INFLUENCE ON SOIL BACTERIA 209 



senic is much greater than it is in a similar soil without arsenic, 

 this activity continues to grow less and less, until at the end of 

 several weeks it is no greater than in soil containing no arsenic. 



It may be that part of the stimulation can be accounted for by 

 the protozoan theories of Russell and Hutchinson. We would 

 have to assume that arsenic acts more readily upon the protozoa 

 than upon the bacteria. After the arsenic has been in the soil 

 for some time it may become insoluble or some of it may be 

 changed by molds into a gas arsine and pass into the air. Then 

 the few protozoa which have not been destroyed by its presence 

 may rapidly multiply and soon hold the bacteria in check. 



This, however, is not the only way in which arsenic acts, for 

 pure cultures of the Azotobacter have been obtained from these 

 soils, and it is found that they also are stimulated. This is due to 

 the action of the arsenic upon these minute specks of living pro- 

 toplasm, causing them to utilize their food more economically in 

 the presence of arsenic than in its absence. Hence, this is similar 

 to the influence of the arsenic upon the cells of the horse. 



Other experiments have demonstrated that the addition of ar- 

 senic to a soil increases the quantity of soluble plant-foods in the 

 soil, especially the phosphorus. Thus arsenic by various means 

 stimulates all the bacterial activities of the soil, and these in- 

 creased activities, as experiments have shown, are reflected in 

 greater crops grown upon that soil. This increased growth must 

 be looked upon as being due to a stimulant and not to the direct 

 nutritive value of the substance added. Soils so treated would 

 produce larger crops and wear out more quickly than would un- 

 treated soils. It is interesting and important to know that arsenic 

 has to be applied to a soil in enormous quantities before it retards 

 microscopic plant life, and probably before it retards the growth 

 of higher plants. 



