QUESTION OP THE SOURCES OF THE NITROGEN OF VEGETATION. 
37 
the gradation of effect under the varying conditions as to strength, time, and 
temperature, and in the evidence as to the proportion of the total nitrogen taken up, 
which is found as ammonia. The proportion of ammonia-nitrogen to amide-nitrogen 
ranges from about I to 5 to 1 to 3, dependent on tlie conditions. According to the 
figures, it would seem that the proportion of the total nitrogen dissolved which is 
determined as ammonia is the greater, the stronger the acid and the longer the 
contact. 
As in Loges’ experiments, so in these of MM. Berthelot and Andre, the strength 
of acid used was in all cases much greater than that in any of the Rothamsted experi¬ 
ments, and very much greater tlian is likely to occur in any root-sap. Indeed, not 
only was the soil operated upon by MM. Berthelot and Andre about four times as 
rich in nitrogen as the Rothamsted subsoils, but in the most extreme case, that with 
the strongest acid, and a temperature of 100° G., nearly one-third of the total nitrogen 
of the soil was dissolved. Hence, although their results are of great interest as 
indicating the character of the nitrogenous bodies existing in soils, and of the changes 
to which they are subject when acted upon by acids, they, like those of Loges, have 
only an indirect bearing on the question whether by the action of the organic acids of 
the root-sap, the insoluble organic nitrogen of the soil, and especially of the subsoil, 
is rendered available as a source of nitrogen to the plant, Supposing this to be the 
case, as already said, the further question still remains—whether the dissolved amide is 
taken up as such, or whether it is subject to further change within the soil before 
serving as food for the plant ? 
The fact that the formation of ammonia seems to be an essential element in the 
reaction, points to the conclusion that at any rate part of the nitrogen liberated from 
the insoluble condition is available in other forms than as soluble or dissolved amide ; 
and, as our experiments show that nitric acid, as well as ammonia, is a constituent 
of the root-sap, the question arises—whether the liberated ammonia is oxidated 
into nitric acid before being taken up Then, again, is the soluble amide taken up 
as such, or subjected to further change, perhaps first yielding ammonia, and this again 
nitric acid ? 
On this supposition we are met again with the difficulty as to the sufficient aeration 
of the subsoil for such a purpose. It has already been pointed out, that such evidence 
as exists on the subject clearly shows that the amount of oxygen vfithin the soil at 
any one time is totally inadequate for the nitrification of the amount of nitrogen taken 
up by some plants within the season ; whilst the replacement by air of the Avater 
evaporated would still be quite insufficient. With what rapidity, or to Avhat extent, 
the oxygen of the subsoil air would be replenished from above as it is used up, there 
is no exnerimental evidence at command to show. But Avhether this would take 
i. 
place adequately or not, it must be supposed that it would occur to some extent. 
Turning to the alternative aspect, inasmuch as the insoluble organic nitrogen of the 
soil exists in the condition of amide bodies, and the chief first |)roduct of the action of 
