INTRODUCTION 3 



been verified by many investigators, and are substan- 

 tially 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 half a century later, when 

 Boussingault, Liebig, and others repeated the investi- 

 gations of DeSaussure, that they were finally accepted 

 by chemists and botanists. 



From the time of DeSaussure to 1835, scientific 

 experiments relating to plant growth were not actively 

 prosecuted, but the scientific facts which had accumu- 

 lated 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 " Es- 

 sentials of Agricultural 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 same period, Thaer published an important 

 work entitled " Principes Raisonnes d' Agriculture." 

 Thaer believed that 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 re- 

 garding the source of plant food, and for a time it 

 prevented the actual value of humus as a factor of soil 

 fertility from being recognized. The writings of Thaer 

 were of a most practical nature, and they did much to 

 stimulate later investigations. 



