604 PRINCIPLES OF SOIL MICROBIOLOGY 



Maclntire, Gray and Shaw 8 have also attempted to prove that 

 sulfur is oxidized chemically in the soil, but considerably more sulfur 

 was found to be oxidized under unsterile than under sterile conditions. 

 A careful comparison of the chemical and biological oxidation of sul- 

 fur led to the conclusion that the process is chiefly biological in nature. 9 

 Of the various compounds, the soluble sulfides and especially K 2 S and 

 CaS are rapidly oxidized in the soil by chemical agencies. 10 



Elementary sulfur and various sulfides are, however, oxidized in 

 the soil more actively by microorganisms. It was thought originally 

 that this process is limited to certain specific groups of bacteria which 

 are capable of utilizing the energy obtained in the process of oxidation 

 for chemosynthetic purposes. We are coming more and more to 

 recognize that the property of slow oxidation of sulfur or of incompletely 

 oxidized sulfur compounds, such as sulfides, is probably widely dis- 

 tributed among microorganisms. Not only various common hetero- 

 trophic soil bacteria (Bac. mycoides, Bad. fluorescens) are capable of 

 oxidizing small amounts of elementary sulfur, in nutrient solutions 

 containing organic nitrogen and sources of energy (glycerol), 11 but 

 also various common soil fungi and even actinomyces are reported as 

 oxidizing small amounts of sulfur in artificial media and in soil. 12 Pen. 

 luteum seems to be particularly active in this connection. However, 

 neither the oxidation of sulfur by chemical agencies nor its transforma- 

 tion by heterotrophic microorganisms can compare with the rapidity 

 with which sulfur is oxidized, when used as a source of energy by 

 autotrophic bacteria. The mechanism of assimilation of elementary 

 sulfur by heterotrophic microorganisms is still unknown. It may be 



8 Maclntire, W. H., Gray, F. J., and Shaw, W. M. The non-biological oxida- 

 tion of sulfur in quartz media. Jour. Ind. Eng. Chem., 13: 310-313. 1921. 



9 Boullanger, E., and Dujardin, M. Mecanisme de Taction fertilisante du 

 soufre. Compt. Rend. Acad. Sci., 155: 327-329. 1912; Brioux, Ch., and Guerbet, 

 M. L'action fertilisante du soufre. Son evolution dans le sol. Ann. Sci. Agr. 

 (4), 2: 384-396. 1913; Evolution du soufre dans le sol; etude sur son oxydation. 

 Compt. Rend. Acad. Sci., 156: 1476. 1913; Demolon, M. A. Recherches sur 

 Taction fertilisante du soufre. Compt. Rend. Acad. Sci., 156: 725-728. 1913. 



10 Brown, P. E., and Kellogg, E. H. The determination of the sulfofying power 

 of soils. Jour. Biol. Chem., 21: 73-89. 1915. 



11 Demolon, A. Sur le pouvoir sulfoxydant des sols. Compt. Rend. Acad. 

 Sci., 173: 1408-1410. 1921. 



12 Abbott, E. V. The occurrence and action of fungi in soils. Soil Sci., 16: 

 207-216. 1923; Rippel, A. tlber einige Fragen der Oxydation des elementaren 

 Schwefels. Centrbl. Bakt. II, 62: 290-295. 1924; Guittonneau, 1926 (p. 609). 



