22 THE HANDBOOK FOR PEACTICAL FARMERS 



exhausted by the production and removal of croi)S. These 

 materials are known as the essential jolant food materials or the 

 golden tripod upon which successful farming must rest. 



The value of a commercial fertilizer is determined solely by 

 the amount and the form of the nitrogen phosphoric acid and 

 potash which it contains Since the needs of the soil and the 



requirements of the crops vary, it 

 follows that not all soils or all crops 

 will respond to the use of fertiliz- 

 ers containing these three elements. 

 A sandy soil is usually lacking in all 

 three of the essential plant foods 

 while clay soils contain more of the 

 mineral elements and are especially 

 rich in potash. Black muck soils are 

 rich in organic matter but lacking in 

 the mineral elements, Avhile soils de- 

 rived from certain geological forma- 

 tions may be rich in phosphorus or 

 potash if these elements are found in 

 abundance in the parent rock. 



Plants of the clover family differ 

 from other plants in being able to 

 take up and use the nitrogen of the 

 air, hence they are not dependent 

 upon the soil nitrogen for their 

 growth. On the other hand grass 

 and grain crops are wholly depend- 

 ent upon the soil nitrogen and must 

 have an abundance during their 

 period of most rapid groAvth if good 

 crops are to be harvested. This vari- 

 ation in the needs of soils and the re- 

 quirements of crops makes it neces- 

 sary that every farmer should know 

 his own i)articular recpiirements before he can make an eco- 

 nomical use of commercial fertilizers. 



Fertilizer materials. — From what has been said it will be 

 understood that any material that supplies one or more of the 

 three essential elements may be used as a commercial fertilizer, 

 but, as a matter of fact, only a few materials are available for 

 use as fertilizers because of the prohibitive price of many of the 

 substances carrying plant food materials. The manufacturers 



¥iG. 10. — Nodules on roots of 

 alfalfa by means of which 

 nitrogen is absorbed from 

 the air. 



