124 INFLUENCE OF ARSENIC ON BACTERIAL ACTIVITY 



zinc became toxic. Lipman and Burgess have shown that it stimu- 

 lates the nitrifying organisms and that the influence is shown in the 

 crop yield. The great variation in the results reported by the differ- 

 ent investigators for zinc, arsenic, and lead is probably due to the 

 fact that it modifies the bacterial flora of the soil. When heated 

 soil or water cultures are used a different result is noted. This, 

 however, is not the only factor, for these results show a marked 

 difference in soil and in water culture. The lead arsenate stimulates 

 the nitrogen-fixing organisms when placed in soils but becomes 

 highly toxic to the same organisms when placed in nutritive solutions. 

 The difference is due in part to adsorption by the soil, but this 

 would have to be attributed to the silica compounds of the soil, for 

 the nitrogen-fixing organisms are stimulated by arsenic in quartz 

 sand free from organic colloids. In this case the arsenic becomes 



FIG. 18. Graph showing the effect of aeration on the nitrogen-fixing activity of soil- 

 containing compounds of arsenic. (Soil Science.) 



concentrated at the surface, layers of the silica leaving the inner 

 part of the water film comparatively free from arsenic, in which 

 the microorganisms multiply and carry on their metabolic processes. 

 This being the case, it is probable that a water solution weak enough 

 to stimulate bacteria could be found. A great difference, however, 

 between the solution and the sand-culture method is the greater 

 aeration in the latter than in the former. That the aeration of a 

 cultural medium does play a great part in determining the activity 

 of the nitrogen-fixing powers of a soil is strikingly brought out in 

 Fig. 18. 



It is remarkable how the aeration of the soil or the filtering of the 

 soil extract can prevent the high loss of nitrogen which is noted at 

 first in the unaerated soil. This cannot be attributed directly to the 

 denitrifying organisms; otherwise, it would not be removed by filtra- 



