English soils: an Important Factor in Soil Biology . 57 
higher plants ; while still more recently Pringsheim 1 has shown that the 
ability of bacteria to fix nitrogen is closely dependent upon the presence of 
blue-green algae. 
Esmarch 2 and Robbins 3 both incline to the belief that in certain 
soils at any rate the presence of algae, especially of blue-green algae, is an 
important factor in the fixation of nitrogen by bacteria. In certain of the 
Colorado soils large quantities of nitrate are built up through the agency of 
Azotobacter chroococcum despite the fact that these soils are poor in organic 
matter, and Robbins considers that the bacteria probably derive their 
organic material from the mucilaginous investments of the blue-green algae, 
which he shows to be present in considerable numbers. 
Recently Nakano 4 investigated the relationship existing between 
algae and Azotobacter and shows that it holds not only for blue-green algae 
but also for certain green algae. He points out that the pure cultures of 
Azotobacter fix less nitrogen than similar cultures to which pure algae have 
been added, and also shows that these green algae are able to build up 
chlorophyll in the dark not only on organic culture-media but also in 
aqueous culture-solutions of mineral salts into which sugar has been intro- 
duced. Nakano did not deal specifically with soil-forms, but both of these 
observations have a good deal of significance in considering the possible 
economic functions of soil-algae. 
According to Russell 5 6 the activities of Azotobacter are limited to well- 
aerated soils that are sufficiently provided with calcium carbonate, potassium 
salts and phosphates, carbonaceous material of the right kind, and moisture, 
and do not take place except at a comparatively high temperature. He 
admits, however, that is impossible to argue from a culture-soltition to 
a soil, and Robbins’s statement that Azotobacter chroococcum As the chief 
nitrogen-fixing bacterium in Colorado soils that are poor in organic material 
rather suggests that Azotobacter may work more efficiently in conjunction 
with blue-green algae than with the sugar and other carbohydrates used in 
the cultures of Koch, Pringsheim, and other investigators. 
In their researches on the fixation of nitrogen by the soil Gautier and 
Drouin 0 observed that in unsown soils which contain only ammonia as 
their source of nitrogen and are destitute of organic matter, there is a con- 
stant decrease in the amount of ammonia present which is due partly to its 
slow conversion into organic substances in the soil and partly to its escape 
into the air. But if algae are present the amount of nitrogenous organic 
1 Pringsheim, E. : Kulturversuche mit chlorophyllfiihrenden Mikroorganismen, III : Zur Physio- 
logie der Schizophyceen. Cohns Beitrage z. Biol. d. Pflanzen, Bd. xii, pp. 99-107. 
2 Esmarch: loc. cit., 1914. 8 Loc. cit. 
4 Nakano, H. : Untersuchungen liber die Entwicklungs- und Ernahrungsphysiologie einiger 
Chlorophyceen. Journ. of Coll, of Science, Imperial Univ., Tokyo, vol. xl, 1917, Art. 2, p. 66 &c. 
6 Russell, E. J. : Soil Conditions and Plant Growth. Monographs in Biochem., 1915, p. 97. 
6 Gautier and Drouin: loc. cit., pp. 1174-6, 1232-4. 
