PREFACE TO THE SECOND EDITION 



A CONSIDERABLE number of additions and alterations 

 have been incorporated in the present edition. These 

 include a revision of the method recommended for the 

 mechanical analysis of soils, the method now given 

 being that adopted by the members of the Agricultural 

 Education Association in this country. ' Owing to re- 

 searches which have appeared since the publication of 

 the first edition, I have greatly modified the views I 

 then expressed on the nature of clay, and on the part 

 played by zeolitic silicates in the retention of ammonium 

 and other salts by the soil During the last six years, 

 however, the greatest additions to our knowledge of the 

 soil are those dealing with bacteria; in consequence, 

 the chapter on the living organisms of the soil has been 

 largely rewritten and added to. A number of minor 

 corrections have been made in the text, some of which 

 represent the removal of errors, and others modifications 

 due to more recent research. For the mistakes which 

 must still remain, and which will become evident in the 

 course of time, I must ask my readers' pardon before- 

 hand ; in dealing with so complex a subject as the soil 

 we are still far from final conclusions, many of our most 

 trusted conclusions are only rough approximations to 

 the truth, and by the progress of research they may at 

 any time require remodelling until they are hardly 



recognisable. 



A. D. HALL. 



The Rothamsted Experimental Station, 

 May 1908. 



