INTRODUCTION 3 



believed that the nitrogen of the soil was the main 

 source of the nitrogen found in plants. These views 

 have since been verified by many investigators, and are 

 substantially those held at the present time regarding 

 the fundamental principles of plant growth. They were 

 not, however, accepted as conclusive at the time, and it 

 was not until nearly a half-century later, when Bous- 

 singault, Liebig, and others repeated the investigations 

 of De Saussure, that they were finally accepted by chem- 

 ists and botanists. 



From the time of De Saussure to 1835, scientific 

 experiments relating to plant growth were not actively 

 prosecuted, but the^ facts which had accumulated were 

 studied, and attempts were made to apply the results 

 to actual practice. Among the first to see the relation 

 between chemistry and agriculture was Sir Humphry 

 Davy. In 1813 he published his "Essentials of Agri- 

 cultural Chemistry," which treated of the composition 

 of air, soil, manures, and plants, and of the influence 

 of light and heat upon plant growth. About this 

 period, Thaer published an important work entitled 

 " Principes Raisonnes d' Agriculture." He believed 

 humus determined the fertility of the soil, that plants 

 obtained their food mainly from humus, and that the 

 carbon compounds of plants were produced from the 

 organic carbon compounds of the soil. This gave rise 

 to the so-called humus theory, which was later shown 

 to be an inadequate idea regarding the source of plant 

 food, and for a time it prevented the actual value of 



