B 



growing without any direct supply of it by manure, must be derived, 

 in some way or other, from atmospheric sources. 



The assumption which is most in favour with some prominent 

 writers is, that whilst some plants derive most or all of their nitrogen 

 from the stores of the soil itself, or from manure applied to it, others 

 derive a large proportion from the free nitrogen of the atmosphere. 

 We, on the other hand, whilst freely admitting that the facts of 

 production are not conclusively explained thereby, have maintained 

 that such collateral evidence as the determinations of nitrogen in our 

 soils afford, is in favour of the supposition that the soil may be the 

 source of the otherwise unexplained supply of nitrogen. This latter 

 conclusion we have frequently stated in general terms ; but we have 

 not hitherto published the numerical results upon which it is based. 

 Fairly enough, it has been objected that such an important conclusion 

 cannot be accepted without the numerical evidence to support it. 

 Further, erroneously interpreting our statements, calculations have 

 been made to show that it is quite beyond the reach of present 

 methods of determination of nitrogen in soils to afford results 

 justifying the conclusions we have drawn. 



Since this subject of the sources of the nitrogen of our crops 

 has been much discussed in America, it has been thought that it 

 would not be inappropriate tO answer the challenge by bringing for- 

 ward some of the numerical evidence we have accumulated before 

 this meeting of the American Association for the Advancement of 

 Science, and to do this is the object of the present communication. 



Before calling attention to the special results in question, it will 

 be necessary, in order to convey a clear idea of the problem to be 

 solved, to recapitulate some of the important facts which have been 

 established as to the amount of nitrogen yielded over a given area by 

 different crops. 



In his original inquiries, Boussingault estimated the amounts of 

 nitrogen supplied by manure, and removed in the crops, in ordinary 

 agricultural practice. This mode of estimate is also the one generally 

 adopted by others, and we have ourselves not neglected it. But it is 

 obvious that the results of experiments in which different crops have 

 been grown for very many years in succession on the same land, both 

 separately and in an actual course of rotation, and both without nitro- 

 genous manure and with known quantities of such manure, must 

 afford very important data as to the amounts of nitrogen available to 

 vegetation, from soil and atmosphere, over a given area. The 

 Biothamsted field experiments are pre-eminently adapted to provide 

 such data. Thus, wheat has now been grown for thirty-nine years 



