42 



HOW ROCKS AJN-D CLIMATE AFEECT SOILS 



Carbonates. — Such as washing soda. 



Phosphates, — ^As Ume phosphate. 



From Granite Into Soil. — ^A residual soil resulting from the 

 weathering of a granite is usually a sandy loam or loam. The 

 transformation of granite into soil is illustrated and summarized 

 in the following diagram: 



From Rock into Soil 



The rock 











Products of 



Resulting soil 





Chemical composition of the 



rock 



(classes) 



Mineral composition 



rock minerals 



weathering 







'Quartz 



Si,0 



Sand, silt, 

 clay * 



Sandy loam 





Feldspars 



Al, Si, Na, Ca, K, 



True clay 





Granite- 



Micas 



Al,Si,Na,Ca,Mg,Fe,K,0 



Carbonate 



or 





Hornblende 



Al, Si, Na, Ca, Mg, Fe, 



of lime 







Apatite 



Ca, P, 



Salts 



loam 





[Pyrite 



Fe,S 







* Sand, silt and clay particles are mamly the products of the breaking up of rocks, while 

 true clay, carbonate of hme and salts are products of rock decay 



Soils From Other Rocks. — Sandstone usually gives rise to sand 

 or sandy soils; shale and slate to heavy clays, and limestone to 

 silt loams and clay loams. 



THE EFFECT OF CLIMATE 



Soils in Humid Climates Are Leached. — In humid^ climates 

 the heavy rainfall causes much soil to be washed away and carried 

 into the ocean. Sand, silt and clay are not the only materials 

 carried into the sea — carbonate of lime and salts are also 

 thus carried. 



In a humid region much of the rainfall sinks into the groimd, 

 and as the water passes down through the soil it dissolves carbon- 

 ate of lime and salts and carries them in solution until it comes 

 to the surface in the form of springs; thence into rivers, and finally 

 into the sea. All drainage waters, therefore, carry to the sea, 

 carbonate of lime and salts in solution and sediments. 



* When a region has an average annual rainfall of more than 30 inches 

 it is regarded as having a humid climate. A region having an average annual 

 rainfall of between 20 and 30 inches the climate is commonly referred to 

 as sub-humid. 



