FIXATION OF ATMOSPHERIC NITROGEN 581 



According to Voicu, 94 organic matter will influence the sensitiveness 

 of Azotobacter to poisons such as boron. Urea, glycocoll, formamide 

 and allantoin depress nitrogen-fixation; this was attributed, 95 not to a 

 direct toxicity but to the fact that those substances furnish an available 

 nitrogen source. Among the substances acting injuriously upon nitro- 

 gen-fixing organisms, we find caffeine, alloxan, betaine, trimethyl- amine, 

 legumin, cinnamic acid, aspartic acid, hippuric acid, creatine, creatinine, 

 xanthine and hypoxanthine. The first two have a stimulating effect in 

 dilute solutions. 



Influence of reaction upon the growth of non-sytnbiotic nitrogen- fixing 

 bacteria. Lime exerts such a favorable influence upon the activities of 

 Azotobacter in the soil that Christensen 96 suggested using the presence 

 of this organism as an index of the lime requirement of the soil. When 

 the reaction of the soil is more acid than pH 5.7, Azotobacter is absent 

 and the soil needs lime; when the pH of the soil is above 7.4, the soil does 

 not need any lime. But when the pH of the soil is between 5.8 and 7.3, 

 an Azotobacter test is made. A definite weight of soil (5 or 10 grams) is 

 added to a definite amount of mannite solution free from calcium carbon- 

 ate (50 or 100 cc.) ; the flasks are sterilized, inoculated with a fresh cul- 

 ture of Azotobacter and incubated. The greater the lime or buffer 

 content of the particular soil, the more abundant will be the growth of 

 Azotobacter. The amount of pellicle development is an index of the 

 buffer action of the soil and can yield information on its lime require- 

 ments. Out of 100 soils used, the Azotobacter test for the lime require- 

 ment of soils agreed in 90 per cent of the cases with the known soil 

 condition, while the ammonium chloride and litmus tests agreed only in 

 50 and 40 per cent of cases respectively. The amount of CaC0 3 which 

 should be added to the soil, to obtain maximum nitrogen fixation, varies 

 with the soil. This is due to differences in the buffer content of soils, in 

 addition to differences of the initial reaction. Soils of a different buffer 

 content, even of the same initial reaction, will require different quantities 



95 Reed, H. S., and Williams, B. The effect of some organic soil constituents 

 upon nitrogen-fixation by Azotobacter. Va. Agr. Exp. Sta. Tech. Bui., 4: 81-95. 

 1915; Walton, J. H. Azotobacter and nitrogen-fixation in Indian soil. Mem. 

 Dept Agr. India. Bact. Ser., 1: 97-112. 1915. 



96 Christensen, 1907 (p. 576); 1915 (p. 578); Experiments in methods for deter- 

 mining the reaction of soils. Soil Sci., 11: 115-178. 1917; Untersuchungen 

 iiber einige neuere Methoden zur Bestimmung der Reaktion und des Kalkbediirf- 

 nisses des Erdbodens. Intern. Mitt. Bodenk., 13: 111-146. 1923; Christensen, 

 H. R., and Larsen, O. H. Untersuchungen iiber Methoden zur Bestimmung 

 des Kalkbediirfnisses des Bodens. Centrbl. Bakt. II, 29: 347-380. 1911. 



