74 SOILS AND FERTILIZERS 



ACID-FORMING ELEMENTS 



81. Silicon. The element silicon makes up from 

 a quarter to a third of the solid crust of the earth and 

 next to oxygen is the most abundant element found in 

 soils. Silicon never occurs in the soil in the free state. 

 It either combines with oxygen to form silica (SiO 2 ), or 

 with oxygen and some base-forming element or elements 

 to form silicates. Silica and the various silicates are 

 by far the most abundant compounds 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. 



When two or more base-forming elements are united 

 with the silicate radical, a double silicate results. The 

 double silicates are the most common compounds 

 present in soils. There are also a number of forms of 

 silicic acid which greatly increase the number of sili- 

 cates, and a study of the composition of soils is largely 

 a study of these various silicates. 



82. 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, and 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 compounds of 



