per square inch, and continues until the 
seed is completely saturated. The great vigor with 
which water is thus absorbed from the soil explains 
how seeds are able to secure the necessary water 
from the thin water film surrounding the soil grains. 
The following table, based upon numerous investiga- 
tions conducted in Germany and in Utah, shows the 
maximum percentages of water contained by seeds 
when the absorption is complete. These quantities 
are reached only when water is easily accessible : — 
