BACTERIA IN NATURAL PROCESSES. 117 



trogen compounds in abundance, together with 

 sulphates, phosphates, sugar, and many other sub- 

 stances. It is this which makes the garden soil 

 different from sand, or the rich soil different from 

 the sterile soil. If the soil is cultivated year after 

 year, its food ingredients are slowly but surely 

 exhausted. Something is taken from the humus 

 each year, and unless this be replaced the soil 

 ceases to be able to support life. To keep up a 

 constant yield from the soil the farmer under- 

 stands that he must apply fertilizers more or less 

 constantly. 



This application of fertilizers is simply feed- 

 ing the crops. Some of these fertilizers the farm- 

 er purchases, and knows little or nothing as to 

 their origin. The most common method of feed- 

 ing the crops is, however, by the use of ordinary 

 barnyard manure. The reason why this material 

 contains plant food we can understand, since it 

 is made of the undigested part of food, together 

 with all the urea and other excretions of animals, 

 and contains, therefore, besides various minerals, 

 all of the nitrogenous waste of animal life. These 

 secretions are not at first fit for plant food. The 

 farmer has learned by experience that such excre- 

 tions, before they are of any use on his fields, 

 must undergo a process of slow change, which 

 is sometimes called ripening. Fresh manure is 

 sometimes used on the fields, but it is only made 

 use of by the plants after the ripening process 

 has occurred. Fresh animal excretions are of 

 little or no value as a fertilizer. The farmer, 

 therefore, commonly allows it to remain in heaps 

 for some time, and it undergoes a slow change, 

 which gradually converts it into a condition in 

 which it can be used by plants. This ripening is 



