CHEMICAL NATITEE OF SOILS 



357 



lime carbonate out of the surface soil into the sub-soil. The 

 accumulated clay is thus frequently more or less cemented into 

 a 'hardpan' by lime partly in the form of carbonate and partly 

 in that of zeolitic (hydrous silicate) compounds, adding to the 

 compactness of the sub-soil, and therefore to the usual specific 

 difference between the soil and sub-soil; viz., the deficiency or 

 absence of humus and the difficulty of penetration by an aera- 

 tion of the roots of plants. ' ' 



For these reasons the soils of arid regions, even though con- 

 taining the same materials, are often of uniform physical and 

 chemical character to great depths. The soluble salts, as car- 

 bonate of lime and salts of potash and soda, which are leached 

 away in regions of great average humidity, remain in those 

 where the annual precipitation is less, or where, on account of 

 its uneven distribution throughout the warmer months of the 

 year, its permeability and consequent leaching action is less. 

 Hilgard brings out this fact prominently in tables from which 

 that below is condensed, the original being compiled from sev- 

 eral hundred analyses of soils from the humid regions of North 

 and South Carolina, Georgia, Florida, Alabama, Mississippi, 

 Arkansas, Kentucky, and the arid regions of California, "Wash- 

 ington, Montana, Utah, Colorado, Wyoming, and New Mexico. 



Showing the Proportional Amounts of Soluble Salts in Soils of Aeid 



AND Humid Eegions 



Constituents 



Insoluble residue . . . , 



Soluble silica 



Potash 



Soda 



Lime 



Magnesia 



Brown manganese oxide , 



Iron peroxide 



Alumina 



Phosphoric acid . , . . 

 Sulphuric acid . . . , 

 Water and organic matter 



Total 



98.635% 



Arid Region 



Humid Eegion 



69.681 % 



84.472 % 



6.289 



3.873 



0.826 



0.187 



0.261 



0.071 



1.645 



0.112 



1.384 



0.209 



0.066 



0.126 



6.431 



3.455 



7.309 



4.008 



0.144 



0.114 



0.035 



0.066 



6.585 



3.657 



100.149% 



Discussing these figures, Professor Hilgard says: "^^ Concern- 



