BONE FERTILIZERS 185 



eral supply of available phosphoric acid. As an illus- 

 tration, the following example of two soils from ad- 

 joining farms, which have been cropped and manured 

 differently, may be cited : 3 ° 



Soil well manured No manure and 



and crops continuous wheat 



rotated. raising. 



Per cent. Per cent. 

 Total phosphoric acid ... . 0.20 0.20 



Humus 4.25 1.62 



Humic phosphoric acid . . 0.06 0.02 



It is more economical to keep the insoluble phos- 

 phoric acid of the soil in available forms by the use of 

 farm manures than it is to purchase superphosphates. 



