166 THE SOIL: INORGANIC MATTER 



is present. Where calcium bicarbonate is in solution in the 

 soil water these phosphates are slowly changed as follows : 



2FeP0 4 + CaH 2 (C0 3 )2+ 4H«0 = 2Fe(OH),+ CaH 4 (P04) 2 + 2C0 2 . 



This emphasizes the importance of having calcium carbonate 

 or lime in the soil. Aluminium phosphate acts similarly. 



(6) Potassium Minerals. — Since these minerals, and in 

 addition most of those of calcium, magnesium, and iron, 

 belong to that principal group of minerals called silicates, 

 it will help to understand their structure if a brief survey 

 is made of the silicic acids from which they are derived. 



Silicic Acids. — Normal or orthosilicic acid is H 4 Si0 4 , 

 written graphically: 



H— O O— H 



\ / 



Si 



/ \ 



H— O O— H 



By the elimination of one molecule of water there is formed 

 metasilicic acid, thus: 



H 4 Si04— H 2 =H 2 Si0 3 or: 

 O— H 



/ 

 0=Si 



\ 



O— H 



metasilicic acid. 



By the elimination of three or more molecules of water from 

 two or more molecules of orthosilicic acid there are formed 

 polysilicic acids, of which the disilicic and trisilicic acids are 

 the commonest. They are formed thus : 



1. 2H 4 Si0 4 — 3H 2 = H 2 Si 2 5 or: 

 O— H 



/ 

 0=Si 



\ 



O 



/ 

 0=Si 



\ 



O— H 



disilicic acid. 



