142 ILLINOIS STATE ACADEMY OF SCIENCE 



THE AGRICULTURAL SIGNIFICANCE OF THE TIGHT 

 CLAY SUBSOIL OF SOUTHERN ILLINOIS 



Prof. R. S. Smith and F. A. Fisher* 

 Illinois Agricultural Experiment Station 



The upland prairie soils of 27 counties in southern Illi- 

 nois, as well as considerable areas in other states located 

 in the Glacial and Loessial Province, among which are Iowa, 

 northern Missouri, and southern Indiana, have an imter- 

 vieus subsoil known as "tight clay." The exact nature of 

 this impervious stratum, which averages from 8 to 12 in- 

 ches thick, and lies at a depth of about 17 inches, has n.t 

 been determined. Its impei-vious nature is probably due o 

 a high inorganic colloidal content. The comparatively low 

 agricultural value of this section of the state is due to the 

 presence of the tight clay, because of its interference with 

 underdrainage. The economic importance as well as the 

 scientific interest of the problem presented by the presence 

 of this unfavorable substratum is apparent, and the dis- 

 covery of a method of ameliorating the unfavorable con- 

 dition would in time add very greatly to the resources of 

 the state as well as to the resources of similar sections in 

 adjoining states. 



The causes of this formation, so far as the writer is 

 aware, are not well understood. One theory accounts lor it 

 on the assumption that the percolating water carried the 

 fine particles down from the surface soil and deposited them 

 in the subsoil. This theory does not account for the failure 

 of this substratum to form excepting in limited areas in 

 glaciated sections farther north in the state. The forma- 

 tion is apparently associated with a high water table. It 

 is found to occur in the bottom lands and terraces as well 

 as in the prairie uplands in practically every portion of the 

 state, but only locally in the northern two-thirds, while it 

 occurs almost universally in the prairie soils of the southern 

 third, which comprises the area covered by the lower Illi- 

 noisan glacial lobe. 



•Mr. Fisher is now Farm Adviser in Wabash County, Illinois. 



