ACID-FORMING ELEMENTS 63 



pounds present in the soil. Silicon is not one of the 

 elements absolutely necessary for plant growth, and 

 even if it were, all soils are so abundantly supplied 

 that it would not be necessary to use it in fertilizers. 



71. Double Silicates. — When two or more base- 

 forming elements are united with the silicate radical, 

 a double silicate is formed. In fact the double sili- 

 cates are the most common forms present in soils. 

 There are also a number of forms of silicic acid which 

 greatly increase the number of silicates, and a study of 

 the composition of soils is largely a study of these 

 various silicates. 



72. Carbon is an acid-forming element and belongs 

 to the same family as silicon ; it is found in the soil 

 as one of the main constituents of the volatile or 

 organic compounds. Carbon also unites with oxygen 

 and the base-forming elements, producing carbonates, 

 as calcium carbonate or limestone. The carbon of the 

 soil takes no direct part in forming the carbon com- 

 pounds of the plant. It is not necessary to apply 

 carbon fertilizers to produce the carbon compounds of 

 plants because the carbon dioxide of the air is the 

 source for crop production. It is estimated that there 

 are 30 tons of carbon dioxide in the air over every 

 acre of the earth's surface. 21 The carbon in the soil is 

 an indirect element of fertility, because it is usually 

 combined with elements, as nitrogen and phosphorus, 

 which are absolutely necessary for crop growth. 



I UNIVERSr 



