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IOWA ACADEMY OF SCIENCE Vor. XXIX, 1922 
large quantities of carbon dioxide for an appreciable time after 
sterilization and there is still a measurable amount given off up 
to twelve weeks. On the other hand unsterilized soils will give 
off amounts decidedly in excess of the sterilized. This indicates 
very definitely the possibility of a joint chemical and biological 
action of the agencies in the soil responsible for the decomposition 
of the organic matter and the liberation of its plant food con- 
stituents. 
The soil itself may be considered as a great reservoir that 
ultimately receives all plant and animal life living upon or within 
it. Also it may be considered as a culture medium for the very 
numerous and complex microscopic population that habitually lives 
in it. But viewing the soil from the standpoint of its crop pro- 
ducing power we find that we are not so much interested in the 
particular kinds of microorganisms living in it as we are in the 
results of the activities of these organisms. We cannot measure 
these results except by chemical methods, hence the soil biologist 
studies his problems by chemical means through the use of micro- 
biological technic. These problems involve primjarily the deter- 
mination of the activities of the soil microflora in liberating or 
producing those elements essential for plant growth either wholly 
or in part from the decomposable organic matter in the soil. 
In describing the various microbiological activities in the soil, 
the soil biologist uses a number of terms that more or less describe 
the end point of the chemical reaction involved but do not indicate 
how the action has been accomplished. For instance he uses the 
term “Ammonification” to describe the reduction of higher nitro- 
gen compounds to ammonia and “nitrification” to describe the 
oxidation of ammonium compounds to nitrates. The term “azo- 
fication” describes the fixation of elementary nitrogen from the 
air. Whether it is directly assimilated or is acted upon before 
assimilation is not known ; most assuredly the elementary nitrogen 
is oxidized at some stage in the process. The term “rhizofication” 
is used to designate the fixation of nitrogen in the roots of the 
legume. Evidently the organism has some ability that corresponds 
closely to the activities of the Azofying organism, yet it has a 
different name. If we look at the names we find that the terms 
“azo” and “nitra” have the same meaning while the term “rhizo” 
means root. The first two terms mean practically the same thing 
while the latter means simply that the root has the same ability 
to fix nitrogen as the infecting organism. 
On the other hand the chemist has definite and exact reactions to 
