INFLUENCE ON SOIL BACTERIA 203 



There is the dark green color and the vigorous development of 

 the plants together with the decided tendency of grain crops to 

 lodge just as if too great quantities of nitrogen were at their dis- 

 posal. These facts led Heinze to conclude that on the whole we 

 must seek the cause of the beneficial effect of carbon bisulfid on 

 the soil in the enormous increase of soil microorganisms at the 

 proper time, thus rendering available, or possibly increasing, the 

 nitrogen supply to growing plants. 



The large amounts of nitrogen thus made available to the crops 

 are derived partly from soil sources and partly from atmospheric 

 sources. Kruger and Heinze not only demonstrated that soils 

 treated with carbon bisulfid showed an increase in their total ni- 

 trogen content but also that the increase was the result of the 

 more vigorous growth of the nitrogen-fixing Azotobacter species. 

 This, Heinze considers, resulted from the initial suppression of 

 amid-ammonia formation and nitrification which would create 

 favorable conditions for the development of the nitrogen-fixing 

 flora. Later there would be more intense transformation of the 

 bacterial proteins and of other nitrogenous organic substances into 

 amino- and ammonia compounds which would result in a more 

 vigorous nitrification, thus placing at the disposal of the plant an 

 abundant and uniform supply of soluble nitrogen compounds. 

 The various organic materials in the soil, such as plant residues, 

 pectins, pentosans, humic substances and the like, together with 

 the rapid growth of algae and molds, may furnish the carbon food 

 for the Azotobacter species. In short, we have a cycle of changes 

 similar to those outlined as occurring in a heated soil. 



How Account for These Conditions? — Numerous theories 

 have been advanced to account for the observed phenomena. 

 However, only two of the more likely and interesting ones can 

 be considered here. 



Hiltner and Stormer explain the changes as follows: 



"(i) By destroying the existing bacterial equilibrium in the 

 soil the carbon bisulfid opens the way for an entirely new bacterial 

 development. This is achieved through the unequal retardation 

 in the growth of the different groups of bacteria. Hence, certain 



