SOIL COMPOSITION 81 



In case of the most important soil types the averages reported 

 in Tables 15, 16, and 17 are based upon analyses of a large number 

 of composite soil samples. The two most extensive soil types in 

 Illinois are the gray silt loam prairie (330) of the lower Illinoisan 

 glaciation (the so-called hard-pan soil of " Egypt "), and the 

 brown silt loam prairie (1126) of the early Wisconsin glaciation. 



The averages reported for the gray silt loam on tight clay of the 

 lo\ver Illinoisan glaciation represent 57 different soil samples. 

 In 2 million pounds of the surface soil the potassium (the most 

 constant constituent) varied from 23,120 to 26,440 pounds, the 

 phosphorus varied from 700 to 1000 pounds, and the nitrogen 

 varied from 2140 to 3500 pounds; and in every case the surface, 

 subsurface, and subsoil were found to be acid. 



The data reported for the brown silt loam of the early Wisconsin 

 glaciation are averages obtained by analyzing 90 different samples 

 of soil collected in ten different counties, and representing more 

 than 500 different borings. In 2 million pounds of the surface soil 

 of this type the potassium varied from 31, 980 to 43,100 pounds, the 

 phosphorus varied from 980 to 1620 pounds (or, if we disregard 

 four samples, from 1020 to 1340 pounds), and the nitrogen varied 

 from 3980 to 7520 pounds (or from 3980 to 6340, if we disregard 

 two samples). 



The limestone has not been leached out of the early Wisconsin 

 brown silt loam to such a depth as in the older gray silt loam. In 

 one case limestone was present in the surface of the brown silt 

 loam, and in three cases it was found in the subsurface, while it 

 was more often present in the subsoil, although in many cases even 

 the subsoil was found to be acid, but never to such a degree as is 

 common for the subsoils of the older brown silt loams (middle and 

 upper Illinoisan, pre-Iowan, and lowan). 



With unimportant exceptions, all samples of surface, subsur- 

 face, and subsoil of the yellow silt loams (and the yellow fine sandy 

 loam) of the hill lands were distinctly acid, the degree varying with 

 the age of the soil. 



In the late Wisconsin glaciation both the brown silt loam and 

 the yellow-gray silt loam samples were almost invariably slightly 

 acid in the surface and subsurface, but exceedingly well supplied 

 with carbonates in the subsoil. 



