PERMANENT SOIL FERTILITY 



The Illinois system of permanent soil fertility recognizes 

 six positive factors of crop production. These may be desig- 

 nated briefly as seed, temperature, moisture, light, a home for 

 the plant, and food for its use. No one of these factors is more 

 essential than another in the production of crops, but since 

 they are not equally susceptible of control certain of them are 

 more often the limiting factors. It is impossible, for example, 

 to change the temperature conditions of winter so as to make 

 that season suitable for crop production, and the practical 

 means available for modifying the temperature conditions of 

 the soil during the growing period of the crop are very lim- 

 ited. Of all six factors that of plant food is the most com- 

 pletely within the control of the farmer, for it is fully pos- 

 sible for him to completely change in an economic way the 

 amount of food available within the soil for the plant. Yet 

 it is frequently true that the food supply of the soil is the 

 limiting factor of crop production, especially under humid con- 

 ditions such as prevail in Illinois. It is therefore to this factor 

 that Doctor Hopkins has given large consideration in develop- 

 ing a system of permanent soil fertility. 



There are ten essential elements of plant food in the ab- 

 sence of any one of which the plant cannot function normally 

 and produce good yields. These elements are : carbon, oxy- 

 gen, hydrogen, iron, sulfur, calcium, magnesium, potassium, 

 nitrogen, and phosphorus. The great bulk of the plant — ap- 

 proximately 96 percent — consists of three elements, carbon, 

 oxygen, and hydrogen. The supply of these elements, how- 

 ever, as Doctor Hopkins pointed out, is automatically taken 

 care of by nature, the carbon and oxygen being obtained by 

 the plant directly from the carbon dioxid of the atmosphere 

 and the hydrogen obtained from the soil moisture, which in 

 turn is, under humid conditions, constantly being replenished 

 by the rainfall. The farmer, therefore, need not concern him- 

 self with these three elements. 



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