250 SOILS AND MANURES 



are slower in action than ammonium salts and nitrates. 

 Some of them, indeed, act so slowly as to be unsuitable 

 for direct application to the soil. They can be rendered 

 more quickly available by subjecting them to a prelim- 

 inary process of fermentation or composting. Treatment 

 with sulphuric acid is still more effective, and they are 

 chiefly used as a source of nitrogen in the manufacture 

 of the so-called compound artificial manures. The fol- 

 lowing are among the most important. 



Ground Leather. Ordinary leather is prepared from 

 the hides of animals, which, in the dry state, contain 

 about 16 per cent, of nitrogen. The hides are treated 

 with tannin, and impregnated with grease in order to 

 preserve the nitrogenous matter from decomposition. 

 This, of course, greatly reduces the manurial value, which 

 depends, in large measure, upon the rate at which the 

 substance decomposes. The scrap leather which is col- 

 lected, is generally found to have lost a considerable 

 proportion of the grease, and when dried and ground to 

 powder is occasionally used as manure. It contains 

 about 5 per cent, of nitrogen, but its action is so slow 

 that it cannot be economically used directly in the un- 

 treated condition. Terrified leather is the name given 

 to samples from which the fat has been extracted by the 

 action of superheated steam. It contains a somewhat 

 larger proportion of nitrogen, and decomposes more 

 readily than the untreated samples, but even then it 

 cannot be recommended for direct use as manure. 



Hoof and Horn Meals. Hoofs and horn, like hides, 

 flesh, etc., belong to the class of albuminoid substances, 

 and have a similar composition, but are much drier and 

 therefore, in the natural state, contain a larger propor- 

 tion of nitrogen. They decompose, however, more slowly 

 and are less suitable as manure, even when very finely 

 ground. They are occasionally mixed with meat meals 



