FERTILIZATION AND FERTILIZERS 145 



Phosphorite rocks. About the same time it was found 

 that in many places what had been taken for simple lime- 

 stone contains so much phosphoric acid that this " phosphate 

 rock" can be used in place of bones, which are often scarce. 

 So it has become possible to make up for the insufficient 

 supply of stable manure, from rich mines in Florida, South 

 Carolina, and Tennessee, and elsewhere. 



The supply of phosphoric acid is now also being increased 

 by using "phosphate slag" from iron works smelting certain 

 ores. And several methods of getting the abundant nitrogen 

 from the air for use in fertilizing have been invented and are 

 being operated. 



So we can make up the deficiency of stable manure by 

 means of these mineral fertilizers, and we can strengthen 

 what manure we have by adding them to it. Only, Chile 

 saltpeter must never be used at the same time as stable 

 manure, but some weeks after. And in general it may be 

 said that good production cannot be permanently kept up 

 with these mineral fertilizers alone. They^ must be supple- 

 mented by the use of stable manure, greenmanuring (see 

 p. 141), or the use of some other material that will put 

 humus into the soil, such as muck or peat. Meat scraps, 

 dried blood, and other offal from the slaughterhouses, called 

 tankage, may also be used with great advantage. 



Fish scrap, the offal from fisheries of various kinds, espe- 

 cially on the North Pacific Coast, is an important material for 

 the manufacture of fertilizers; and fish may also be put 

 directly on the land, to supply especially nitrogen and 

 phosphoric acid. On the shores of New England a small 

 kind of fish called menhaden is caught by the ton in nets 

 and used as a fertilizer by farmers. Seaweeds also are often 

 used to add to the supply for compost heaps. Slaughter- 

 houses also supply "superphosphates/ 7 bones made soluble 



