2o6 



BACTERIA AND SOIL FERTILITY 



Metallic Poisons. — Copper, lead, and arsenic at times occur 

 naturally in native agricultural soil and at times are added to soils 

 by sprays and from smelter smokes. Hence, the question nat- 

 urally arises as to what influence they are going to have upon 

 the microflora of the soil. Moreover, it is of economic impor- 

 tance as any factor 

 which modifies the 

 bacterial flora must 

 of necessity influ- 

 ence the crop. 



Extensive studies 

 have been made on 

 the influence of 

 various arsenic 

 compounds upon 

 the bacterial flora 

 of the soil with the 

 result that arsenic 

 was found to be a 

 stimulant in low 

 concentration and toxic only in larger quantities. The extent of 

 stimulation and toxicity varies greatly with the specific type of 

 microorganism and the form in which the arsenic is applied. 



Ammonifiers. — Experiments on ammoniiiers demonstrated 

 that this class of bacteria are not at first injured by the arsenic, 

 but their speed of action is increased. The actual results showed 

 that whereas the untreated soil produced in unit time lOO parts 

 of ammonia, soil to which sixty pounds of arsenic in the form 

 of lead arsenate an acre was applied produced 103 parts of am- 

 monia in the same length of time. And it was not until 2,500 

 pounds of arsenic an acre was applied to the soil that the produc- 

 tion of ammonia was reduced to one-half normal. The Paris 

 green, on the other hand, retarded the action of this class of bac- 

 teria even in the lowest concentration added, and by the time 600 

 pounds an acre had been applied the ammonia produced in unit 

 time had been reduced to one-half normal. The poisonous action 



Fig. 41. — Arsenic obtained from various soils. 



