PREFACE TO THE THIRD EDITION. 



CONSIDERABLE alterations have been made in the text 

 and a new chapter has been added discussing the col- 

 loidal properties of the soil. It is abundantly clear that 

 the soil investigator of the future will have to be 

 thoroughly familiar with the ways of colloids, and I fully 

 expect that much of the older work will require careful 

 re-examination in the light of what has been done in 

 this direction by chemists and physicists. 



Although the volume has necessarily expanded I 

 have tried to keep it as a monograph : I have not 

 attempted to turn it into an extended card index by re- 

 ferring to every paper published on the subject since the 

 first edition came out. Many papers have been omitted ; 

 the guiding principle has been to include only those that 

 brought in some new idea or profoundly modified an old 

 one. Some of the papers omitted from the last edition 

 have been included in this because they now fall into 

 their place, while before they did not. Doubtless this 

 will happen again. 



Continued progress is being made. Since the book 

 was first begun two Journals have sprung up devoted 

 entirely to soil : Soil Science, under the editorship of 

 the indefatigable J. G. Lipman, and the International 

 Mitteilungen fur Bodenkunde. Another Journal, the 

 Journal of Ecology, has also arisen and is vigorously 

 developing another aspect of the same subject, while the 

 older agricultural journals are finding more and more of 

 their space taken up by soil papers. The subject now 

 only lacks a name, and though many have been pro- 

 posed pedology, agrogeology, edaphology, etc. I have 

 not felt drawn to any of them. 



