10 



BULLETIN 551. U. S. DEPARTMENT OF AGRICULTURE. 



phases of soil composition concerning which it is extremely desirable 

 to reach conclusions, but with this limitation in mind it is thought 

 that the following topics may be discussed from the point of view of 

 chemical composition: Relation of soil to subsoil; extreme variation 

 in composition; variation in soils of the same province; variation in 

 soils of the same type ; and the relation of the limit of error in analysis 

 to quantities of constituents. 



RELATION OF SOIL TO SUBSOIL. 



It has been commonly observed that where there is a distinct sub- 

 soil, differing in color and texture from the surface soil, there may 

 be a marked difference in the chemical composition of the two and 

 that this difference is most marked in the content of silica, iron, and 

 alumina. Usually it is found that the silica is higher in the surface 

 soil and the iron and alumina in the subsoil. 



The analyses presented here are in accord with this experience. 

 Thirty-four locations are represented by soil and subsoil, and in all 

 but two cases the silica is higher in the surface soil. The iron is 

 higher in the subsoil in all but five cases, and the alumina in all but 

 one. 



Allowing a limit of error for all determinations of 0.05 per cent 

 and regarding all figures that do not differ by more than 0.05 as equal, 

 the relations of soil to subsoil in composition are summarized in 

 Table 4. 



In this table are stated the number of locations where soil and 

 subsoil are the same in composition with regard to any constituent 

 and the number of locations where they were found to differ by 

 amounts of 0.05 to 0.5, 0.5 to 1, and more than 1 per cent. 



Table 4. — Comparison of soils and subsoils, as regards chemical composition. 



Group. 



Si0 2 . 



Fe 2 3 . 



A1 2 3 . 



CaO. 



MgO. 



K 2 0. 



Na 2 0. 



P 2 5 . 



S0 3 . 



Soil higher, more than 1 per 



2S 

 3 



1 



1 

















Soil higher, 0.5 to 1 per cent.. 

















Soil higher, 0.05 to 0.5 per 



4 



1 



8 

 19 



6 



1 



3 



7 



21 

 3 



5 

 12 



IS 



1 



7 

 22 



5 



3 



29 



2 



5 





24 



Subsoil higher, 0.05 to 0.5 









5 



Subsoil higher, 0.5 to 1 per 





11 



l c 



1 

 32 





Subsoil higher, more than 1 



o 























EXTREME VARIATION. 



The soils analyzed represent a wide variation in kind of material 

 from which they have been derived, processes of formation, topog- 

 raphy, and present climatic conditions, and probably represent for 

 some of the constituents nearly the extremes in composition of soils 

 in this country. 



