Purchase of Fertilizers 155 



of this, but because they do not vary widely in their com- 

 position. A definite amount from each class can be 

 depended upon to furnish practically the same amount 

 of available phosphoric acid. For example, a ton of 

 South Carolina rock superphosphate, from whatever 

 manufacturer obtained, will not vary widely in its con- 

 tent of phosphoric acid, and will always act in the same 

 way under similar conditions. The various German 

 potash salts are also standard and high-grade, since the 

 composition of each grade and kind is practically uniform 

 in its content of potash, which will always act in the same 

 way under the same conditions, and since they are richer 

 in the specific element, potash, than other potassic com- 

 pounds suitable for the manufacture of fertilizers. 



These various standard, high-grade products, when 

 used in the manufacture of fertilizers, make what are 

 called "chemical fertilizers/' because they are really 

 crude chemical compounds, and furnish the particular 

 fertilizer elements in their most concentrated and active 

 forms. 



FERTILIZING MATERIALS WHICH ARE VARIABLE IN COM- 

 POSITION 



The products which are included in the second group 

 differ from the others, in that they not only vary in their 

 content of the specific constituent, or in their composition, 

 but they are also variable in the sense that the constitu- 

 ents contained in them do not show a uniform rate of 

 availability. For example, ground bone varies in its 

 composition owing to its source and the method of treat- 

 ment, and the availability of the constituents, nitrogen 

 and phosphoric acid, also varies because of these condi- 



