CHEMICAL COMPOSITION OF SOILS 6l 



Acid-forming elements Ba^e-forming elements 



Oxygen O Aluminum . . Al 



Silicon Si Potassium K 



Phosphorus P Sodium Na 



Sulphur S Calcium Ca 



Chlorine CI Magnesium Mg 



Nitrogen N Iron Fe 



Hydrogen H 



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 : 



i. 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. 



69. 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 combined 

 with oxygen and other elements to form compounds. 

 When considered as oxides the acid- and base-forming 

 elements may form various compounds as : 



