SOIL COMPOSITION 61 



In level upland areas, such as the loess-covered prairies of the 

 Central West, which neither receive deposits from overflow nor 

 lose partially depleted soil by erosion (especially while covered by 

 prairie grasses), the operation of the natural laws tends steadily 

 toward soil depletion, with respect to the valuable mineral elements; 

 and this law has been in operation since the glacial age, or since 

 the loess was deposited, wherever the climatic conditions have been 

 similar to those prevailing in historic time. Thus we find (as 

 hereinafter shown) that the oldest glacial or loessial soils (as in 

 the lower Illinoisan glaciation) are markedly poorer in total phos- 

 phorus, potassium, magnesium, and calcium than are the simi- 

 larly formed soils of more recent formation (as in the late Wiscon- 

 sin glaciation). With some elements the difference is most marked 

 in the surface soil, and with others in the subsoil. 



The accumulation of organic matter in the glacial or loessial 

 soil begins sometime after its deposition and continues until a 

 maximum is reached, after which the organic matter, as well as 

 the valuable mineral elements, tends to decrease, the latter because 

 of leaching, as from the beginning, and the former because the rate 

 of decay finally exceeds the rate of growth or accumulation. 

 Ultimately, under these natural processes, the level lands would 

 become practically barren. All of the level upland soils of southern 

 Illinois were far past the maximum in productive power when this 

 country was first settled. Indeed, much of the land of central and 

 northern Illinois was past the maximum and tending toward 

 depletion. Probably the black clay loam soil of the flat prairie 

 lands in the Wisconsin glaciation was almost at its maximum 

 condition of productiveness when the White Man took possession, 

 but even the soil of this topography (drab silt loam) was far past 

 its prime in the lower Illinoisan glaciation. 



In some of the Southern states there are still to be found level 

 upland virgin soils that are known, as a class, to be too unproduc- 

 tive to justify cultivation. The author has collected representative 

 samples of this class of virgin gray silt loam soils that were found 

 upon analysis to contain less than 400 pounds of total phosphorus 

 in 2 million pounds of surface soil, while the subsoil of adjoining 

 moderately productive slopes contained 1500 pounds of phos- 

 phorus. The carbonates of calcium and magnesium have entirely 



