viii ' PREFACE 



an acre rent, another has to get twice as much out of 

 his land before he touches a profit ; one man's markets 

 are such that he can repay himself for an outlay of £^ 

 an acre for fertilisers on his root area, whereas another 

 man could not afford 20s. ; no one recipe can be 

 handed out to suit all these different men. 



The object, then, of the scientific man should be to 

 lay down principles which the practical man in his 

 turn must learn to apply to his own conditions ; success 

 is only possible when he too does some thinking. 

 Furthermore, the object of experiments should be to 

 provide knowledge that can be thus applied to other 

 conditions, and an experiment is practical just in so far 

 as it carries out its avowed object, which is to lead 

 men into a sound and fruitful way of thinking on 

 the question at issue. 



It is in this respect — the elucidation of general 

 principles — that the Rothamsted experiments have 

 proved so exceedingly valuable ; though initially laid 

 out to test certain definite questions about the 

 nutrition of crops, the answers to which have long 

 since been absorbed into farming practice, the design 

 was so sound and the continuity of the record has 

 been so rigorously maintained that the results now 

 afford an instructive commentary on the whole 

 range of the science of crop production. We have by 

 no means come to the end of the lessons the Rotham- 

 sted experiments can teach : every new theory, each 

 extension of our knowledge, finds an unsuspected 

 criticism or an illustration in the records that are still 

 accumulating. 



I have in consequence throughout this book used 

 very freely the results of the Rothamsted experi- 



