The Groiindwoi^k of the Universe, Sec. 77 



certain "nitrifying bacteria," which have the power of converting 

 the ammonia into mtrous acid, and thence again into nitric acid- 

 The salts of these acids, not the acids themselves, are found in 

 the soil. 



In the form of a nitrate, nitrogen is easily assimilated by 

 plants, and begins again the cycle in Avhich once more it can be 

 utilised by the animal. But in actual practice we find a different 

 state of affairs. Part of the nitrites and nitrates is attacked by 

 another class of bacteria, known as "denitrifying bacteria," which 

 set free elemental nitrogen, of no use for either animal life or 

 that of the majority of plants. Then under our modern system 

 of sanitation, we discharge most of our sewage into the sea, 

 throwing away with it an enormous quantity of nitrogen com- 

 pounds, originally derived from the soil. It is evident that this 

 process of robbing the soil of its nitrogen will render it ultimately 

 incapable of supporting plant-life, and the food supply of humanity 

 will be diminished. 



In the early years of the present centuiy, great regions in 

 the American wheat area had become so exhausted that the 

 cultivators were forced to seek fresh lands in Canada, and 

 Mr. A. J. Balfour has suggested that the secret of the decline 

 and fall of the Roman Empire is to be found in the decreasing 

 productivity of the soil. 



It is thus necessary to return to the soil the nitrogen of 

 which it has been robbed, and there are three main sources of 

 supply. 



Ammonia is a by-product of gas manufacture. In 1913 the 

 United Kingdom produced in this way from its gas works, iron 

 works, coke ovens, &c., no less than 432,618 tons of ammonium 

 sulphate. Again, in Chili and Peru there are vast deposits of 

 naturally-formed nitrates which can be exported to countries in 

 need of combined nitrogen for their land. These have been 

 drawn upon very heavily. The figures during the war would be 

 misleading, but for the ten years 1904 — 1913 inclusive, the total 

 exports amounted to 20,850,000 tons. This demand is a steadily 



