130 SOILS AND MANUBES 



to that condition by complete decomposition of the 

 felspar. That portion which is soluble in concen- 

 trated acids, but insoluble in dilute acids, is almost 

 certainly a product of partial decomposition, and that 

 which is not attacked by concentrated acids consists of 

 the original or only very slightly altered mineral. Dilute 

 and concentrated acids are, of course, relative terms, and 

 it is impossible to distinguish sharply between the products 

 of complete and partial decomposition by this means. 

 Also, the amount of potash that can be extracted from soil 

 by acid of any strength depends, to some extent, upon the 

 temperature and time of extraction ; there appears to 

 be no absolute finality about it. Even after prolonged 

 extraction, renewed application of the solvent will gener- 

 ally extract a little more. 



The presence of lime in the soil favours the liberation 

 of potash. Three hypotheses are proposed in explanation 

 of the fact, any or all of which may be correct. 



1. The lime, by reason of its affinity for silicic acid, 

 favours the splitting off of Si0 2 with which it unites form- 

 ing calcium silicate, thus 



H 2 Si0 3 + CaC0 3 = CaSi0 3 + H 2 + C0 2 



2. The lime acts simply as a carrier of C0 2 , thus 



2 KA1 Si 3 8 + CaH 2 (C0 3 ) 2 + H 2 = 



Si 2 5 A1 2 (OH) 4 + K 2 C0 8 + 4 Si0 2 + CaC0 3 



The first of these two reactions may be held to involve 

 the second, and they may be represented jointly thus 



2 KA1 Si 3 8 + Ca C0 3 + 2 H 2 = 



Si 2 5 Al a (OH) 4 + K 2 C0 3 + CaSi0 3 + 3 Si0 2 



3. The lime simply displaces the potash, thus 



2 KA1 Si 3 8 + Ca C0 3 = CaAl 2 (Si 3 8 ) 2 + K 2 C0 3 



