204 SOIL CONDITIONS AND PLANT GROWTH 



that the relative distribution of azotobacter and Clostridium at 

 Rothamsted depended on the amount of calcium carbonate 

 in the soil ; wherever any notable quantity was present, azoto- 

 bacter invariably occurred : otherwise Clostridium alone was 

 found. This result appears to be general.^ Remy suggested 

 in 1 906 that azotobacter was a good organism for the bacterial 

 diagnosis of soils — absence showing some harmful factors and 

 a rich development showing favourable conditions. Christensen 

 (67^) gave definite form by designing a workable method (see 

 p. 242), while Gainey ^ has ascertained that azotobacter occurs 

 in soils with a Ph value 6"0 or more, but not in those with Ph 

 value 5 '9 or less. 



Nitrogen Fixation by Bacteria in Symbiosis with 

 Leguminosse. 



After Hellriegel and Wilfarth's great discovery of the 

 relationship between bacteria and leguminosae (p. 24) many 

 unsuccessful attempts were made to isolate and study the 

 organisms by the methods then in vogue. In 1888 Beijerinck 

 (13) broke away from the ordinary meat-bouillon-gelatin plate 

 and substituted a slightly acid medium made up of infusion 

 of pea leaves, gelatin (7 per cent), asparagine ("25 per cent.) 

 and sucrose ('5 per cent). Growth readily took place and 

 the colonies yielded rods i ^ wide and 4 to 5 //, long, some 

 of which showed signs of bacteriod formation, and " swarmers " 

 0"9 /i long and 0'i8 yu, wide, these being among the smallest 

 soil organisms known. ^ 



The life cycle has been shown by Bewley and Hutchinson 

 (36) to include non-motile and motile stages : conditions 

 were ascertained under which one passed into the other. 



^ Hugo Fischer (1905) found azotobacter on the limed plots at Bonn- 

 Poppelsdorf but not on the unlimed. Burri (1904) found it in only one-third of 

 the Swiss soils examined. 



'' Journ. Agric. Res., 1918, 14, 265. E. B. Fred and A. Davenport found the 

 limits in culture solutions to lie between 6*5 and 8*6 (jfourn. Agric. Res., 1918, 



14.317)- 



* Golding has shown that they will even pass through a porcelain filter and 

 has prepared pure cultures in this way. 



