SOIL CLASSIFICATION 



15 



Mechanical Analysis and Mechanical Composition. — In order 

 to be sure to which class a soil belongs, the amounts of sand, silt 

 and clay it contains must be determined by laboratory methods 

 designated as ''mechanical analyses/' Such analyses give the 

 ''mechanical composition" of a soil; as, for example: 



Dry soU 



Mechanical Composition 

 Per cent of 



Soil class 



Reason 





Sand 



Silt 



Clay* 





A — ^fonnd to contain 

 B — ^foiind to contain 

 C — ^found to contain 



15.7 

 16.9 

 63.0 



68.1 

 37.9 

 21.0 



15.8 

 44.4 

 12.0 



Silt loam 

 Clay 

 Sa.ndy loam 



Contains more than 

 50 per cent silt. 



Contains more than 

 30 per cent clay 



Contains more than 

 50 per cent sand. 



* Per cents total 99,6 — remaining 0.4 per cent consisted of stone and gravel. 



" Light " and " Heavy " Soils. — A sandy soil, because of its 

 high sand content, is comparatively easy to work — ^for this reason 

 it is usually regarded as a " light ^' soil. A soil like clay, on the other 

 hand, is termed "heavy" because it is much more difficult to till. 

 By weight sand is the heaviest soil — clay is much lighter. 



Soil Classification Based on Mode of Formation. — It is con- 

 venient to study soils in relation to the manner in which they were 

 formed or built up. This gives rfee to quite a different classifica- 

 tion, as follows : 



1. Residual soils — ^remaining on rocks where formed. 



2. Cumulose soils — deposits of partially decayed vegetation. 



3. Alluvial soils — ^built up by alluvium deposited by flow- 

 ing water. 



4. Glacial soils — ^formed through glacial action. 



5. Marine soils — ^formed by sediment carried into the sea. 



6. Lacustrine soils — ^formed by sediment carried into lakes. 



7. Loess (lo'^s)) — ^formed through the accumulation of dust 

 carried by wind. 



8. Colluvial soils — amoved down steep slopes by gravity. 



The first two groups are ^'sedentary'' soils, since they have 

 stayed where they were originally formed. The next five groups 

 are ^'transported" soils — ^the material having been carried and 

 laid down by water, glaciers and wind. The last named soils are so 

 called because they moved down steep slopes, due to gravity. 



