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 
