30 



atmosphere in the open country are entirely insufficient to do more 

 than supply a small proportion of the nitrogen assimilated by crops so 

 grown. 



With regar '^'^her possible supplies of already combined nitrogen 

 from the atmoH^ e to the soil, it has been pointed out that there is 

 no direct quantitative evidence at command, and that such evidence 

 as docs exist leads to the conclusion that such supplies are very limited 

 and inadequate. 



The same may be said, even in a greater degree, of the supposed 

 combination of the free nitrogen of the air within the soil ; also of 

 the supposition that plants take up any material jjroportion of their 

 nitrogen from combined nitrogen in the atmosphere by their leaves. 



Finally, it has been concluded that the balance of direct experi- 

 mental evidence is decidedly against the supposition that plants 

 assimilate the free nitrogen of the atmosphere. Indeed, the strongest 

 argument that we know of in favour of such a supposition is that, in 

 defect of other conclusive evidence, some such explanation of the 

 facts of production would seem to be needed. 



W'i 



The Nitrogen of the Soil as a Soukce of the Nitrogen of 



Crops. 



We now turn to that part of the subject which it is the special 

 object of this communication to bring furward, namely, the determi- 

 nations of nitrogen in the soils of some of the experimental fields at 

 Rothamsted, the yield of nitrogen in which has been given, and to 

 show the bearing of the results on the question of the sources of the 

 nitrogen of the crops. 



We have no wish or intention to ignore the difficulties inherent in 

 the treatment of the subject from this point of vicAv. The difficulty 

 of the problem will at once be recognised when it is borne in mind 

 that a difference of 0"001 in the percentago of nitrogen in the dry soil 

 may represent a difference of from 20 to 25 pounds of nitrogen per 

 acre in a layer of 9 inches in depth. Again, it is farther to be borne 

 in mind that, in the case of the Rothamsted arable soils with which 

 we have to deal, the percentage of nitrogen in the first 9 inches of 

 depth is sometimes only about 0"1, and seldom exceeds 0'14 or 0'15 ; 

 that in the second 9 inches it ranges from under 0'07 to little over 

 0-08; in the third 9 inches from under 006 to about 0'07; and that 

 in the lower depths is rather lower still. 



It will be seen, therefore, that if any quantitative estimates are to be 

 based on the percentage amounts of nitrogen determined in samples 



