CARBON AND NITROGEN CYCLES IN THE SOIL 203 



serve to increase nitrogen fixation, though under other 

 conditions they can, like sugar, depress the nitrate content 

 of the soil. Doryland^ has discussed the possibility of 

 using various waste substances as energy supply, and Emer- 

 son^ has discussed the prospect of soil inoculation with 

 azotobacter. 



Richards (238^) has made the interesting observation that 

 animal fsces also serve for the organism both in culture and 

 in more natural conditions, but there is a sharp connection 

 between the diet and the effect. Horses fed on oats gave 

 faeces which induced the greatest fixation : horses in grass 

 came next : cattle receiving cake were next, while the faeces 

 from cattle fed on grass proved unsuitable. 



The difficulty of material might therefore be overcome 

 because large quantities 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 (281), 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°.^ I 



It seems legitimate to conclude that azotobacter fixes | 

 nitrogen in well-aerated soils sufficiently provided with ^ 

 calcium carbonate, potassium salts and phosphates, carbon- 

 aceous 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 (jc) found 



^ Set. Proc. Soc. Amer. Bacteriologists, 1918. 



^ Iowa Research BuL, 45, 1918. 



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



IO°-I2° C. 20°-22° C. 30°-32° C. 



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



