Supplying Soil Needs 



171 



Nature Study Review 



FIG. 133. The lack of a single plant-food element prevents growth. In. the 

 first jar of pure sand, none of the essential elements has been added; in the 

 second jar, all except nitrogen have been added. The third jar lacks only 

 phosphorus ; the fourth contains phosphorus and lacks only potassium (chemical 

 symbol K). The fifth jar contains all the necessary plant-food elements. 

 Except for these differences, the five lots of corn grew under exactly the same 

 conditions. 



principal plant-food elements each of several different The amount 

 crops requires in one year, the list below is given. It is JJ^ r 

 taken from Dr. Hopkins' work on Soil Fertility and material a 



crop 



Permanent Agriculture. requires 



