PREFACE. 



, Bacteriology lias contributed, "within the past few years, 

 eome important facts to our rapidly expanding knowledge 

 of soils. It has shed a new light on the transformation and 

 accumulation of nitrogen compounds in tilled land, as well 

 as in .uncultivated forest and prairie soils. It has taught us 

 to understand more clearly the decomposition of cellulose and 

 allied compounds, and the conservation of the potential en- 

 ergy of hydrogen and of methane. It has given the student 

 of soils and of soil fertility a number of new methods for 

 measuring differences in the quality and quantity of plant- 

 food. , It bids fair to supply us with still other methods that 

 would be more adequate than any chemical methods yet de- 

 vised, for the investigation of fundamental problems of soil 

 fertility. 



In view of the past achievements of soil bacteriology and 

 of the promise of even greater usefulness in the future, no 

 apology is needed for the publication of this little volume. 

 The authors have felt, for some time, that there is room in 

 every well-arranged course in soils for a series of exercises 

 in soil bacteriology, and have, therefore, made the attempt 

 to provide them. It is their hope that after preliminary in- 

 struction in general bacteriology and laboratory technique 

 the "Guide" will prove useful to students of soils, and that 

 time and service will permit the elimination of the imper- 

 fections and inadequacies that undoubtedly exist in it. 

 Toward this end, friendly criticism will be appreciated. 

 Meanwhile, the authors wish to express their indebtedness 

 ro Dr. E. E. Buchanan, of the Iowa State College, for many 

 helpful suggestions. 



