34 



nitrate of soda. The reverse of this is often the case, owing 

 to variations in prices ; and, indeed, for some years sulphate 

 of ammonia has been generally the cheapest source of 

 nitrogen in the market the price per unit of nitrogen having 

 been as a rule lower (sometimes as much as 20 per cent, 

 lower) than in any other manure supplying nitrogen in a 

 condition equally ready for the use of crops. 



IMPURITIES AND ADULTERANTS. 



Good samples of sulphate of ammonia generally contain 

 little impurity except moisture, with a very small amount of 

 sandy matter, some free acid, and often still less common salt. 

 In impure specimens, however, the proportion of both earthy 

 impurity and of common salt is sometimes considerable. The 

 well-known test of putting a little of the sulphate of ammonia 

 on a red-hot shovel, shows their presence ; for the sulphate of 

 ammonia disappears as a vapour, leaving the impurities 

 behind. A good sample will, therefore, leave practically no 

 residue when tested in this way. Sulphate of soda, and 

 sometimes sulphate of iron also, are occasionally found, being 

 used as adulterants; but their presence is, of course, detected 

 by the same test. 



Now and then cases are reported in which sulphate of 

 ammonia is put on the market containing considerable 'quan- 

 tities of free sulphuric acid (oil of vitriol). It need hardly be 

 said that such manures are harmful to crops, and should be 

 avoided. 



An even worse impurity though a comparatively rare 

 one is the thiocyanate or sulphocyanate of ammonia, which 

 is one of the products from the distillation of coal in the 

 manufacture of gas. It is so powerful a poison to plants 

 that even a few pounds per acre of it in a dressing of sul- 

 phate of ammonia will do great damage to the crop. The 

 thiocyanate is easily detected by adding a solution of ferric 

 chloride to a solution of the manure, when, if it be present, 

 a deep red colour will appear. 



At one time, arsenic was not uncommonly found in 

 sulphate of ammonia derived from the impure acid used in 

 its manufacture ; but it is now rarely present. 



EFFECT OF SULPHATE ON INDIVIDUAL CROPS. 



Having dealt with the general facts and principles con- 

 nected with the use of sulphate of ammonia, we will next 

 consider its action upon the chief farm crops, as shown by 

 the results of experiments. As nitrate of soda is the only 

 other manure practically comparable with sulphate of am- 

 monia, we will, where possible, compare the action of the 

 two substances. 



