208 Bacteria in Relation to Country Life 



of the clovers and their ability to forage for food and 

 moisture through a much greater thickness of soil. 

 Part of the food, it was thought, derived from the sub- 

 soil, was concentrated in the roots and stubble near the 

 surface and was later made available to succeeding 

 crops. 



When the composition of plants was begun to be 

 studied, it was learned that certain chemical elements 

 are always present in them. The number of analyses 

 increased with the improvement of the chemical methods 

 and led to the further investigations on the origin of 

 plant-food. The source of nitrogen, one of the constitu- 

 ents always found in plants, became a subject for much 

 study and discussion, particularly in view of the fact 

 that so large an amount of nitrogen gas is present in the 

 air. Some of the chemists in the early years of the nine- 

 teenth century were inclined to think that the gaseous 

 nitrogen of the air serves directly as plant-food. 



Liebig's theory. — The existing uncertainty was seem- 

 ingly dispelled for a time by the declaration of Liebig 

 that plants can derive not only their carbon but also 

 their nitrogen from the air. According to him, however, 

 the nitrogen is not supplied to crops in the elementary 

 state, but in the form of ammonia. Liebig thought all 

 crops capable of securing their nitrogen from the air, 

 but that legumes and other broad-leaved plants were 

 particularly capable. His position was based on the fact 

 that legumes not only benefited the succeeding cereal 

 crops, but that they did not respond to applications of 

 nitrogenous fertilizers. 



Liebig's conclusions were not accepted by all of the 



