646 PRINCIPLES OF SOIL MICROBIOLOGY 



num silicates into hydrated aluminum oxide. 7 Miintz 8 suggested in 

 1890 that bacteria are concerned in rock decomposition, their action 

 being confined not only to the surface but often entering into the depth 

 of the rock mass; nitrifying organisms were always demonstrated in 

 decomposed rocks. Other investigators 9 also suggested that bacteria 

 are possible agents in the decomposition of rocks. 



Certain bacteria were found 10 capable of deriving their necessary 

 mineral nutrients from feldspars, bringing considerable quantities 

 of undecomposed orthoclase into solution, probably by means 

 of the carbon dioxide which is formed. The action of a number of 

 bacteria, including B. extorquens, nitrate-forming and butyric acid or- 

 ganisms, as well as yeasts, upon twelve different silicates and upon 

 apatite was investigated. The bacteria were able, by means of their 

 products of respiration, to dissolve considerable amounts of pulverized 

 silicates; the formation of organic acids by Bac. amylobacter markedly 

 influenced the solubility of silicates. The intensity of contact of the 

 organism with the mineral to be acted upon was found to be of even 

 greater importance than the other agents of solubility. Thus, B. 

 extorquens, which produced only carbon dioxide but which formed a 

 close and firm envelope around the mineral particles, had the strongest 

 solvent action. Yeasts which do not form as close a contact, although 

 they produce more carbon dioxide, brought about less solubility. 

 Nitrite forming bacteria, as a result of the production of a strong in- 

 organic acid (HNO2), caused a considerable solubility of the silicates. 

 Minerals rich in alkaline earths were most readily acted upon. Apa- 

 tite dissolves only to a limited extent in carbonic acid and only those 

 bacteria which produce organic acids can bring about a considerable 

 solubility. The chemical constituents of the minerals were found 10 in 

 the filtrates of the bacterial cultures, especially in case of B. extorquens. 

 The alkalies came into solution first, followed by the alkaline earths 

 and iron; silicic acid and aluminum oxide came into solution last. 



7 Vernadsky, W. J. Sur le probleme de la decomposition du kaolin par les 

 organismes. Compt. Rend. Acad. Sci., 175: 450-452. 1922; see also Stoklasa, J. 

 tlber die Verbreitung des Aluminums in der Natur. G. Fischer, Jena. 1922. 



8 Miintz, A. Chimie agricole. Sur la decomposition des roches et la forma- 

 tion de la terre arable. Compt. Rend. Acad. Sci., 110: 1370-1372. 1S90. 



9 Merrill, G. P. Disintegration of the granite rocks of the District of Colum- 

 bia. Bui. Geol. Soc. Am., 6: 321-332. 1895. 



10 Bassalik, K. Tiber Silikatzersetzung durch Bodenbakterien. Ztschr. 

 Garungsphysiol., 2: 1-32. 1912; 3: 15-42. 1913. 



