854 O. D. von Engeln — Factors influencing Percentages of 



Illinois Soils — Lower Illinoian Glaciation. 



Gray Silt Loams on Tight Clay, Undulating Prairie Lands. 



Depth <SP a O. #K a O 



0-7" -096 1-49 



7-20" '085 1-61 



20"-40" -091 1-68 



An appreciably and consistently lower percentage of these 

 constituents is shown by these averaged analyses to exist in the 

 older than in the younger material, and at all depths. This 

 difference in the percentage of the mineral plant foods present, 

 and in the physical conditions, i. e., brown and gray color, can 

 only be ascribed to changes due to natural processes, operative 

 since the glacial age. In other words, as a result of the devel- 

 opment of drainage, leaching and weathering in general, under 

 a natural vegetation cover, the soils of the earlier glaciation are 

 notably poorer in phosphorus and potassium. 



The important point to be noted here is that the concentra- 

 tion of the soluble plant-food constituents in soils seems to be 

 a characteristic of the age of the soil, or better, of the stage in 

 an erosion cycle in which the soil occurs. Soil of the youngest 

 glaciation has higher percentages than the oldest, while the per- 

 centage average for the Maryland soils lies between the two, 

 but approaches the condition indicated for the oldest glaciation. 

 According to this, glacial soil, young and old, occurring on dis- 

 sected uplands, where the runoff is rapid, and leaching cor- 

 respondingly active, should show a much closer agreement in 

 plant-food constituents for young and old glaciations than is 

 shown by glacial soils from areas of prairie topography. Such 

 an assumption finds confirmation in the figures given by 

 Hopkins and Pettit (loc. cit.) for the yellow silt loam type of 

 soil, which occurs on "sloping hillsides and on the broken lands 

 adjoining water courses in all parts of the state " : thus on the 

 divide areas of the main glacial deposits of various ages, as well 

 as in the unglaciated section. An exception is the youngest, 

 or Late Wisconsin glacial deposit, where ayellow-#T«?/silt loam 

 occurs on "undulating timber uplands." 



In this table the percentage values for the phosphorus are 

 strikingly consistent; such discrepancies as are shown in the 

 case of this element, and the larger variations for potassium, 

 are within the limits of error introduced by the number and 

 character of samples from the individual areas. Grlacialists, 

 moreover, are not agreed as to the verity and succession of all 

 the advances here differentiated. 



