INFLUENCE ON SOIL BACTERIA 205 



applied in the fall there is enough time left until the planting of 

 the following spring crop for the mineralization of the bacterial 

 nitrogen. It is quite evident, of course, that the nitrogen com- 

 bmed in the bodies of generations of bacteria is not all made 

 soluble within a single year, but only in the course of several 

 growing seasons, so that we may readily account for the increased 

 harvests secured for two or more successive years after strong ap- 

 plications of carbon bisulfid, even though the bacterial transfor- 

 mations had by that time declined. The exhaustion of the soil 

 finally manifesting itself after a shorter or longer time may be 

 explained by the deep-seated changes in the bacterial soil flora, 

 which does not return so easily to its normal state. It is quite 

 possible that the return to the normal conditions is prevented by 

 the exhaustion for years to come of the more available portions of 

 the plant nutrients." 



The theory of Russell and Hutchinson is both interesting and 

 probable. They maintain that within the soil are microscopic 

 plants, bacteria, and also microscopic animals, protozoa. The 

 protozoa are many times larger than the bacteria, and when ex- 

 amined under the microscope they are seen moving about and 

 creating, viewed from the standpoint of the bacteria, great 

 waves. They can be seen continually feeding upon the minute 

 bacteria. This would keep down the number of bacteria in the 

 soil. Now, when a weak antiseptic is applied to the soil it kills 

 many of the protozoa as they are more sensitive to poisons than 

 are the bacteria. The bacteria, being no longer preyed upon by 

 their natural foe, rapidly multiply. The ammonifiers are not in- 

 jured to as great an extent as the nitrifiers, the result being that 

 ammonia at first accumulates. But later the nitrifiers rapidly 

 multiply as there are no protozoa to feed upon them; hence, we 

 have a rapid production of nitrates. As the antiseptic evaporates 

 the few remaining protozoa multiply and soon are able to keep 

 the bacteria in check. Hence, the struggle for existence goes on in 

 the soil; one species preys upon the other. It is possible that micro- 

 scopic life wage battles within the soil proportionately as terrific 

 as those waged by the higher animals upon the earth's surface. 



