PREFACE. 



HAVING had occasion to study somewhat closely the work 

 of the Rothamsted investigators, it occurred to me that 

 their valuable memoirs, dealing as they so largely do 

 with actually ascertained results, might be advan- 

 tageously condensed into the form of a text-book. 

 Hence arose the present volume which, though it is 

 concerned with a portion only of the many questions 

 that have been brought within the range of experimental 

 inquiry at Rothamsted, yet deals with subjects of first 

 class importance, equally in their scientific bearing as in 

 their economic aspect. To the student the discussion of 

 concrete results should prove at least as useful as the 

 consideration of abstract assertions, whilst it is possible 

 that it may be even more suggestive. 



In the endeavour to make each subject as far as 

 possible complete in itself, a certain amount of repetition 

 has been unavoidable. Many questions, again, which 

 are but lightly touched upon in these pages, are more 

 fully dealt with in other of the Rothainsted memoirs 

 than those which the writer has laid under contribution. 

 Even as this work is passing through the press a fresh 

 memoir issuing from Rothamsted throws further light 

 upon the classical inquiry as to the sources of the 

 nitrogen of vegetation. 



