INFLUENCE ON SOIL BACTERIA 207 



of arsenic on bacteria is in a direct relationship to its solubility. 

 An extremely large quantity of lead arsenate would have to be 

 applied to a soil before it would interfere with ammonification. 



Nitrification. — The nitrifying flora of a soil are stimulated 

 to a greater extent by small quantities of arsenic and are more 

 resistant to larger quantities of arsenic than are the ammonifiers. 

 Tests made in soil showed that whereas untreated soil produced 

 100 parts of nitrates in unit time, the same soil to which had 

 been added arsenic in the form of lead arsenate at the rate of 120 

 pounds an acre produced 178 parts of nitrates. In other words, 

 in place of being injured by the arsenic the bacteria were nearly 

 twice as active in the presence of this quantity of arsenic as they 

 were in its total absence. It was not until more than 700 pounds 

 of arsenic per acre, in the form of lead arsenate, had been applied 

 to the soil that the bacterial activity sank back to I CO. Even 

 when arsenic in the form of lead arsenate was applied at the rate 

 of 3,500 pounds an acre there was 68 per cent as much ammonia 

 produced as in the untreated soil. The Paris green gave similar 

 results. The untreated soil produced 100 part of nitrates in given 

 time whereas similar soil to which arsenic in the form of Paris 

 green was added produced, under the same condition, 129 parts of 

 nitrates. When higher concentrations of arsenic in the form of 

 Paris green, however, were added it became toxic, and eventually 

 stopped all bacterial activity. The quantity added had to be so 

 large that it is not likely that sufficient would ever occur under 

 agriculural practice. 



Arsenic, then, does not injure the ammonifying or nitrifying 

 organisms of the soil. But how about the other beneficial bac- 

 teria of the soil? What effect has it upon them? 



Nitrogen Fixation. — How does arsenic influence the nitrogen 

 fixers? Are they also stimulated or injured by it? The an- 

 swer to these questions came when arsenic in the form of lead 

 arsenate, zinc arsenite, and arsenic trisulfid were added to the 

 soil and their influence noted. When arsenic in the form of 

 lead arsenate was applied to the soil at the rate of 500 pounds 

 an acre, the azofiers gathered twice as much nitrogen in unit 



