SOIL COMPOSITION 71 



composite samples of subsoil from different places in the deep 

 loess areas of Illinois, 35,070 pounds of potassium were found in 

 2 million of loess. 



In the loess which was not mixed with carbonates, or from which 

 most of the carbonates have been removed, presumably by leach- 

 ing (Dubuque and Kansas City samples), the total supply of mag- 

 nesium and calcium is markedly smaller than the supply of potas- 

 sium; but when compared with the average requirements of a 

 general crop rotation (see Table 13) the supply of magnesium and 

 calcium is still somewhat more ample than that of potassium. 



While the average supplies of sulfur and phosphorus are about 

 equal, the requirement for phosphorus is five times as great as for 

 sulfur in the total produce of the average crop rotation, and forty 

 times as great if the grain only is removed and not returned. 



A partial analysis of loess from Cheyenne, Wyoming, reported 

 by Eakin, shows, in 2 million pounds of loess, 960 pounds of phos- 

 phorus, 44,500 of potassium, 14,880 of magnesium, 69,700 of 

 calcium, and 20,000 pounds of carbonate carbon. 



It is suggested that most of the carbonates contained in deep 

 loess deposits may have a different origin than the silicates, which 

 constitute the bulk of the material. In many deep loess deposits, 

 pieces of limestone shells (usually of light weight) are a characteris- 

 tic, indicating that a part of the loessial material may have come 

 from areas which were at times covered with water and at other 

 times dried on the surface and exposed to wind action. Thus, 

 while such loess may have been derived from glacial drift, more or 

 less of it has had some intermediate resting place where carbonates 

 tend to accumulate, as in ponds, shallow lakes, swamp areas, 

 bottom lands, or on seepy slopes; and from the dried surfaces of 

 such areas much of it has been transported by wind action over 

 bluffs and upland plains. It is noteworthy that the broadest deep 

 loess deposits along the river bluffs are found where the valley is 

 correspondingly wide (Illinois Bulletin 123, page 238). 



The most definite lesson to be drawn from these analyses of this 

 most important soil material is, that phosphorus is clearly the most 

 limited element of plant food; whereas among the four elements, 

 potassium, magnesium, calcium, and sulfur, it is difficult to de- 

 termine which is likely to be the most limited. 



