4 SOILS AND FERTILIZERS 



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 inaccurate idea re- 

 garding the source of plant food. The writings of 

 Thaer were of a most practical nature, and they did 

 much to stimulate later investigations. 



About 1830 there was a renewed interest in scientific 

 investigations relating to agriculture. At this time 

 Boussingault became actively engaged in agricul- 

 tural research. He was the first to establish a chemical 

 laboratory upon a farm and to make practical inves- 

 tigations in connection with agriculture. This marks 

 the establishment of the first agricultural experi- 

 ment station. Boussingault's work upon the as- 

 similation of the free nitrogen of the air is reviewed 

 in Chapter IV. His study of the rotation of crops 

 was a valuable contribution to agricultural science. 

 He discovered many important facts relating to the 

 chemical characteristics of foods, and was the first to 

 make a comparative study of the amount of nitrogen 

 in different kinds, of foods and to determine the value 

 of foods on the basis of the nitrogen content. His 

 study of the production of saltpeter did much to pre- 

 pare the way for later work on nitrification. The 



