[Chap. XLIII BACTERIA OF THE SOIL 539 



sulfur from sulfates of the soil. Upon the death of the plant the proteins 

 containing sulfur are acted upon by bacteria, with the production of 

 hydrogen sulfide. The hydrogen sulfide is oxidized through the agency 

 of sulfur bacteria to elemental sulfur and sulfur dioxide. The latter is 

 further oxidized to sulfuric acid which may react with a base, such as 

 calcium carbonate, and form calcium sulfate. This formation of sulfates 

 is known as sulfofication, and takes place through the agency of the 

 bacteria. The sulfates are then available to green plants and the cycle 

 is complete. 



Sulfates in the soil may be depleted through the action of another 

 group of bacteria, forming hydrogen sulfide. Reduction of elemental 

 sulfur may also result in the formation of sulfides. This reduction process 

 is known as desulfofication. 



Sulfofication is an oxidation process through which the sulfur bacteria 

 secure energy that is used by them in the synthesis of sugar and other 

 foods. Such microorganisms are thus autophytes, and in this respect 

 resemble the nitrifying bacteria. 



Bacteria and phosphates. Organic compounds of phosphorus are pres- 

 ent in plant and animal residues. Before the phosphorus is usable by 

 green plants, these complex compounds must be resolved by soil micro- 

 organisms. Such decomposition involves a number of reactions; phos- 

 phoric acid is finally formed, it accumulates in the soil as phosphates of 

 calcium, magnesium, iron, and aluminum. 



Iron bacteria. Bacteria are associated with the transformation of iron 

 compounds in the soil through oxidation-reduction reactions. Certain 

 iron bacteria are able to bring about the oxidation of ferrous to ferric 

 iron, therebv securing energv bv which thev synthesize their sugars and 

 other foods; thus they are autophytes. Under anaerobic conditions ferric 

 iron may be reduced to ferrous iron by other iron bacteria. Although 

 it is rarely necessary to add iron to soils, iron deficiencies sometimes 

 occur in alkaline regions owing to the formation of insoluble compounds 

 of iron both by colloidal aggregation and by bacterial action. 



If we could review all the activities of the many kinds of bacteria we 

 would appreciate still more their fundamental importance in our bio- 

 logical world. Parasitic bacteria may cause the death of living organisms. 

 Numerous saprophytic bacteria in turn digest the organic compounds in 

 the dead bodies, and oxidize the carbon compounds in respiration. 

 Through these disintegrative processes the inorganic ions, such as those 

 of phosphorus, iron, and magnesium, that are chemically bound in the 



