SOIL COMPOSITION 



Iowa, shows 35,600 pounds of total potassium in 2 million (see 

 Table n). 



Since the above was written the author has secured, through the 

 kindness of Professor Stevenson, the unpublished data shown in 

 Table 18.1 which present the average results of from one to six 

 analyses of the most important soil types in the great soil areas of 

 the state. Professor Stevenson writes: " The samples are believed 

 to represent the most widely distributed type of the respective 

 areas. We did not determine total potassium." (Two types, the 

 uncovered glacial till, and the shallow loess, are included for the 

 Kansan area.) 



TABLE 18.1. PLANT FOOD IN SURFACE SOILS OF IOWA 

 Pounds per Acre in 2 Million (about 6f -inch Stratum) 



1 Only two analyses for phosphorus in the Kansan till 



The Kansan drift is the oldest and the poorest in both nitrogen 

 and phosphorus, while the Wisconsin is the newest and the richest, 

 with the lowan intermediate in both respects. The Mississippi 

 loess and the shallow loess on the less rolling parts of the Kansan 

 glaciation are similar in composition and probably of similar origin 

 (of the lowan age) , but the Mississippi loess is much deeper and 

 of a more rolling topography, which insures a much better subsoil, 

 physically, and may also account for the somewhat lower nitrogen 

 content of the surface, through loss of organic matter by washing. 

 The higher phosphorus content of the Missouri loess suggests that 

 it owes its origin in part to deposits from the semi-arid plains of the 

 northwest, the richer mineral soil having encouraged the more 

 recent accumulation of nitrogen. 



