COMMERCIAL FERTILISERS 383 



Besides dissolved bone and dissolved boneblack, 

 which are the common bone superphosphates, 

 "plain superphosphate," "acid phospnate," and 

 "dissolved rock" are standard sources of this 

 plant food. These are all superphosphates made 

 from rock phosphate. That made from South 

 Carolina rock is most common; it contains 12 to 

 14 per cent, of soluble phosphoric acid. The 

 "double superphosphate," containing about 45 

 per cent, of available phosphoric acid, is not 

 used much in this country. 



It is more diffi&ult to decide what material to 

 buy as a source of phosphoric acid than either of 

 the other plant foods. The first question that 

 arises is whether a raw bone or a raw mineral 

 phosphate should be bought, or a superphosphate. 

 The relative cost of the plant food in the different 

 materials, its availability, and the special needs of 

 the soil and crop must determine this. A pound 

 of phosphoric acid can be bought in raw rock 

 phosphate for two and a half cents; in raw 

 ground bone for four cents and in a super- 

 phosphate for five to six cents. Crops that 

 mature quickly, including most vegetables, need 

 quickly available fertilisers; while plants which 

 grow for several seasons, as fruit trees and 

 grasses, are able to get along with a certain 

 amount of slowly available fertiliser. The cru- 

 ciferous plants, as cabbage and turnip, ap- 

 pear to do very well on the raw phosphates. 

 Use as much of the slow-acting, but cheap, 

 raw phosphates as practicable. Oftentimes 

 a combination of superphosphate, for quick 

 results, and raw phosphate, for general en- 

 richment, is the best solution of the prob- 

 lem. 



