248 ON THE CHEMICAL CONSTITUENTS OF SOILS. 



of ammonia, and of fixing the ammonia, is not pecu- 

 liar to gypsum, but is shared also by other salts of 

 lime (chloride of calcium, for example). But it acts 

 also as a sulphate, and when useful as such cannot 

 be replaced by any other salt of lime which does not 

 contain sulphuric acid. 



Hence gypsum can be replaced as a manure only 

 by a mixture of a salt of lime with ammonia, and a 

 salt of sulphuric acid. Sulphate of ammonia can 

 therefore be substituted for gypsum, and exerts a 

 more ' rapid and effectual action. In France, sul- 

 phuric acid has been poured upon the fields after the 

 removal of the crops, and has been found to form a 

 good manure. But this is merely a process for form- 

 ing gypsum in situ; for the soils upon which it is 

 applied contain much lime, which enters into com- 

 bination with the sulphuric acid. It would certainly 

 be much more advantageous to form sulphate of am- 

 monia by adding the acid to putrefied urine, and to 

 apply this mixture to the field. 



