CHAPTER XXII 



ILLINOIS FIELD EXPERIMENTS 



ASIDE from the old experiments on the University Farm, the 

 Illinois field experiments have been in progress only for a few years, 

 but they are of special interest and value because they are conducted 

 in widely separated places and on different definite soil types of 

 great extent and importance. 



Brown silt loam constitutes the most common prairie soil in the 

 middle and upper Illinoisan, pre-Iowan, and early Wisconsin 

 glaciations, and is found also in the lowan and late Wisconsin. It 

 is called " the ordinary prairie land " by farmers throughout the 

 corn belt, extending from Mattoon, Illinois, into Wisconsin, and 

 from north-central Indiana into Nebraska and South Dakota. 



While the different brown silt loams are similar in many respects, 

 they differ somewhat in chemical composition, varying with age 

 or formation of the different areas, and it is noteworthy that in 

 the older soil areas the brown silt loam is either no longer repre- 

 sented (as in the lower Illinoisan glaciation), or it is replaced to 

 some extent by a type of soil intermediate in character and value 

 between brown silt loam and gray silt loam on tight clay. This 

 intermediate type is well developed in places in the southern part 

 of the middle Illinoisan glaciation and in the western part of the 

 upper Illinoisan, but it is only one of many minor types whose 

 exact location requires a detail soil survey. 



The top soil of the brown silt loam consists of a friable dark- 

 colored and fairly uniform soil to a depth of 1 6 to 20 inches, with 

 appreciably less organic matter at the lower depth. Below the top 

 soil, from 16 or 20 inches to 40 inches and more, is the yellow, silty 

 subsoil, somewhat less porous or friable than the top soil, but not 

 very compact. 



This soil and subsoil have great capacity to absorb and retain 

 water from heavy rains, and later to deliver the moisture to grow- 



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