THE COMPOSITION OF THE SOIL 



121 



soil with very different effects in either case. He instances 

 two soils of nearly equal potash content, one of which from 

 la Creuse responds to potassic fertilisers, while the other from 

 Grignon does not : in the former case the potash is present 

 mainly in the coarser material, in the latter mainly in the fine 

 (Table XXXIL). 



TABLE XXXII. DISTRIBUTION OF POTASH AMONG SOIL PARTICLES (DUMONT). 



The calcium compounds of a number of soils have been 

 studied by E. C. Shorey, Fry and Hazen (263). The follow- 

 ing were detected by petrographic methods ; 



Frequently 



Hornblende, chiefly Ca(MgFe) 3 Si 4 O 12 with Na 2 Al 2 Si 4 O 12 

 and (MgFe) 2 (AlFe) 4 Si 2 12 . 



Plagioclase, isomorphous mixtures of CaAlSi 2 O 8 and 

 NaAlSi 3 O 8 . 



Epidote, Ca 2 H(AlFe) 8 Si 3 O 18 . 

 Occasionally 



Calcite, CaCO 3 . 



Titanite, CaTiSiO 5 . 



Garnet (Ca, Mg, Fe, Mn) 3 (Al, Fe, Cr, Ti) 2 (SiO 4 ) 8 . 

 Rarely 



Dolomite (Ca, Mg)CO 3 . 



Augite, CaMgSi^A with (Mg, Fe)(Al, Fe) 2 SiO 6 . 



Gypsum, CaSO 4 + 2H 2 O. 



Chemical analysis showed that pairs of soils containing the 

 same total CaO might differ considerably in their content of 

 the various calcium compounds (Table XXXIII.): 



