90 THE PRINCIPLES OF AGRICULTURE 



duce large leaves and stalks, while the hardi- 

 ness may suffer. On the other hand, insuf- 

 ficient nitrogen is almost certain to result in 

 dwarfing and loss of vitality. It must receive 

 attention, also, because one form, the nitrate, 

 tends to leach from the soil. 



135. In a pure or elemental state, nitrogen 

 is an invisible gas. It comprises four -fifths 

 of the atmosphere. And yet, with this vast 

 amount about us, it is the most expensive ele- 

 ment of plant -food. The nitrogen of the air 

 can not be used by the great majority of plants, 

 because it is in what is known as a free or un- 

 combined state. The sources of nitrogen for 

 plants are ammonia, nitrates, or in some 

 compound formed by animals or plants (that 

 is, in some organic form). 



136. If the gas nitrogen be combined with 

 the gas hydrogen, there will be formed am- 

 monia (N H 3 ) . From this the plants can 

 derive, indirectly, their supply of nitrogen. 

 Another compound of nitrogen is called nitric 

 acid, which is composed of nitrogen, hydrogen, 

 and oxygen (H N Os) . When some mineral 

 element takes the place of the hydrogen in 

 this combination, the compound is called a 

 nitrate : as Na N Os, nitrate of soda ; K N 3 , 

 nitrate of potash, or saltpetre. Both ammonia 

 and nitrates are found in the soil in small 



