IIO AGRICULTURE 



tivated on a field are small, there is probably need 

 of nitrogen in the soil. A crop of cowpeas or clover 

 usually leaves the soil rich in nitrogen. Clay soils gener- 

 ally contain more potash than sandy soils. Whether a 

 soil is rich in phosphoric acid cannot be told by looking 

 at it. 



How to find what fertilizer the soil needs. Even when 

 a chemist analyzes soil and finds out just what it contains, 



FIG. 69. WHEAT FROM EQUAL AREAS 

 On left, fertilized with nitrate of soda; on right, no nitrogen in the fertilizer. 



he cannot tell how much of every precious element is in a 

 condition for plants to use. The chemist's analysis does 

 not, therefore, show what fertilizer to apply. 



The only way to determine the kind of fertilizer a soil 

 needs is to make an experiment on that soil with different 

 fertilizers. 



It will pay to make this experiment with the principal 

 crop of any farm. The following diagram shows how to 

 make such a test. The areas must be of exactly the same 



