COMMERCIAL FERTILISERS 371 



soil water can be available, or useful to plants. 

 This valuable kind of phosphoric acid has but one 

 part of lime with it and two parts of water: 



Lime ^^ 



Water ^Phosphoric acid. 



Water^ 



This is the kind of phosphoric acid that is found 

 in superphosphates. It quickly dissolves in water 

 and plants can use it at once. In some guarantees 

 it is called " water-soluble phosphoric acid." 



"Insoluble phosphoric acid," on the other hand, 

 has three parts of lime to one of phosphoric acid, 

 and has no water, thus: 



Phosphoric acid. 



This is the kind of phosphoric acid found in fresh 

 bones and in the various "rock phosphates" taken 

 from the mines. Since it cannot be dissolved in 

 water, insoluble phosphoric acid has no value 

 whatever as a plant food unless it can be changed 

 into soluble phosphoric acid. As bones rot in the 

 ground the insoluble phosphoric acid in them 

 slowly becomes soluble, losing part of its lime. 

 A quicker way to change this into plant food is to 

 treat it with acids, which is the way superphos- 

 phates are made. 



In studying fertiliser guarantees, remember 

 that the "insoluble phosphoric acid" is not plant 

 food and cannot become so until it has been made 

 soluble. When applied to the soil, insoluble 

 phosphoric acid may gradually become soluble. 

 Hence it is customary, when figuring on the value 

 of a fertiliser, to count the insoluble phosphoric 

 acid as worth about one-half as much as the 

 soluble. Some chemists, however, do not con- 

 sider it worth even that much. 



