78 SCIENCE AND SOIL 



vegetation is not due to original differences in the soil materials ; 

 but rather the difference between prairie land and timber land is 

 due to the influence of the vegetation upon the soil. The existence 

 of prairies over areas naturally well surface-drained is due very 

 largely, if not entirely, to the prairie fires, which were, as a rule, of 

 annual occurrence and often a source of danger to the early settlers 

 in prairie regions. 



The annual destruction of any seedlings that may have started 

 effectually prevented the growth of forests on the prairie lands, 

 and it is noteworthy that level areas or valleys on the northeast 

 side of streams were usually timbered, while corresponding areas 

 on the southwest were usually prairie, because of the prevailing 

 southwest winds during summer and autumn. (See " Soils of Clay 

 County, Illinois.") Prairie fires have no tendency to run down hill, 

 and they make but little progress against the wind. 



The wild prairie grasses and weeds, including native legumes, 

 developed an abundant root system to an average depth of 16 to 

 20 inches, varying somewhat with the latitude or length of season, 

 the depth being greater in the latitude of central Illinois than in 

 northern Illinois; and, with the partial decay of these roots, 

 followed the marked accumulation of humus which characterizes 

 the " black soil " of the prairie; while the smaller amount of humus 

 is the chief characteristic of the timber soils. Rotting tree roots 

 are subject to very complete decay, because of the large cavities 

 and ready admission of air. Boring insects and burrowing animals 

 also hasten the destruction, so that the small amount of leaf mold 

 that remains constitutes the main source of humus for timber 

 soils, and even this is exposed to rapid decay. Being poorer in 

 organic matter, the upland timber soils are correspondingly poorer 

 in nitrogen than the prairie soils. 



The prairie lands may be classified according to topography, 

 as undulating prairies and flat prairies. 



The undulating prairie soil covers the nearly level or gently 

 rolling prairie lands that were naturally fairly well surface-drained. 

 It is usually markedly uniform in a given formation, and consti- 

 tutes the most important soil of the corn belt. More or less of the 

 clay and finer silt has been carried downward from the surface and 

 accumulated in the subsoil, and some has been carried away by 



