103 The Sweet Potato 



manufacture of steel from phosphatic iron ores. None 

 of these forms, however, is used in the raw state for 

 sweet potatoes. 



Superphosphates, of which acid phosphate is the most 

 common commercial form, are obtained from the in- 

 soluble or very slowly soluble raw products just de- 

 scribed. The process of manufacture consists in treat- 

 ing the finely pulverized raw products with sulfuric acid 

 which renders soluble the phosphorus which they con- 

 tain. In this process the phosphoric acid obtained is a 

 definite chemical compound and its composition is the 

 same irrespective of its raw state. Any material con- 

 taining soluble phosphoric acid as its chief constituent 

 may, therefore, be termed properly a superphosphate. 



Acid phosphate, containing on an average 14 to 15 

 per cent of soluble phosphoric acid, is manufactured by 

 treating 1000 pounds of raw phosphate rock with 1000 

 pounds of a dilute solution of sulfuric acid. The raw 

 rock, containing about 30 per cent phosphorus which is 

 very slowly soluble, is thereby converted into a com- 

 pound having about one-half as much total phosphorus 

 as did the raw rock but which is vastly more valuable 

 as a fertilizer for ordinary crops because of the availa- 

 bility of the plant-food. This phosphate is the form 

 used almost exclusively on sweet potatoes. 



Potash exists chiefly in two forms, as chlorids or 

 murates and as sulfates. Though the availability of 

 the potash in these forms is about equal, the respective 

 elements with which it is combined in the two forms 

 exert considerable influence on its use for particular 

 crops. 



It has been found that the potash combined with 

 chlorin is not so valuable for potatoes as the sulfate 



