REACTION OF MEDIA 255 



certain that large quantities of calcium carbonate may be present 

 in soil without injury to the azofiers. 



The author found numerous Azotobacter and a very active nitro- 

 gen-fixation in a soil 43 per cent, of which was calcium and mag- 

 nesium carbonate. 



The organisms develop normally in the presence of either calcium 

 or magnesium carbonate, but in liquid cultures the film develops 

 earlier and it contains less foreign organism in the presence of mag- 

 nesium carbonate than in the presence of calcium carbonate. The 

 actual nitrogen fixed, as reported by Ashby, is also greater where 

 the magnesium carbonate is used. This he attributes to the sup- 

 pression by the magnesium of foreign organisms, especially of the 

 butyric acid ferments. 



There is, however, a marked difference in the action of calcium 

 carbonate and magnesium carbonate when they are applied in large 

 quantities. Lipman and Burgess found the calcium carbonate stimu- 

 lating and never toxic to Azotobacter chroococcum in concentrations 

 up to 2 per cent, in mannite solution. The magnesium carbonate 

 was sharply toxic in higher concentrations up to 2 per cent, in 

 mannite solution. The magnesium carbonate was sharply toxic 

 in higher concentrations above 0.1 to 0.2 per cent, in such cultures. 

 The calcium salt is without effect when added to most soils up to 

 1.4 per cent., but the magnesium carbonate is even more toxic in 

 soils than in solutions. Moreover, their work indicates that calcium 

 exerts a protective influence, in both soils and solutions, against 

 the toxic influence of magnesium. The best ratio of calcium to 

 magnesium varies with solution and soil. 



In many soils lime increases the nitrogen fixed, for Krzemieniewski 

 found limed soil to fix in ten days 17.52 mgm. of nitrogen, whereas 

 adjoining unlimed soil fixed only 7.15 mgm. There is, however, the 

 possibility of applying too large a quantity of the caustic lime and 

 thereby decreasing nitrogen-fixation, a condition which has never 

 been experienced in the use of the carbonate. 



Von Feilitzen, however, found neither a direct relationship be- 

 tween lime content of moor soil and the development of Azotobacter, 

 nor relationship between their development and the reaction of the 

 soil. But this only serves to illustrate the fact that/although lime 

 and neutral or slightly alkaline media are essential, they will not" 

 ensure a rich Azotobacter flora in a soil unless all other conditions 

 are optimum. 1 Remy found sodium and potassium carbonate less 

 favorable for nitrogen-fixation than was calcium or magnesium. 



So far as the writer is aware, Krainsky is the only worker who has 

 found sodium carbonate more favorable than calcium carbonate. 

 This may have been due to the sodium carbonate's liberating plant- 

 food which was in the soil in an insoluble form but which was essen- 

 tial to the development of Azotobacter, Mockeridge has found that 



