4 THE PRINCIPLES OF SOIL MANAGEMENT 



2. General abundance of the plant-food elements. 



Having now in mind the essential food elements it is 

 of interest to know their general abundance in the earth's 

 cjust. The following table is given by Clark: 



Oxygen 47.02 Phosphorus 0.09 



Silicon 28.06 Manganese 07 



Aluminum 8.16 Sulfur 07 



Iron 4.64 Barium 05 



Calcium 3.50 Strontium 02 



Magnesium 2.62 Chromium 01 



Sodium 2.63 Nickel 01 



Potassium 2.32 Lithium 01 



Titanium 41 Chlorin 01 



Hydrogen 17 Fluorine 01 



Carbon 12 



100.00 



The first eight elements form 98.8 per cent of the 

 earth's crust. In this list are found all of the food ele- 

 ments except nitrogen, which forms four-fifths of the 

 atmosphere. All of the food elements except nitrogen 

 appear among the first thirteen, and in amounts of not 

 less than .07 per cent. This gives assurance that none 

 of the food elements are rare. It will appear later that 

 they are all very generally distributed. The ultimate 

 source of the elements of the first or so-called incom- 

 bustible groups is the minerals of the earth's crust. 



II. IMPORTANT SOIL-FORMING MINERALS 



Minerals are the units of which soils and rocks are 

 primarily composed. A mineral is a compound occurring 

 in nature having approximately a definite chemical 



