72 SOILS AND FERTILIZERS 



ACID-FORMING ELEMENTS BASE-FORMING ELEMENTS 



Oxygen O Aluminum Al 



Silicon Si Potassium K 



Phosphorus P Sodium Na 



Sulphur S Calcium Ca 



Chlorine Cl Magnesium Mg 



Nitrogen N Iron Fe 



Hydrogen H 



Carbon C 



Boron, fluorine, manganese, and barium are usually 

 present in small amounts, besides others which may be 

 found in traces, as the rare elements lithium and 

 titanium. 



For crop purposes the elements of the soil may be 

 divided into three classes : 



1. Essential elements most liable to be deficient; 

 nitrogen, potassium, phosphorus, and calcium. 



2. Essential elements usually abundant; iron, mag- 

 nesium, and sulphur. 



3. Unnecessary and accidental elements, usually 

 abundant; as chlorine, silicon, aluminum, and man- 

 ganese. 



80. Combination of Elements. In dealing with the 

 composition of soils, the percentage amounts of .the 

 individual elements are not given, except in the case of 

 nitrogen, but instead, the amounts of the correspond- 

 ing oxides. The elements do not exist in a free state 

 in soils, but are combined with oxygen and other 

 elements to form compounds. When considered as 



