330 A CENTURY OF SCIENCE 



fertilization of soils depends upon replenishing the 

 inorganic substances that have been exhausted by the 

 crops. 



The fundamental principles set forth by Liebig have 

 been confirmed, and it has been found that the fertilizing 

 constituents most commonly lacking in soils are nitrogen 

 compounds, phosphates, and potassium salts, so that 

 these have formed the important constituents of artificial 

 fertilizers. Liebig himself found that humus is valuable 

 in soils, because it absorbs and retains the soluble salts. 



The foundation established by Liebig in regard to arti- 

 ficial fertilizers has led to an enormous application of 

 these materials, much to the advantage of the world's 

 food-supply. 



It was Liebig 's belief, in accordance with the prevail- 

 ing views, that decay and putrefaction as well as 

 alcoholic and other fermentations were spontaneous 

 processes, and when the eminent French chemist, Pas- 

 teur, in 1857, explained fermentation as directly caused 

 by yeast, an epoch-making discovery which led to the 

 explanation of decay and putrefaction by bacterial action 

 and to the germ-theory of disease, the explanation was 

 violently opposed by Liebig and other German chemists. 

 Pasteur's view prevailed, however, and since that time 

 it has been found that various kinds of bacteria are 

 responsible for the formation of ammonia from nitro- 

 genous organic matter and also for the change of ammo- 

 nia into the nitrates that are available as plant-food. 



The long-debated question as to the availability of 

 atmospheric nitrogen for plant-food was settled in 1886 

 by the discovery of Hellriegel that bacteria contained in 

 nodules on the roots, especially of leguminous plants, are 

 capable of bringing nitrogen into combination and fur- 

 nishing it to the plants. 



No more than an allusion can be made to agricultural 

 experiment stations where soils, fertilizers, foods and 

 other products are examined, and where other problems 

 connected with agriculture are studied. 



The late S. W. Johnson of Yale studied with Liebig 

 and subsequently did much service for agricultural chem- 

 istry in this country, by his investigations, his teaching, 

 and his writings. His book, "How Crops Grow," pub- 



