168 THE SOIL: INORGANIC MATTER 



but in water containing carbon dioxide it decomposes as 

 follows : 



K 2 Al2(Si 3 08)2+ C0 2 +2H 2 = Al 2 (OH)4Si 2 5 + 4Si0 2 + K 2 C0 3 . 



Its solubility in pure water is 1 part in 37,000; in water 

 saturated with carbon dioxide, 1 part in 4000. 



From the above equation it is to be noted that the potas- 

 sium goes into solution as potassium carbonate and it is 

 this form which supplies the plant with most of its potassium. 

 It is also to be observed that a new silicate is formed. This 

 is a hydrated disilicate of aluminium and is called kaolinite, 

 the graphic formula of which is: 



o— H 



O— Al 



0=Si O— H 



\ 

 O 



/ 

 0=Si O— H 



\ / 



O— Al 



\ 



O— H 



It is a compact or mealy mass with greasy feel when wet, 

 very plastic, and white, yellow, brown, red, or blue in color. 

 This particular process of decomposition by which kaolinite 

 is formed is called Jcaolinization and is common to very many 

 silicates. It is one of the most important soil reactions, for 

 not only is plant food made available by it, but in addition 

 the soil structure is modified by kaolinite, the basis of clay. 

 The mechanical mixture of kaolinite and silica formed in 

 the above reaction is called kaolin or potter's clay. Varying 

 amounts of silica are present, some being dissolved and 

 washed away in the soil. 



The potassium in orthoclase is also made available by 

 another reaction due to calcium bicarbonate, thus: 



K 2 Al 2 (Si 3 8 ) 2 + CaH 2 (C0 3 ) 2 = CaAl 2 (Si 3 0«) 2 + 2KHC0 3 . 



This shows the importance of having calcium carbonate in 

 the soil. (Compare the effect of calcium bicarbonate on 

 iron phosphate, Section 135, a). 



