14 THE FERTILITY OF THE SOIL [CH. 



or farm-yard manure (which is essentially the same 

 thing) added to the soil are not the actual food of 

 plants, but only the raw materials out of which food 

 is made. The true food is the nitrate to which the 

 organic matter gives rise, and our first business is with 

 this. 



O^ing to the losses which the nitrate suffers there 

 is rarely any great stock of it in the soil, frequently 

 not enough for the current season's growth. For- 

 tunately the process of nitrate production, commonly 

 called nitrification, goes on fairly readily so that fresh 

 supplies are forthcoming whenever the conditions are 

 suitable. The process has turned out to be very 

 wonderful. It was formerly supposed to be entirely 

 chemical, but a remarkable piece of work by Schloes- 

 ing and Miintz in 1877 showed that it was brought 

 about by bacteria. In studying the purification of 

 sewage by land filters they caused a stream of sewage 

 to trickle slowly down a column of sand and lime- 

 stone, the experiment being continued for some weeks. 

 For the first 20 days the ammonia in the sewage 

 remained unaltered, then it began to change into 

 nitrate, and finally the issuing liquid contained no 

 ammonia but only nitrate. Why, asked the authors, 

 was there this delay of 20 days before nitrifica- 

 tion began ? If the process were a purely chemical 

 oxidation it should begin at once. If, however, it 

 were bacterial, they could readily explain the delay, 



