46 SOILS 



SOILS. 



Soil is the uppermost layer of the earth's surface, and 

 consists of the more weathered portions of the rocks of which 

 the earth is composed. It is immediately underlain by the 

 subsoil. It consists of stones, gravel, sand, silt, clay and 

 organic matter in varying proportions. 



Classification of Soils. 



Per cent clay. 



Sandy under 10 



Sandy loam 10—20 



Loam 20—30 



Clay loam ... 30-50 



Clay over 50 



Besides this we may have calcareous soils, containing over 

 20 per cent calcium carbonate, humous soils with more than 

 5 per cent organic matter, and gravelly soils containing vary- 

 ing quantities of gravel or kunkur. 



Residual or sedentary soils are those formed in situ from 

 the disintegration and decomposition of rocks. 



Transported soils are formed from disintegrated and partly 

 decomposed rock, but instead of remaining in the place previ- 

 ously occupied by the rock, they have been transported and 

 retransported by various agencies such as wind and water to 

 the place where they are now found. 



ROCK-FORMING MINERALS. 



Felspar, an anhydrous double aluminium silicate with 

 potash, soda or lime. Orthoclase is a potash felspar and is 

 the commonest. Felspars on decomposition give rise to clay 

 which is a hydrated aluminium silicate, or laterite. 



Quartz. — Silica or an oxide of silicon. It is found in all 

 crystalline rocks and forms the bulk of ordinary sand where 

 it is often coloured red with oxides of iron. 



Mica is found in many volcanic rocks and is a constant 

 constituent in the gneissic rocks of Southern India. It is 

 extensively quarried in Nellore. 



Carbonate of lime commonly found as accretionary depo- 

 sits of kunkur or nodular limestone. 



