QUESTION OF THE SOURCES OP THE NITROGEN OF VEGETATION. 
101 
some cases small, and in others very large, but the modes of explanation are so 
different, indeed so conflicting, that it seems essential to hold final judgment in 
abeyance for the present. 
The various modes of explanation of the observed gains of nitrogen are:—that 
combined nitrogen has been absorbed from the air, either by the soil or by the plant; 
that there is fixation of free nitrogen within the soil by the agency of porous and 
alkaline bodies; that there is fixation by the plant itself; that there is fixation within 
the soil by the agency of electricity; and finally that there is fixation under the 
influence of micro-organisms within the soil. The balance of the evidence recorded, is 
undoubtedly in favour of the last-mentioned mode of explanation. Indeed, it seems 
to us that, if there be not experimental error, there is fixation within the soil, under 
the influence of micro-organisms, or other low forms. 
Assuming that definite decision on the point must wait for further evidence and 
discussion, it will nevertheless be well, in the meantime, to consider the facts of 
agricultural production in their bearing on the question, with a view of forming a 
judgment as to how far the establishment of the reality of the fixation of free nitrogen, 
either by the plant or by the soil, is so essential for the solution of the problems which 
such production presents, as is by some supposed. 
It has been seen that much of the investigation that has been undertaken in recent 
years, has been instigated by the assumption that there must exist natural compensa¬ 
tion for the losses of combined nitrogen which the soil suffers by the removal of crops, 
and for the losses which result from the liberation of free nitrogen from its combina¬ 
tions under various circumstances. In some cases, however, the object seems to have 
been for the most part limited to an attempt to solve the admitted difficulty as to the 
explanation of the source of the whole of the nitrogen of the Leguminosse. 
As to the losses which the soil sustains by the removal of crops, Berthelot for 
example assumes that 50 to 60 kilog. of nitrogen will be annually removed from a 
hectare of meadow (= 45 to 54 lbs. per acre), and that as only 10 kilog., or less, of 
this will be restored as combined nitrogen in rain, &c., there will be an annual loss of 
from 40 to 50 kilog. per hectare (= 36 to 45 lbs. per acre); so that, if there were not 
compensation from the free nitrogen of the air, the soil would become gradually 
exhausted. Further, he considers that the fact of the fixation of free nitrogen, not 
only explains how fertility is maintained, but how argillaceous soils which are sterile 
when first brought into contact with the air, gradually yield better crops, and at 
length become vegetable moulds. Frank again, assumes that the average loss of 
nitrogen by the removal of crops is 51 kilog. per hectare (=45 lbs. per acre). 
It is quite true, that a good hay crop may contain as much as 50 to 60 kilog. of 
nitrogen per hectare, but it may safely be affirmed that, in ordinary practice, even in 
the case of an unusually fertile meadow, such an amount is not annually removed for a 
number of years in succession, without the periodical return of manure supplying 
nitrogen; whilst, taking the average of soils, the annual yield will not reach the 
