176 SOILS AND FERTILIZERS 



tration, the following example of two soils from ad- 

 joining farms, which have been cropped and manured 

 differently, may be cited : 31 



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, lime, rotation of crops and thorough 

 cultivation, than it is to purchase superphosphates in 

 commercial forms. 



