6o FERTILISERS CONTAINING NITROGEN [chap. 



about 95 per cent, purity, it is usually guaranteed to 

 contain 20-2 per cent, of nitrogen, or 24-5 per cent, of 

 ammonia. Adulterations are infrequent and can 

 readily be detected, because sulphate of ammonia is 

 the cheapest substance which is wholly volatile. A 

 handful of sulphate of ammonia placed on a fire shovel 

 and heated to redness over a fire should leave no 

 appreciable residue. Samples of the salt are occasion- 

 ally found containing ammonium sulphocyanide (thio- 

 cyanate), a substance actively injurious to vegetation. 

 Its presence can be readily detected by adding to a 

 solution of the salt a little ferric chloride, with which a 

 sulphocyanide produces an intense red coloration. Like 

 all salts of ammonia, the sulphate reacts with lime and 

 even with carbonate of lime, giving off free ammonia as 

 a gas. For this reason sulphate of ammonia should 

 never be mixed with lime or with basic slag, which 

 contains a certain amount of free lime, lest a loss of 

 nitrogen should ensue. A lightly calcareous soil in dry 

 weather may induce a similar loss of free ammonia. 



As a nitrogenous manure sulphate of ammonia is 

 practically as effective, nitrogen for nitrogen, as nitrate 

 of soda ; it is also to all intents and purposes as rapid 

 in its action, because the process of nitrification, which 

 generally precedes the utilisation of the ammonia by the 

 plant, takes place very rapidly in suitable soils. The 

 fact is well illustrated in the following table (XIV.), 

 showing the composition of the water draining from one 

 of the Rothamsted wheat plots to which a mixture of 

 sulphate and chloride of ammonia had been supplied on 

 25th October, followed the next day by heavy rain, so 

 that on the 27th the drains began to run. It will be 

 seen that at this early date the ammonia had not been 

 wholly caught up by the soil, so that a little found its 

 way into the drains; at the same time, however, the 



