158 Plant Physiology 



required in order to determine what is, for any soil, the 

 most effective fertility. 



Many analyses of tillable soils have been made through- 

 out the United States, and it is shown that the storage of 

 nutrient elements therein is most diverse. Calculated to 

 pounds per acre in the upper seven inches of soil many 

 complete analyses afford extremes as shown in the follow- 

 ing tabular summary : 



When the distribution of the roots and the availability 

 or lack of availability of the compounds are taken into 

 consideration, it is evident that with respect to the minima 

 one may speak of exhaustion or lack of nutrients, but with 

 respect to the maxima there may be sufficient, conserva- 

 tively used, for generations. 



A very thorough study is being made of the soils of 

 Illinois, and the table on the opposite page gives the 

 average amount of plant food for a variety of soil types. 



These represent, except in the last two cases, the total 

 plant food in 2,000,000 pounds of dry surface soil, this 

 weight being approximately that of 7 inches of ordinary 

 soil. In the case of sand, which is heavier, about 

 2,500,000 pounds are concerned in the same depth, and 

 in the case of the light peat only about 1,000,000 pounds. 



