Bacteria and Sulfur 299 



monly employed in the preparation of culture media, 

 serves as the source of sulfur to the organisms in such 

 cultures. In field and pot experiments, sulfates have 

 been found to produce favorable results in encouraging 

 the development of certain groups of soil bacteria. It 

 has been shown that sulfates of lime, magnesia, potash 

 and soda favor nitrification in most soils. It has also 

 been shown that in the decomposition of manure, gyp- 

 sum (sulfate of lime) exerts a retarding effect at the 

 beginning. Carbon dioxid is evolved less rapidly, and 

 the losses of nitrogen are smaller. When, however, the 

 nitrification processes once set in, the change of the 

 organic nitrogen into nitrate proceeds more economi- 

 cally and more rapidly. The resulting product is of 

 better quality and yields a greater crop increase than 

 the untreated manure. 



Sulfate of iron, another compound of sulfur, has like- 

 wise been found in some instances to stimulate crop 

 growth. The effects in this case are undoubtedly of a 

 bacteriological character, although scarcely anything is 

 known of the exact nature of this action. There is rea- 

 son to believe that, at times, the stimulating action 

 may be due to the iron rather than the sulfur in the iron 

 sulfate, while at other times the sulfur plays a predom- 

 inating role. 



BACTERIA IN RELATION TO IRON 



Substances capable of uniting with the element 

 oxygen are said to possess potential energy. Such po- 

 tential energy is possessed, among other elements, by 

 nitrogen, carbon, hydrogen, and sulfur. We have seen 



