82 



CROP PRODUCTION AND SOIL FERTILITY 



Presence of helpful soil organisms. (5) Sufficient available plant- 

 food elements, nitrogen, phosphorus, potassium, calcium (carbon- 

 ate of lime). 



A negative factor: (1) The absence of harmful agents in the soil. 



FIG. 32. Having power for maximum production. Yield 550 bushels to the acre 

 following clover. A complete fertilizer was used at the rate of 2000 pounds per 

 acre. (Maine.) 



Soil Fertility Illustrated. Fig. 33 will help the student to 

 gain a clearer idea concerning fertility. 



Let the tank represent a soil. 



The capacity of the tank for holding water represents fertility, 

 or the power of the soil for producing a good or high yield. 



The staves which determine the water-holding capacity of the 

 tank represent the positive factors which determine soil fertility. 



The tank being free from any leaks represents the negative 

 factor, viz., the absence of harmful agents. 



