BACTERIA OF THE SOIL 



29 



they live to transfer them, while others are able to migrate by- 

 means of their organs of locomotion, namely, the flagella. (3) 

 The spirilla are thin spiral-shaped forms, which have different 

 numbers of turns characteristic for the different species; those 

 forms which produce only one-half turn are intermediate between 

 rods and spirilla, and are spoken of as vibrios. Numerically, the 

 rod-shaped forms are much the largest group of the soil bacteria. 



Figs. 7 to 16 show several typical soil bacteria. Fig. 7 repre- 

 sents the general group of spore-forming, rod-shaped, hetero- 

 trophic bacteria, living by the processes of decomposition of 

 various organic compounds in the soil. Figs. 8 and 9 represent 

 two typical bacteria capable of decomposing cellulose, one being 

 aerobic and one anaerobic in nature, living in the presence and 

 in the absence of atmospheric oxygen, respectively. The aerobic 

 and anaerobic non-symbiotic nitrogen- 

 fixing bacteria are shown in Figs. 10 

 and 11. Species of symbiotic legume 

 bacteria are shown in Fig. 12. The 

 important groups of nitrifying bacteria, 

 namely, the nitrite- and nitrate-forming 

 organisms, are shown in Figs. 13 and 14. 

 Bacteria capable of obtaining their en- 

 ergy from the oxidation of elementary 

 sulfur to sulfuric acid are shown in 

 Fig. 15. A bacterium which can re- 

 duce sulfates to sulfides is represented 

 in Fig. 16. The activities of some of 

 these bacteria and their roles in soil 

 processes will be described in subse- 

 quent chapters. 



Nearly all the bacteria are so small 

 that they are able to pass through ordi- 

 nary filters. To remove them from liquids, filtration is accom- 

 plished by the use of Chamberland candles made of unglazed 

 porcelain, or Berkefeld filters made of diatomaceous earth. 

 The pore spaces of these filters are so small that they will pre- 

 vent the passage of bacteria which are \isible with the micro- 

 scope. It has been estabUshed, however, that there are certain 

 ultra-visible bacteria which can even pass through these filters. 

 These are spoken of as ultramicroscopic orga7iis7ns. Such tiny 



Fig. 16. — Sulfate-reducing 



bacterium, Spirillu7n desul- 



furicans (from Beijerinck 



and Omeliansky). 



