56 The Story of the Bacteria 



and seed, and his task is to get a crop. This 

 seems simple enough. In fact he has oxygen, 

 hydrogen, and carbon a plenty. The trouble 

 is with the nitrogen, which is so abundant 

 that it makes up about four fifths of the vol- 

 ume of the air. But plants can't eat pure 

 nitrogen. So the farmer has to secure it 

 in a roundabout way. As all animal and 

 vegetable substances contain nitrogen, and, 

 when dead and decaying under the in- 

 fluence of bacteria, give up those nitrogen 

 compounds which living and growing plants 

 require, he gets this dead stuff wherever he 

 can. His great domestic supply is from his 

 barnyard manure. This is in truth the most 

 valuable product of his farm, if he knows how 

 to care for and use it. But he often spends 

 a great deal of money in buying imported 

 nitrogen in the form of Chile saltpetre or 

 other compounds. There are commercial 

 laboratories which prepare artificial nitrogen 

 compounds for the farmer. But for all these 

 things he has to pay out his hard-earned cash. 

 Some crops, notably grain, take a great 

 deal of nitrogen out of the soil, so that if a 



