SUITING THE FERTILIZERS TO THE SOIL 109 



more of this than the plant requires is not applied. For 

 fields where soil-improving crops have grown and on stock- 

 farms, often there is no 

 need to buy any nitrogen, 

 or very little at the most. 



The agricultural value of 

 a fertilizer. The agricul- 

 tural value of a fertilizer is 

 the value of the increase 

 in the crop caused by using 

 the fertilizer. 



Suiting the fertilizer to 

 the crop. Different plants 

 require different kinds of 

 fertilizer. Since legumi- 

 nous plants get nitrogen 

 from the air by the work 

 of their root tubercles they 

 generally do not need nitro- 

 gen. To add a fertilizer 

 containing nitrogen is 

 therefore a useless expense. 



Suiting the fertilizer to 

 the soil. What a soil 

 needs cannot be told by 

 looking at it. There are, 

 however, some helpful 

 rules. Generally, a soil that is black or very dark con- 

 tains much vegetable matter, which in turn contains much 

 nitrogen. On the other hand, if the stalks of crops cul- 



FIG. 68. SORGHUM FROM EQUAL AREAS 



On left, no nitrogen in the fertilizer; on 

 right, fertilized with nitrate of soda. 



