SOIL-FORMING PROCESSES 35 



thereby favors the accumulation of silica, alumina and iroix, 

 all soils as they age tend to approach each other in chemical 

 composition. Yet, owing to a difference in the adjustment 

 of the forces at work and to the time element, no two soils 



% 



550a l^-^g 



AI2O3 (28.4fflHi 



CaO+ 

 MgO ' 



f46.0 

 1 • I 



KaO+r 

 Na^O 1 





4t.6C 



m i 1 umestqjNe 



Fig. 7. — Diagram showing the composition of limestone and its residual 

 soil. Note the excessive loss of lime and carbon dioxide in soil 

 formation. 



will ever be exactly alike. Soils will differ from the original 

 rock and from one another according to the intensity and 

 character of the weathering and erosive forces and to the 

 constitution of the parent minerals. 



