66 



Occurrence of Microorganisms in Soil 



upper few inches and rapidly disappear in the subsoil. In arid soils, 

 they occur to a depth of many feet. 



Sulfur Bacteria. The sulfur bacteria, or those bacteria which 

 are capable of obtaining the energy necessary for their growth from 

 the oxidation of sulfur or its compounds, should be distinguished 

 from other bacteria taking part in the sulfur cycle, such as those 

 liberating hydrogen sulfide in the hydrolysis of proteins or in the 

 reduction of sulfates. 



The sulfur bacteria do not form any uniform morphological or 



physiological group of organisms, 

 as do the nitrifying bacteria. 

 Morphologically they are found 

 among the Desmobacteriaceae 

 and among the Bacteriaceae. 

 Physiologically they oxidize hy- 

 drogen sulfide and other sulfides, 

 elementary sulfur, or thiosulfate; 

 they act either in an acid or in an 

 alkaline reaction. Some are obli- 

 gate autotrophic and some are 

 facultative. They are widely dis- 

 tributed in nature, occurring in 

 water basins, soils, and other nat- 

 ural substrates. Those sulfur bac- 

 teria that are found in normal, 

 fertile soils, or that become active 

 in such soils when introduced, are limited chiefly to the genus Tliio- 

 hacillus. 



At least eleven species of TJiiobacilliis are found in the literature, 

 and twelve others have been described but not named. There is 

 considerable overlapping among the various forms, many of them 

 having been only incompletely described. Some, like TJi. tliiooxi- 

 dans, also oxidize sulfiu and are obligate autotrophic. The thio- 

 sulfate-oxidizing bacteria have been separated into the strictly auto- 

 trophic (Th. thioparus), facultative autotrophic (TJi. noveUus), and 

 heterotrophic (Pseudomonas fluorescens) forms: the first two groups 

 increase the acidity of the medium, and the third group decreases 

 its acidity. 



When sulfur is mixed with soil, it is oxidized slowly at first and 

 then, as the soil becomes acid, more rapidly. If powdered rock phos- 

 phate is added to the mixture of soil and sulfur, the insoluble phos- 



FiG. 29. Sultur-uxidizing bacterium, 



Thiobacillus thiooxidans (from Waks- 



man and Joffe). 



