THEORIES CONCERNING SOIL FERTILITY 307 



leaves of plants under the influence of sunlight; 1 Senebier had 

 shown that the carbon of the plant is derived from the carbon dioxid 

 of the air; and De Saussure had analyzed the ash of many plants, 

 had shown that these ash constituents were derived from the soil, 

 and that, though small in quantity as compared with the amount 

 of material furnished to the plant by the air and water, the ash 

 constituents were also essential to plant growth. 



De Saussure's publication in 1804 of his " Reserches Chimique 

 sur la Vegetation" gave to the world the first definite and approxi- 

 mately correct statement concerning the requirements and sources 

 of plant food. While Davy's lectures on Agricultural Chemistry 

 (first published in 1813) did much to extend the existing knowledge, 

 and the investigations of Bousingault and Lawes began to develop 

 (about 1835), it remained for Liebig to bring together the work of 

 all and present it in a more comprehensive form in his " Organic 

 Chemistry in its Application to Agriculture and Physiology," 

 published in 1840. 



Thus, while Liebig is popularly known as the "Father of Agri- 

 cultural Chemistry," the more fundamental contributions to knowl- 

 edge concerning soil fertility and plant growth have been made by 

 Senebier (of Switzerland), De Saussure (of France), Lawes and 

 Gilbert (of England) , and Hellriegel (of Germany) , the last being 

 the discovery (in 1886) of nitrogen fixation by the root-tubercle 

 bacteria of legumes. 



Liebig devoted much effort toward the proof of his theory that 

 the ammonia of the air is the source of nitrogen for plants; but 

 in this he failed, and Lawes and Gilbert's laboratory and field 

 investigations at Rothamsted, which were in part planned for the 

 purpose of disproving Liebig's nitrogen theory, clearly established 

 the fact that in the main the soil must furnish nitrogen as well as 

 the mineral elements of plant food. 



The following quotations from Liebig's writings are interesting; 

 and they are also instructive, in that they well illustrate the weak- 

 ness of drawing quantitative deductions and specific conclusions 

 from qualitative data and general observations. Thus wrote 

 Liebig: 



1 Any one may observe the bubbles of oxygen formed upon fresh leaves placed 

 under water in the sunlight. 



