338 



PRINCIPLES OF SOIL MICROBIOLOGY 



Smith and Alekseiev, and even ciliates, as shown by Peters. In most 

 instances, of course, no direct evidence has been submitted to show that 

 the protozoa took an active part in the decomposition of the organic 

 matter; the statements were often based upon the fact that protozoa 

 were found to occur in soils in which decomposition was taking place. 

 Goodey has also shown that when various protozoa are added to the soil, 

 bacterial activity has not been limited, as seen in table 18. This seems 

 to be contrary to the latter results of Goodey 98 mentioned previously; 

 however, they are explained by the fact that soil treated with an anti- 

 septic does not afford a suitable medium for the development of pro- 

 tozoa. The drop in the numbers of bacteria follows the exhaustion of 

 available plant food in the soil. 



TABLE 18 

 Bacteria in millions per gram 



Few attempts were made to demonstrate whether protozoa actually 

 injure important soil biological processes. Nasir" determined the in- 

 fluence of the presence of protozoa (Colpidium colpoda) upon the fixation 

 of nitrogen by Azotobacter in mannite cultures, both in solution and in 

 sand. In 31 experiments out of 36, the presence of protozoa resulted in an 

 increase in the amount of nitrogen fixed by Azotobacter. The feeding 

 action of protozoa upon Azotobacter seems to stimulate the further 

 development of the latter and thus maintain its nitrogen-fixing 

 efficiency for a longer period." 3 Very little soluble nitrogen was 



98 Goodey, 1915 (p. 323). 



99 Nasir, S. M. Some preliminary investigations on the relationship of pro- 

 tozoa to soil fertility with special reference to nitrogen fixation. Ann. Appl. 

 Biol., 10: 122-123. 1923. 



99a Cutler, D. W., and Bal, D. V. Influence of protozoa on the process of 

 nitrogen fixation by Azotobacter chroococcum. Ann. Appl. Biol. 13: 516-534. 1926. 



