206 THE FARMER OF TO-MORROW 



tassium, 18,500 pounds; magnesium, 3,480 

 pounds; calcium, 1,000 pounds; phosphorus, 

 160 pounds. 



"The brown silt loam prairie soil of the early 

 Wisconsin glaciation is the most common type 

 of the greatest soil area in the Illinois Corn 

 Belt. Two million pounds of this surface soil 

 contain as an average: Potassium, 36,250 

 pounds; magnesium, 8,790 pounds; calcium, 

 11,450 pounds; phosphorus, 1,190 pounds. 



"The older gray silt loam prairie, the most 

 extensive soil of Southern Illinois, contains 

 in two million pounds of soil: Potassium, 

 24,940 pounds ; magnesium, 4,690 pounds ; cal- 

 cium, 3,420 pounds; phosphorus, 840 pounds. 



"These data represent averages involving 

 hundreds of soil analyses, and they emphasize 

 the fact that normal soils are rich in potassium 

 and poor in phosphorus. This is to be ex- 

 pected, for most soils are made from the earth's 

 crust, and normal soils should bear some re- 

 lation in composition to the average of the 

 earth's crust, which contains in two million 

 pounds 49,200 pounds of potassium and 2,200 

 pounds of phosphorus, as shown by the 

 weighted averages of analyses involving about 

 two thousand samples of representative rocks, 

 reported by the United States Geological 

 Survey. 



"The plant food required for one acre of 

 wheat yielding 50 bushels, one acre each of 



