PREFACE TO THE NEW EDITION. 



THE second impression of this book was called for so soon 

 after the first was published that there seemed no occasion to 

 make any alterations beyond the correction of a few misprints, 

 but now that a third is required the opportunity is taken of 

 adding a new chapter on the relationship between the micro- 

 organic population of the soil and the growth of plants, and 

 also numerous sections dealing with recent developments of 

 other parts of the subject. It is satisfactory to be able to 

 record that continued progress has been made, and that some 

 of the difficulties that obstructed the way when the book was 

 first written have now been overcome. 



In this revision I have been greatly helped by Dr. Hans 

 Brehm, who translated the first Edition into German, and who 

 very kindly placed at my disposal his complete and critical 

 knowledge of the continental work on the subject. 



ROTHAMSTED EXPERIMENTAL STATION, 

 HARPENDEN, 1915. 



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