On the Sources of the Nitrogen of Vegetation. 109 



and conclusions of others which have recently been put forwarJ, as 

 above alluded to. 



In our earlier papers we had concluded that, excepting the small 

 amount of combined nitrogen annually 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 

 comparable conditions without a crop. In the case of gramineous 

 crops the evidence pointed to the conclusion that most, if not the 

 whole, of their nitrogen was taken up as nitric acid. In the experi- 

 ments with leguminous crops the evidence was in favour of the suppo- 

 sition that, in some cases, the whole of 'the nitrogen had been taken 

 up as nitric acid, whilst in others that source seemed to be 

 inadequate. 



It was further shown that, under otherwise parallel conditions, 

 there was much more nitrogen as nitric acid in soils and sub- 

 soils 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 influence 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, at every depth of 9 inches down to a 

 total depth of 108 inches, the Vicia soil contained much less nitric 

 acid than the Trifolium repens soil ; and it was concluded that much 

 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 experiments 

 with Trifolium repens as a shallow rooting and meagrely yielding 

 plant, to Helilotus leucantha as a deeper rooting and freer growing 

 one, and to Medicago sativa as a still deeper rooting and still freer 

 growing plant, very strikingly illustrated and confirmed the result of 

 the exhaustion of the nitric acid of the subsoil by the strong, deep- 

 rooting, and high nitrogen-yielding Leguminosaa. For example, at 

 each of the twelve depths of the Medicago soil there remained very 

 much less nitrogen as nitric acid than where very much less 

 nitrogen had been removed in the Trifolium repens crops ; there being 

 on the average not one-twelfth as much in the lower ten depths 

 of the Medicago soil as in the corresponding depths of the Tri- 

 folium repens soil. Still, the figures did not justify the conclusion 

 that the whole of the large amount of nitrogen taken up by the 



