6 SOILS 



SOILS 



Soils are formed by the gradual decomposition or wearing 

 away of rocks, under the influence of weather, glacial or volcanic 

 action, assisted by the presence of plant-life. Variations of temper- 

 ature have an important effect on the formation of soils, for rocks 

 expand by heat and contract by cold. Thus in the hot tropical 

 sunshine, the surfaces of the rocks expand and are again contracted 

 by the cool air of the nights ; this causes them to crack, and par- 

 ticles crumble off, thereby giving rise to a small but constant addi- 

 tion to the soil. Plant- life also assists in the formation of soils 

 in two ways, i.e., mechanically and chemically. The roots enter 

 crevices of rocks, and by their gradual increase in bulk act like 

 wedges in forcing the two sides apart ; the force thus exerted, 

 even by the roots of very small plants, is considerable, being 

 sometimes sufficient to break through concrete, or lift stone 

 pavements. Roots have also, by means of their acid sap, a solvent 

 action on rocks, and thus assist in their decomposition. Vegetation 

 further contributes to the formation of soil by depositing dead 

 leaves and stems on the surface in the form of humus, to the 

 presence of which is usually due the black colour of garden soils. 



Other important factors in the formation of soils are rivers and 

 streams, the sea, and, in some cases, wind. Rivers grind down 

 rocks and stones, and carry particles of rock or gravel to the 

 plains or valleys, depositing it in the form of mud, which is known 

 as alluvial soil. Such deposits are usually rich in plant-food. 

 The wind occasionally adds fertile matter to soils by carrying sand, 

 dust, straw, leaves, etc., and depositing them in hollows. 



All soils may be classed as either (1) transported, when they 

 are carried from a distance, as by any of the agencies named 

 above ; or ( 2 ) sedentary, local, or indigenous when they remain 

 where formed. To the latter belong by far the greater portion of 

 Ceylon soils. In some cases the surface soil is transported, while 

 the sub-soil is sedentary, or vice versa. Transported deposits, as 

 alluvial soils, are, as stated above, usually characterized by greater 

 fertility than those which are sedentary. 



CONSTITUENTS OF SOIL 



The five substances of which all soils may be said to be 

 composed of are : sand, clay, lime, vegetable matter or humus, and 

 stones. 



