6 GENERAL BIOCHEMISTRY 



fundamental principles of plant nutrition. In the days of De Saussure, 

 however, his opinions were not accepted as true, and it was nearly 

 half a century later that Boussingault, Liebig, and others repeated the 

 investigations of De Saussure and confirmed his results, which then 

 were finally accepted by the scientific world. 



Between 1802 and 1812 Sir Humphry Davy delivered lectures an- 

 nually on agricultural chemistry. These lectures were published in 

 book form in 1813 and entitled Essentials of Agricultural Chemistry. 

 This was the first textbook of the modern period; it treated of the 

 composition of the air, soil, manure, and plants, and of the influence 

 of heat and light on plant growth. Although some of the views ex- 

 pressed in this work were incorrect, it was for the most part a carefully 

 prepared summary of the accepted knowledge obtained from the re- 

 sults of previous investigations. 



Thaer also published an important work entitled Principles of Ra- 

 tional Agriculture, in which he proposed the so-called humus theory of 

 soil fertility. He believed that plants obtain their nourishment from 

 the humus, and consequently it is this material which determines the 

 productivity of the soil. Thaer's idea, however, was shown to be in- 

 adequate in accounting for the sources of plant food, and it prevented, 

 for a long time, the recognition of the actual value of humus as a fac- 

 tor in soil fertility. 



About 1830 Boussingault became actively engaged in agricultural 

 research and began a series of field experiments on his farm in Alsace. 

 He was the first investigator to have a chemical laboratory on a farm 

 and to make thoroughly practical investigations in connection with 

 agriculture. The establishment of this laboratory marked the start 

 of the first agricultural experiment station. In his work Boussingault 

 is credited with reintroducing the quantitative methods of De Saus- 

 sure. He also did very important work upon the assimilation of the 

 free nitrogen of the air by plants. He ascertained many important 

 facts relating to the chemical characteristics of foods, made a compari- 

 son as to the quantity of nitrogen in different kinds of feeding stuffs, 

 and compared their values on the basis of the nitrogen content. His 

 study on the production of saltpeter did much to prepare the way for 

 later work on nitrification. 



In 1840, Justus von Liebig's report to the British Association (pub- 

 lished later as Chemistry in Its Application to Agriculture and Physi- 

 ology) stimulated a great amount of discussion in scientific circles. Von 

 Liebig delighted in taking plant physiologists to task for their lack of 

 chemical knowledge and for ignoring the accumulating experimental 

 evidence that plants receive their carbon supply from the atmosphere 



