THE SOURCES OF THE NITROGEN OF VEGETATION, ETC. 



435 



and conclusions of Boussingault, and others, in connexion with this question, from the 

 date above mentioned up to the present time, we shall have to refer pretty fully further 

 on. It may here be mentioned, however, that already at that early period Bous- 

 SINGAULT had so far advanced in his inquiries into the chemical statistics of certain agri- 

 cultural practices on the large scale, as to be apparently led by them to see the import- 

 ance of investigating much more closely the sources of the Nitrogen periodically yielded 

 by a given area of land, over and above that which was artificially supplied to it. 



We fully admit the pertinence of the considerations, and the sagacity of the observa- 

 tions adduced on this head, more than twenty years ago, by Boussingault. It will, 

 nevertheless, be well to preface the discussion of our own experimental evidence regard- 

 ing the sources of the nitrogen of plants, by the statement of a few prominent and 

 striking facts, established by investigations conducted here, at Kothamsted, illustrative 

 of the amounts of nitrogen yielded by different crops over a given area of land, and of 

 the relation of these amounts to certain measured, or known, sources of it. Of these 

 points, however, we profess to speak only in a very brief and summary manner on the 

 present occasion. The discussion in detail, of the evidence relating to them, would 

 indeed itself exhaust the limits of our Paper. Moreover, we have already treated of 

 this subject in a separate form, elsewhere*; and it is our intention to consider it much 

 more fully at some future opportunity. 



Section II.— ANNUAL YIELD OF NITROGEN PER ACRE, IN DIFFERENT CROPS f. 



A. — Yield of Nitrogen per eicre vohen the same Crop is grown year after year 



on the same Land. 



The following Summary Table shows the average annual amounts of nitrogen yielded 

 per acre, in the crops enumerated, when each was grown for a number of years con- 

 secutively on the same land, without manure. 



Table I. 



There were obtained, then, in each of the Cereal crops (wheat and barley) about 

 24 1 lbs. of Nitrogen per acre, per annum, without manure. In the case of each of the 

 crops the land was, in an agricultural sense, exhausted at the commencement of the 



* British Association for the Advancement of Science, Leeds Meeting (1858), Section B. 

 t The results given in this Section have been revised, and in some cases the periods over which the 

 estimates are taken extended, since the reading of the Paper. 



