The Physical Character of Soils 27 



Combinations The most abundant element in nature is 

 cirton'*''' probably silicon. The quartz rocks so plen- 

 Oxygen, and tifully distributed over the earth are corn- 

 Hydrogen posed of silicon and oxygen, making what 

 is called siUca, and the breaking down of 

 these rocks makes the sharp quartz sand used in plas- 

 ter for building. SiHca, either in the form of sand more 

 or less fine, or in the forms of pebbles and broken frag- 

 ments of rock, makes the greater part of the bulk of all 

 the soils, and without a Hberal amount the cultivation 

 of soil would be almost impracticable. 



Carbon gets into the soil through the decay of vegeta- 

 tion. Vegetation gets it from the air, through the green 

 leaves that possess power to gather carbon from the at- 

 mosphere, where it is always present in combination with 

 oxygen. When it combines with oxygen and forms car- 

 bon dioxide, or as it is commonly called, carbonic acid, it 

 becomes one of the great decomposing agencies in the soil, 

 and also combines with hme to form the carbonate of lime 

 known as limestone, or with other elements, makhig other 

 carbonates. 



In the soil this carbonic acid plays a very important 

 part as a decomposer of combinations that are insoluble 

 in plain water, and so brings into use for plants potash 

 and other plant foods that exist in the soil. While carbon 

 in its various combinations is plentiful in all fertile soils 

 where humus or vegetable decay abounds, no experiments 

 have proved that plants ever take carbon through their 

 roots from the soil. In building up their structure, in 

 which carbon plays a very important part, they get their 

 supply rather, as we shall explain further on, through 



