ORIGIN AND COMPOSITION OF SOILS 17 



account the rare elements which may occur in the 

 soil, but only those need be considered which are 

 present in some quantity and which enter as an 

 important factor into plant growth or modify in 

 some manner its physical properties. Of the 

 whole number of chemical elements less than 

 one-third are of any importance in soil investi- 

 gations. These elements may be grouped into 

 two classes, the non-metals and the metals as 

 follows : 



Non-metals. Metals. 



Oxygen Aluminum 



Silicon Calcium 



Carbon Magnesium 



Sulphur Potassium 



Hydrogen Sodium 



Chlorine Iron 



Phosphorus Manganese 



Nitrogen Barium 



Fluorine Titanium 



Boron ' Chromium 



Oxygen exists in the free gaseous state in the 

 atmosphere of which it constitutes about one-fifth 

 by bulk, and in combination with other elements 

 it forms nearly half the weight of the solid earth, 

 and eight-ninths by weight of water. It enters 

 into combination with most of the other elements, 

 forming what are known as oxides, and with many 

 of the elements it unites in several proportions, 

 forming oxides of different composition. Com- 



