68 FERTILISERS CONTAINING NITROGEN [chap. 



which are not utilisable in other ways, are available for 

 manure. Again, all industries dealing with wool, silk, 

 hair, feathers, skins, give rise to highly nitrogenous 

 waste material, and other residues of vegetable origin 

 occur from time to time. 



From their origin and mode of preparation it follows 

 that these substances must be of very variable composi- 

 tion : again, the supply is apt to be irregular and 

 limited, so that their use is somewhat local and confined 

 to particular classes of farmers. 



Most of the manures of organic origin will contain 

 phosphoric acid as well as nitrogen, but as a matter 

 of convenience some of them may be treated as purely 

 nitrogenous fertilisers, leaving others to be dealt with 

 among the compound substances. 



Of these waste materials the most generally used 

 is soot ; its value, which is due as much to its physical 

 effects upon the soil as to its fertilising constituents, 

 has been known for the last three centuries at least. 

 It has already been pointed out that coal contains one 

 per cent, or more of nitrogen ; in a fire some of this 

 is evolved as ammonia when the coal is heated, and if 

 it escapes combustion in the higher levels of the fire it 

 is afterwards partially arrested by the particles of carbon 

 constituting soot, which possess an exceptional power of 

 condensing gases upon their surface. In the main soot 

 is only an impure form of carbon ; its fertilising value 

 is due to the small and variable proportion of ammonia 

 it has thus absorbed from the gases in the chimney. 

 The percentage of nitrogen present may be as low as 

 0-5, in exceptional cases it may rise to 6, 3-2 being the 

 mean of a large number of analyses. 



Since the nitrogen is present in the form of 

 ammonia, soot as a fertiliser may be regarded as one 

 of the ammonium salts ; its action, however, is pro- 



