CLIMATIC AND GEOC HEMIC AL MELATIONSHIPS 69 



of lime present, and again emphasizeh the point that, 

 while a glacial soil from a limestone is rich in lime, a 

 residual soil from the same rock is usually poor in that 

 constituent. Even loess, which has been subjected to 

 some washing before being deposited, is a considerably 

 richer soil than those of residual origin. 



It must be remembered, however, that these comparisons 

 are of a general character and do not apply to all cases, 

 since many glacial soils may be very much poorer in the 

 plant-food constituents than some of the representative 

 residual soils. Moreover, the physical condition of a 

 soil is a great factor in productivity. As a matter of 

 fact, the mere presence of plant-food is but one of a 

 considerable number of factors that determine the crop- 

 producing power of a soil. Also, the humus content of 

 the soils of various provinces may be variable, due to 

 climatic conditions. Neither are all glacial soils rich in 

 lime, as that constituent is determined largely by the 

 amount in the parent minerals. A rock poor in lime, 

 therefore, must from necessity give rise, when glaciated, 

 to a soil deficient in lime. This is well illustrated by the 

 average analyses ^ of the loam soils of Ashtabula County, 

 Ohio, originating from the giaciation of the lime-poor 

 shales of that region : — 



CaO 25 



MgO 61 



P2O6 04 



-i.\.2^— ' » • . • » • • . . • X«0( 



Humus .1,70 



^ Ames, J. W., and G-aither, E. W. Soil Investigations. Ohio 

 Agr. Exp. Sta., Bui. 261. 1913. 



