58 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* j 



Boron, fluorine, manganese, and barium are usually 

 present in small amounts, besides others which may 

 be present 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. f 



3. Unnecessary and accidental elements, usually 

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

 ganese. 



70. 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 percentage amounts 

 of the various oxides. This is because the elements 

 do not exist as free elements in the soil, but are com. 

 bined with oxygen and other elements to form com. 

 pounds. When considered as oxides, the acid- and base- 

 o rming elements may form various compounds as : 



