8 



due to the carbon dioxide of the air and water of the soil. He be- 

 lieved that the nitrogen of the soil was the chief source of the 

 nitro-gen found in plants. Unfortunately his conclusions were 

 not accepted at that time, and it was not until about fifty years 

 later, when other investigators had repeated his experiments, that 

 his results were finally accepted by botanists and chemists. 



One of the first investigators to see the relation between chem- 

 istry and agriculture was that of Sir Humphry Davy (1813), who 

 published a book entitled, "Elements of Agricultural Chemistry." 

 This treated of the composition of air, soil, manures, plants, and 

 of the influence of heat and light upon the growth of plants. 



Thaer (1809-10) contended 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. These ideas gave rise to his 

 so-called humus theory, which was later shown to be inadequate. 

 His writings, however, did much to stimulate later investigation. 



The French investigator Boussingault verified much of the 

 earlier work of de Saussure and secured many additional facts 

 concerning the chemistry of growth. His predecessors had sought 

 to solve the question as to whether plants assimilate the free or 

 uncombined nitrogen of the atmosphere. Boussingault (1838, 

 1838a) improved the methods for the determination of the point in 

 question, and showed that peas and clover could get their nitrogen 

 from the air while wheat could not. Unfortunately, he did not 

 make as much of this discovery as he might have done. 



Boussingault was the first to have a chemical laboratory lo- 

 cated on a farm and to make investigations along a practical line 

 in connection with agriculture. His was the first agricultural ex- 

 periment station. 



The investigations of de Saussure, Boussingault, Davy, Thaer, 

 and others paved the way for the work and writings of Liebig. He 

 published (1840) "Organic Chemistry in its Application to Agri- 

 culture and Physiology," which was an important factor in at- 

 tracting the attention of the public to> agricultural problems. Many 

 of his investigations and discoveries in the field of organic chem- 

 istry were applied directly to his interpretation of these problems. 



He assailed the humus theory of Thaer and showed that humus 

 could not be an adequate source of the plant's carbon. By applying 

 the exact methods of chemistry to agriculture Liebig succeeded in 

 establishing that plants derive the carbon of their tissues from 

 the carbon dioxide of the air, and not from the carbon compounds 

 that may be present in the soil. He came to regard the ammonia 

 of the air as analogous with the carbon dioxide of the air, and 

 preached the doctrine that plants were able to derive their nitrogen- 

 ous food from the atmosphere. In the Farmer's Magazine, for in- 

 stance, he writes : 



If the soil be suitable, if it contains a sufficient quantity of alkalis, phos- 

 phates, and sulphates, nothing will be wanting. The plants will derive their 

 ammonia from the atmosphere as they do carbonic acid. (Cited by Russell, 

 1912.) 



Although the work of Liebig was not conducted in connection 

 with field experiments, it had a stimulating effect upon agricultural 



