Manuring and Rotations 79 



nitrogen. In making coal-gas by the distillation of coal, 

 a quantity of ammonia is given off. The gas is passed 

 through sulfuric acid in which the ammonia is removed 

 and ammonium sulfate formed. This salt is about 20 

 per cent nitrogen. 



By means of electricity and in other ways, it is possible 

 to combine the nitrogen of the air in such a manner that 

 it can be used as a fertilizer. The chief products of these 

 processes are calcium nitrate and calcium cyanamid. The 

 main difficulty in the way of using these fertilizers more 

 widely is the lack of cheap power which is required in 

 their manufacture. 



Many animal products are used for their nitrogen. 

 Dried blood, dried flesh, ground fish, tankage, hoof-and- 

 horn meal, and wool and hair wastes are all used. The 

 availability of nitrogen in these compounds decreases 

 about in the order named. The nitrogen of dried blood 

 is available at once, whereas in leather and hair it becomes 

 available slowly. 



It is probable that the future supply of nitrogen will 

 come more and more from the use of leguminous plants 

 rather than from the addition to the soil of material from 

 the outside. The supply of these materials is diminishing, 

 but there is no limit to the use that may be made of these 

 nitrogen-gathering crops. 



Phosphorus. 



Fertilizers yielding phosphorus are obtained from both 

 organic and mineral sources. Bones in various forms are 

 extensively used. Formerly they were used chiefly raw, 

 both ground and unground; now most of the bone is 



