INTRODUCTION 9 



While underestimating the value of manure, he has 

 shown the importance of thorough tillage of the soil 

 more clearly than had ever been done before. "The 

 Horse-Hoeing Husbandry" by Jethro Tull is worthy 

 of careful study by all agricultural students. 



During recent years the agricultural experiment 

 stations of this and other countries have made soils a 

 prominent feature of their work. Some of the results 

 obtained are noted in the following chapters. Our 

 knowledge regarding the chemistry, physics, geology, 

 and bacteriology of soils is still far from complete, 

 but many facts have been discovered which are of the 

 greatest value to the practical farmer. The literature 

 relating to soils and fertilizers has become very exten- 

 sive, and in the classification of agricultural subjects 

 for study, the soil forms one of the main divisions of 

 agronomy. 



In soil investigations it has frequently happened, 

 owing to imperfect interpretation of results and to 

 the presence of many modifying influences, that the 

 conclusions of one investigator appear to be directly 

 contradictory to those of another. This is well illus- 

 trated in the investigations relating to the assimilation 

 of free atmospheric nitrogen, where seemingly opposite 

 conclusions now form a complete theory. 



A scientific study of soils is valuable from an educa- 

 tional point of view, as well as because the practical 

 knowledge obtained can be utilized in the production 

 of crops. In the cultivation of soils, complicated physi- 



