76 FIRST PRINCIPLES OF AGRICULTURE. 



made up more largely of extractive animal matter, and 

 is more uniform in composition, and much richer in 

 nitrogen. 



The lack of uniformity in both the chemical composition 

 and physical character of tankage, makes it impossible to 

 give to it a distinct position as a fertilizing product, since 

 the results derived from its use under uniform conditions 

 in other respects must naturally be variable. 



Dried Pish. — Dried fish was formerly an important 

 and considerable source of organic nitrogen. It is derived 

 mainly from the waste resulting from the extraction of 

 the oil from the menhaden, a fish not used for food, though 

 valuable for this purpose. The waste from fish-canning 

 establishments also furnishes a considerable amount of 

 this product. 



The " menhaden pomace " is rich in quickly available 

 nitrogen, and also contains considerable phosphoric acid, 

 derived from the bone contained in it; it is reasonably 

 uniform in composition. The waste from the canneries 

 contains more of the skin and bone, and is more variable 

 in composition. Aside from the source from which it is 

 derived, the chief cause of variability in composition is the 

 content of water. Frequently, too, acid is used to prevent 

 decomposition, which, while useful in making the constitu- 

 ents more available, renders the product less concentrated 

 and less easily dried. Well-dried samples contain on the 

 average seven to eight per cent of nitrogen, and the same 

 of phosphoric acid. Finely ground, it decays rapidly in 

 the soil, and is highly regarded as a manure. 



Fish was one of the first materials used as a fertilizer. 

 The custom in this country in early times, of using a fish 



