QUESTION OF THE SOURCES OF THE NITROGEN OF VEGETATION. 
7 
The difference is the greatest near the surface, but it is very considerable down to 
the lowest depth. Hence it is obvious that any loss by drainage would be much the 
greater from the Trifolium plot, and so the difference between the two plots was 
probably in reality greater than the figures show. In both cases the actual amount is 
the greatest near the surface, indicating more active nitrification ; and the excess is 
much the greater in the Trifolium ropens soil, doubtless due to more nitrogenous crop- 
residue from the leguminous than from the gramineous crop. Indeed, about 74 lbs. 
of nitrogen had been removed in the Trifolium repens crop in 1882, and none in 1883, 
the year of the soil collections. On the other band, only about one-fourth as much 
was removed in the wheat crop of 1882, and the land was fallow in 1883. Unless, 
however, there was considerably more nitrogen in the crop-residue than in the 
removed crops of the Trifolium repens, the excess of 93 lbs. of nitrogen as nitric 
acid found in the Trifolium repens soil, together with the increased amount lost by 
drainage, could not have had its source entirely in the nitrification of recent nitro¬ 
genous crop-residue. Some of the increased amount in the lower layers was indeed 
doubtless due to washing down from the surface. But as, notwithstanding much 
more nitrogen had been removed in crops from the leguminous than from the 
gramineous crop soil during the previous 30 years, the surface soil of the leguminous 
plots remained slightly richer in nitrogen than that of the gramineous plot, it cannot 
be supposed that the whole of the nitrogen of the crop, and of the nitric acid found, 
had its origin in the surface soil. If, therefore, nitrogen has not been derived from 
the atmosphere, the conclusion must be that some has come from the subsoil. 
The indication was that nitrification had been more active under the influence of 
the leguminous than of the gramineous growth and crop-residue. In fact, under the 
influence of leguminous growth, not only will there be increased nitrogenous matter 
for nitrification, but it would seem that the development of the nitrifying organisms 
will be favoured. The question is, therefore, whether part of the result be not due 
to the passage downwards of the nitrifying organisms, and the nitrification of the 
nitrogen of the subsoil. 
The alternative was suggested, that the soil and subsoil might still be the source of 
the nitrogen of the crops, but that the plants may take up, at any rate part of their 
supply, in other forms than as nitric acid—as ammonia, or as organic nitrogen, for 
example. It was pointed out that fungi do take up both organic carbon and organic 
nitrogen; but that, whilst existing direct experimental evidence was conflicting as to 
whether green leaved plants even assimilate carbon taken up by their roots as carbonic 
acid, the evidence was even less conclusive as to whether they take up either organic 
carbon or organic nitrogen as such from the soil. To this question we shall recur 
further on. 
The next point is to compare the amount of nitrogen as nitric acid found in the 
Vida saliva soils with that in the Trifolium repens soil. In the first place it is to 
be observed, that whilst from the Trifolium repens plot only 164 lbs. of nitrogen had 
