CARBON AND NITROGEN CYCLES IN THE SOIL 97 



the addition of sugar to the soil would be out of the question for field 

 work. Pringsheim (231) has shown, however, that certain decomposi- 

 tion products of cellulose also serve as sources of energy to clostridium 

 and presumably also to azotobacter. These particular products (which 

 were not identified) are apparently not always formed in the soil (152), 

 but are readily produced in culture solutions under the action of the 

 mixed bacterial flora from soils, composts, dung and river mud. The 

 difficulty of material might therefore be overcome because large quanti- 

 ties of cellulose are available on the farm in the form of straw. But 

 there still remains the question of temperature. Azotobacter, as we have 

 seen, requires more warmth than many other organisms, and according 

 to Koch's experiments ceases to work at 7 C. Thiele read tempera- 

 tures daily for three years of arable and grass soils at different depths 

 at Breslau (282), and concluded that only rarely were they favourable 

 for azotobacter. But it is impossible to argue from a culture solution 

 to the soil, and indeed Lohnis has shown that the mixed cultures of the 

 soil are almost as effective at 10 as at 20 : x 



IO-I2 C. 20-22 C. 30-32 C. 



3*15 mg. 4-55 mg. 4-27 mg. nitrogen fixed. 



It seems legitimate to conclude that azotobacter fixes nitrogen in 

 well-aerated soils sufficiently provided with calcium carbonate, potassium 

 salts and phosphates, carbonaceous material of the right kind and 

 moisture, so long as the temperature is high enough. Where the air 

 supply is diminished owing to the close texture of the soil there is still 

 the possibility of fixation by clostridium. Ashby (7) found that the 

 relative distribution of azotobacter and clostridium at Rothamsted de- 

 pended on the amount of calcium carbonate in the soil ; wherever any 

 notable quantity was present, azotobacter invariably occurred : other- 

 wise clostridium alone was found. It is not certain whether this result 

 is due to some specific action of calcium carbonate or to the shortage 

 of air supply consequent on the bad mechanical state always induced 

 at Rothamsted when calcium carbonate is absent. 



Nitrogen Fixation by Bacteria in Symbiosis with Leguminosae. 



After Hellriegel and Wilfarth's great discovery of the relationship 

 between bacteria and leguminosae (p. 16) many unsuccessful attempts 

 were made to isolate and study the organisms by the methods then in 

 vogue. In 1888 Beijerinck (n) 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 /j, long, some of which showed 



1 Mitt. Landw. Inst., Leipsic, 1905, vii., 94. 



