SOILS OF THE EASTERN UNITED STATES AND THEIR USE-XI. 



THE KNOX SILT LOAM. 



GEOGRAPHICAL DISTRIBUTION. 



The Knox silt loam has been encountered in 17 different soil survey 

 areas located in 7 States, and the total area mapped in these locali- 

 ties aggregates 1,881,024 acres. 



The type occurs chiefly along the greater stream drainage systems 

 of central and southern Indiana, western and southwestern Illinois, 

 and the extreme southwestern portion of Wisconsin; in the eastern 

 and southern portions of Iowa; in northern Missouri and eastern 

 Nebraska. Like the Marshall silt loam, this type is derived from 

 the loess which overlies the areas of several glacial subdivisions, and 

 also mantles the unglaciated rock surfaces in some portions of south- 

 ern Indiana, of Kentucky, and of southern Illinois. 



CHARACTERISTICS OF THE SOIL AND SUBSOIL. 



The surface soil to an average depth of 10 or 12 inches is a light- 

 brown, ashy gray, or nearly white silt loam. In the better drained 

 areas, where erosion has not been excessive, this surface material 

 may have a depth of 14 to 16 inches, and the typical color is then 

 brown or yellow. On the other hand, in areas which are to the 

 least degree deficient in drainage, and also in areas where the organic 

 matter of the surface soil has been largely exhausted, the color is 

 more nearly gray. The subsoil is a compact light-brown or pale- 

 yellow silt loam, sometimes becoming more stiff and clayey at greater 

 depths. Both soil and subsoil are normally stone free, and very 

 little sand is found in any except local areas, where other materials 

 have been mingled through erosion processes with the materials of 

 the Knox silt loam. The fine silt or silty clay, known as the loess, 

 has given rise to both the soil and subsoil of this type. The depth 

 of this material varies from 40 or 50 feet along the bluffs of such 

 rivers as the Mississippi, the Illinois, and the Missouri to layers 

 only 5 or 6 feet thick along the Ohio River and its northern tribu- 

 taries. 



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