6 SOILS: PROPERTIES AND MANAGEMENT 



. a^ff 



Oxygen .... 47.17 Sodium . . 



Silicon .... 28.00 Potassium . 



Aluminium . . . 7.84 Hydrogen ... .23 



Iron 4.44 Carbon 19 



Calcium .... 3.42 Sulfur 11 



Magnesium ... 2.27 Phosphorus . . . .11 



The briefest scrutiny of this table reveals the fact that 

 the lighter elements are the more abundant hi the earth's 

 crust. The first four elements make up eighty-seven per 

 cent, while the primary elements of plant growth either 

 are lacking or are present only in very small quantities. 



5. Soil-forming rocks. As has been stated, ordinary 

 soil is made up largely of inorganic matter which is derived 

 from ground-up rock material. Therefore, in any study 

 of soil origin or formation, however cursory, the atten- 

 tion must be directed toward geological conditions, not 

 because of their mere geological interest but because of 

 their ultimate bearing on soil fertility and crop growth. 

 In the soil we expect to find, and do find, fragments of 

 the commonest rocks, because those most exposed and 

 those present to the largest extent at the earth's surface 

 must be the ones to break down into soil. Therefore 

 the commonest soil-forming rocks are the rocks that 

 are met so commonly in the field. They may be classified 

 broadly under three heads igneous, sedimentary, and 

 metamorphic. Some of the common types are as follows : 



Igneous Sedimentary Metamorphic 



- Granite Limestone Schist 



-Syenite Sandstone Gneiss 



Diorite Shale Marble 



Diabase Dolomite Slate 



Gabbro Quartzite 

 Peridotite 



