6 Influence of Arsenic in Solls [Oct. 



stimulating effect on ammonification and no toxic influence was 

 noted until the highest proportions were present. Even 1,120 parts 

 of arsenic per million exerted little toxic influence. 



The zinc arsenite and lead arsenate are very similar in their 

 action, for they both apparently stimulate the ammonifying organ- 

 isms of the soil when used in small quantities ; and while in greater 

 concentration they exert a certain toxic influence upon the soil or- 

 ganisms, this influence increases slowly with the added quantity of 

 the Salt. The power of the soil to produce ammonia, after 800 

 parts of arsenic in the form of lead arsenate had been added, was 

 49.43 percent of the power of the original soil, while with the same 

 arsenic concentration in the form of zinc arsenite, it was reduced 

 to 51. II percent. At the highest concentration tested, 1,120 parts 

 per million, the lead arsenate reduced the ammonifying efficiency to 

 37-57 percent of the original soil while the zinc arsenite reduced 

 it to 44.68 percent of the original soil. Furthermore, it may be 

 Seen that neither of these salts retarded very materially the ammon- 

 ifying powers of the soil when present in a quantity equal to that 

 known to occur in soils.^^ This, however, is not the case when 

 the arsenic is applied in the form of Paris green. This substance, 

 even in the lowest concentration, retards very materially the ammon- 

 ifying powers of the soil. 



II. INFLUENCE OF ARSENIC UPON NITRIFICATION 



Method of experimentation. Soil similar to that used in the 

 ammonification tests was employed. The method was that of Lip- 

 man.^** Beakers covered with Petri dishes were sterilized and into 

 these were weighed loo-gm. portions of the air-dried soil and 2 gm. 

 of dry blood. Sodium arsenate was added from a Standard Solu- 

 tion with the proper proportion of sterile water, and the mixture 

 thoroughly stirred with a sterile spatula. The so-called insoluble 

 arsenates were added in the form of the dry powders and then thor- 

 oughly mixed with a sterile spatula. Sufficient sterile distilled 

 water was added to make the moisture content of the soil 18 percent. 

 These portions were weighed and the moisture content made up 

 weekly to 18 percent. 



2aGreaves: Biochemical Bulletin, 1913, ii, p. 519. 

 21) Lipman : Centralhl. f. BakterioL, 1912, xxxii, p. 8. 



