SILICON 59 



The following reactions will explain some of the 

 more elementary forms of combinations : 



CaO + SiO 2 = CaSiO 3 CaO + N 2 O 5 = Ca(NO 3 ) 2 



3 CaO + P 2 5 =Ca 3 (P0 4 ) 2 K 2 O + SO 3 = K 2 SO, 



CaO + S0 3 = CaSO, Na 2 O + SO 3 = Na.SO, 



CaO + CO 2 = CaCO 3 MgO + SO 3 = MgSO 4 



When considered as the oxide, calcium may com- 

 bine with any of the oxides of the acid-forming ele- 

 ments, as indicated by the reactions, to form salts. 

 Each of the compounds formed from the more com- 

 mon elements may have a separate value as plant 

 food, hence it is important to consider the combina- 

 tions of each element separately. 



ACID-FORMING ELEMENTS 



71. 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 the soil. Silicon never occurs in the soil in the 

 free state. It either combines with oxygen to form 

 silica (SiO a ), or with oxygen and some base-forming 

 element or elements to form silicates. Silica and the 

 various silicates are by far the most abundant com- 

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

 elements absolutely necessary for plant growth, and 



