139 
Berthelot shows that vegetable soils continually absorb nitrogen 
from the air, and very much more than exists in the air as ammonia 
or nitrogenous compounds, so that it must be taken directly from 
the free nitrogen and this, too, although the soil contains no growing 
vegetables. (Agr. Sci., Vol. I, p. 120.) Apparently this absorption 
is the work -of the microbes preparing the soil for-future plant growth, 
and much of the irregularity in our crop reports depends not upon 
the climate or the fertilizer, but upon the activity of this form of life. 
Berthelot (1887) shows that the fixation of gaseous nitrogen of the 
atmosphere by the soil takes place continually even when no vegeta- 
tion is presented and that it is greater in soil exposed to rain than in 
soil protected from the rain, this being undoubtedly due to the fact 
that in the exposed soil the minute forms of life by means of which 
nitrogenous compounds are formed can operate more intensely because 
of the greater quantity of air dissolved in and carried down to them 
by the rain. (See Wollny, X, p. 205.) 
_A parallel investigation by Heraeus shows that probably the bac- 
teria may be divided into two classes—those which oxidize and those 
which reduce the oxides, and that in general where an abundance of 
nutrition exists, as in rich soils, the reducing bacteria are in excess, 
and that, on the contrary, where these do not find a sufficiently favor- 
able soil there the oxidizing bacteria have the upper hand. 
Salkowsky (1887), as the result of his own experiments, considers 
it indubitably established that processess of oxidation in water can 
only be due to the vital activity of bacteria, and that this is equally 
true of water permeating the soil, and therefore of the oxidation 
processes in the soil itself. 
Warington (1887), having shown that the process of nitrification 
goes on by means of organisms that are rather uniformly distributed 
at the surface, and that they are less frequent at depths of 9 and 18 
inches, depending on the porosity of the soil, and that none could be 
found at depths of from 2 to 8 feet, has now revised these early 
experiments and finds a few nitrifying bacteria at depths at from 5 
to 6 English feet, but that in general they are less numerous and 
have a feebler activity the deeper they are in the earth. Under 
natural conditions nitrification occurs principally in the highest layer 
of soil, because the conditions of this process—viz, accessibility of the 
air and quantity of nitrogenous compounds—are more favorable here 
than in the lower strata. (See Wollny, X, p. 211.) 
As our views as to the relation of the nitrogen of the atmosphere to 
vegetation have been entirely remodeled within the past five years, 
the following summary by Maquenne (1891) has been selected as 
showing the slow progress of our knowledge up to the brilliant suc- 
cess of Hellriegel and Wilfarth. 
