352 0. D. von Engeln — Factors influencing Percentages of 



!5P,0 6 . £CaO -fKsO 

 Average composition ten mile shell of 



the lithosphere -25 4*82 2-96 



Average composition of representative 



residual soils of Maryland -125 -50 1*73 



Average composition of 10 Tennessee 



soils 'HI -244 1-022 



It will be noted that of P.,0 B and K s O only about one-half the 

 amount was present as compared to the percentage computed 

 for the average rock of the lithosphere. In the case of the 

 CaO the difference is still greater. In an}' event, the differ- 

 ences shown are sufficiently great to indicate that residual soils 

 are notably deficient in such plant food elements as compared 

 to the parent, rock. 



Glacial soils may be regarded as composed of a thorough 

 mixture of mechanically derived material from many rock 

 types ; especially when they comprise a portion of the drift 

 sheet resulting from continental glaciation. Such soils, 

 having been subject to solvent denudation only since their 

 deposition by the glacier, should, according to the above deduc- 

 tions, under favorable conditions, show a higher concentration 

 of the mineral plant food elements than do residual soils. 



In the state of Illinois are found soils from several glacial 

 advances, the earliest of which probabty antedates the youngest 

 by a long time interval. Hopkins and Pettit* have published 

 partial results of a large number of complete analyses of soils 

 from this state and have, moreover, correlated their soil types 

 with the various glacial advances, and with the topographic 

 features of the soil's occurrence. Tables are given " showing 

 the average results of analyses of many representative samples 

 of each soil type." These data afford an excellent opportunity 

 for some interesting comparisons. Soil from the youngest 

 glaciation, the Late Wisconsin, brown stilt loams occurring on 

 undulating prairie lands show percentages of P 2 O s and K 2 at 

 various depths as given below. Calcium is unfortunately not 

 given. 



Illinois Soils — Late Wisconsin Glaciation. 



Brown Silt Loams on Undulating Prairie Lands. 



Depth $P 2 5 #K 2 



0-7" '162 2-71 



7-20" -112 2.90 



20-40" -100 3-21 



* Hopkins, C. G. and Pettit, J. H., The Fertility in Illinois Soils, Ball. 

 123, University of Illinois, Agri. Expt. Station, 1908, pp. 196-200-201. 



