QUESTION OF THE SOUECES OF THE NITROGEN OF VEGETATION. 
89 
PART III. 
Summary, and General Considerations and Conclusions. 
It seems desirable to endeavour to summarise the results, both experimental and 
critical, of this extended inquiry, relating to a very difficult and very complicated 
subject, and involving the consideration of very conflicting evidence, and of equally 
conflicting opinions in regard to it. 
We will first give a resume of the results, and conclusions, as given in Part I. of 
this paper, on— 
1. The Evidence relating to other Sources than Free Nitrogen. 
In our earlier papers we had concluded that, excepting the small amount of 
combined nitrogen coming down in rain and the minor aqueous deposits from the 
atmosphere, the source of the nitrogen of our crops was, substantially, the stores 
within the soil and subsoil, whether derived from previous accumulations, or from 
recent supplies by manure. 
More recently, we have shown that the amount of nitrogen as nitric acid in the soil, 
was much less after the growth of a crop than under corresponding conditions without 
a crop. In the case of gramineous crops it was concluded that most if not the whole 
of their nitrogen was taken up as nitric acid. In the experiments with leguminous 
crops the evidence indicated that, in some cases the whole of the nitrogen had been 
taken up as nitric acid, but that in others that source seemed to be inadequate. 
It has been further shown that, under otherwise parallel conditions, there was very 
much more nitrogen as nitric acid in soils and subsoils, down to a depth of 108 inches, 
where leguminous than where gramineous crops had for some time been grown. The 
indication was, that nitrification had been more active under the infiuence of 
leguminous than of gramineous growth and crop residue. 
At the same time, comparing the amounts of nitrogen as nitric acid in the soil 
where the shallow rooting Trifolium repens had previously been grown, with those 
where the deeper rooting Vicia sativa had yielded fair crops, it was found that, 
down to a depth of 108 inches, the Vicia soil contained much less nitric acid than 
the Trifolium reyens soil; and it was concluded that most, if not the whole, of the 
nitrogen of the Vicia crops had been taken up as nitric acid. 
New results of the same kind, which related to Trifolium repens as a shallow 
rooting and meagrely yielding plant, to Melilotus leucantha as a deeper rooting and 
freer growing one, and to Medicago sativa as a still deeper rooting and still freer 
MDCCCLXXXIX.—B. 
N 
