120 INFLUENCE OF ARSENIC ON BACTERIAL ACTIVITY 



similar soil to which arsenic in the form of Paris green was added 

 produced, under the same condition, 129 parts of nitrates. When, 

 however, higher concentrations of arsenic in the form of Paris 

 green were added it became toxic, and eventually stopped all 

 bacterial activity; but the quantity added had to be so large that 

 it is not likely that sufficient would ever occur under agricultural 

 practice. 



Arsenic, then, does not injure the ammonifying or nitrifying 

 organisms of the soil. But how about the other beneficial bacteria 

 of the soil? What effect has it upon them? 



Nitrogen Fixation. There are 75,000,000 pounds of atmospheric 

 nitrogen resting upon every acre of land, but none of the higher 

 plants have the power of taking this directly from the air. Certain 

 bacteria, however, can live in connection with the legumes and 

 assist them to take nitrogen from the air. Then there is another 

 set of nitrogen-gathering organisms which live free in the soil, and 

 which may, under ideal conditions, gather appreciable quantities 

 of nitrogen. It is rather possible that much of the benefit derived 

 from the summer fallowing of land is due to the growth of this class 

 of organisms in the soil and storage by them of nitrogen for future 

 generations of plants. In such soils they are both more active and 

 are also found in greater numbers. All the work put on soil to 

 render it more porous reacts beneficially upon these organisms. 

 They not only require atmospheric nitrogen and oxygen, which are 

 absolutely essential to their life activities, but they must obtain 

 them from within the soil, for the minute organisms cannot live 

 upon the surface of the soil because to them the direct rays of the 

 sun means death. How does arsenic influence this class of organisms 

 which are so beneficial to the soil, but which are so much more 

 sensitive to adverse conditions than are the other kinds of bacteria? 

 Arsenic in the form of lead arsenate, zinc arsenite, and arsenic 

 trisulphid stimulate these bacteria. When arsenic in the form of lead 

 arsenate was applied to the soil at the rate of 500 pounds an acre, 

 the nitrogen-fixing organism gathered twice as much nitrogen in 

 unit time as it did in the absence of arsenic. The Paris green, 

 however, is poisonous to this group of organisms in the minutest 

 quantities. This is most likely due to the copper rather than to the 

 arsenic in the compound. 



How Does the Arsenic Act? It may, therefore, be concluded that 

 arsenic stimulates all the beneficial bacteria. But how does it act? 

 Will it stimulate for a short time and then allow the organism 

 to drop back to its original or to a lower level as does alcohol and 

 various stimulants when given to animals? Will it act as does 

 caffeine continue to stimulate? From the results on men and 

 horses the former might be expected, for although the arsenic eaters 

 of India and Hungary maintain that the eating of arsenic increases 



