62 SCIENCE AND SOIL 



disappeared from these level upland soils, and in their place marked 

 acidity has developed. Under these conditions the growth of vege- 

 tation and the fixation of nitrogen by legumes become very lim- 

 ited, and level virgin soil, not subject to erosion, was found to 

 contain less than one fifth as much as the average nitrogen content 

 of the black clay loam of the late Wisconsin glaciation in northern 

 Illinois. 



In the progress of geologic time, surface drainage courses are de- 

 veloped, and all level uplands become eroded hills and valleys, thus 

 exposing the lower subsoils with their larger supplies of unleached 

 mineral plant food, more or less of which is spread out over the 

 lower lying slopes or level bottom lands, which sometimes again 

 become depleted, as broad terraces above the deepened channel. 



Thus, moderate soil erosion is not an unmixed evil; and, with no 

 adequate return of mineral plant food, the bottom lands and the 

 sloping hill lands are more permanently productive (with legumes 

 made prominent in the crop rotations) than are the level upland 

 soils. It is doubtful if there has ever been a land on the face of the 

 earth, where the same soil particles have been turned with the plow 

 year after year, that has remained productive for two centuries, 

 with no return of mineral plant food. Even in populous China 

 there are many level upland areas, sometimes of a hundred square 

 miles in extent, where no one lives; and the restoration of these 

 areas has been called the " Problem of China." 



" In nature all things are in equilibrium " is often stated as though 

 it were a self-evident fact. So far as the soil is concerned, the oppo- 

 site is essentially true, that, in nature, there is no equilibrium. 

 Thus an ancient forest land now lies from 10 to 300 feet beneath 

 the Illinois black prairie, which covers the unweathered glacial 

 drift of the most southern lobe of the Wisconsin glaciation. 



It is of first importance that the man who controls land, and who 

 is thus responsible for its future productive power, should have 

 sufficient fundamental knowledge concerning the composition of 

 common soils and the plant-food requirements of common staple 

 crops to furnish him a foundation of absolute facts on which to 

 build possible systems of permanent agriculture. Because of this 

 need, considerable space is devoted to the ultimate composition 

 of soils as they exist on the earth to-day. 



