60 Occurrence of Microorganisms in Soil 



Soil Bacteria 



For the classification of soil bacteria, the Bergey system is now 

 almost universally used. It will also be adopted here, with certain 

 slight modifications. The following five orders are now recognized: 



I. Simple and undifferentiated forms, not producing any tlireads and not 



branching under normal conditions of culture Eubacteriales. 



II. Rod-shaped, clubbed, or filamentous cells, with decided tendency to true 

 branching Actinomycetales. 



III. Filamentous, largely aquatic forms, some showing false branching. 



Chlamydobacteriales. 



IV. Cells enclosed in a slimy mass, forming a pseudoplasmodium-hke aggrega- 

 tion before passing into a cyst-producing resting stage .... Myxobacteriales. 



V. Cells slender, spiral, flexuous Spirochaetales. 



Another system of classification of bacteria based upon their physi- 

 ological activities has frequently been employed in soil studies. 



A. Autotrophic and facultative autotrophic bacteria, deri\ ing their carbon prima- 

 rily from the COj of the atmosphere and their energy from the oxidation of 

 inorganic substances or simple compounds of carbon. 



I. Bacteria using simple nitrogen comiDounds, ammonia and nitrite, as 



sources of energy. 

 II. Bacteria using sulfur and simple inorganic sulfur compounds as sources 

 of energy. 



III. Bacteria using iron (and manganese) compounds as sources of energy. 



IV. Bacteria using hydrogen as a source of energy. 



V. Bacteria using simple carbon compounds (CO, CH4) as sources of 

 energy. 



B. Heterotrophic bacteria deriving their carbon and energy from organic com- 

 pounds. 



I. Nitrogen-fixing bacteria, deriving their nitrogen from the atmosi^here as 

 gaseous atmospheric nitrogen. 



1. Nonsymbiotic nitrogen-fixing bacteria. 



a. Anaerobic, but>'ric acid organism. 



b. Aerobic Azotohacter, Radiohacter, Aerobacter, etc. 



2. Symbiotic nitrogen-fixing, or root-nodule, bacteria. 

 II. Bacteria requiring combined nitrogen. 



1. Aerobic bacteria. 



a. Spore-forming bacteria. 



h. Non-spore-forming bacteria: 



( 1 ) Gram-positi\'e bacteria. 



(2) Gram-negative bacteria. 



2. Anaerobic bacteria, requiring combined nitrogen. 



