10 Influence of Arsenic in Soils [Oct. 



ferent Compounds, exerted very marked toxic influences. This was 

 greatest for arsenic trisulfide and least for zinc arsenite. It is im- 

 probable that sufficiently large quantities of any of these Compounds 

 (lead arsenate, zinc arsenite, arsenic trisulfide, Paris green), would 

 be added to a soll under natural Systems of agriculture to very mate- 

 rially retard nitrification. On the other band, these results Iudicata 

 that a marked stimulating influence may be exerted. 



III. GENERAL CONSIDERATIONS 



Throughout this work there has been noted a wide difference 

 in the quantitative action of arsenic Compounds, but a striking simi- 

 larity in qualitative influence. Some exerted little influence in the 

 lowest concentrations on either ammonification or nitrification, but 

 in the higher concentrations exerted a very marked toxic influence, 

 e. g., Paris green. All, at some concentration, exerted either a 

 slight or very great stimulating influence. This fact suggests that 

 arsenic in small quantities tends to stimulate the bacterial activity of 

 the soil as measured by ammonification and nitrification. On the 

 other band, the absence of any great toxic influence where the sol- 

 uble arsenic was applied and the great variations in results noted 

 with other Compounds, raise the question whether the toxic influence 

 is exerted by the anion or cation. 



In Order to gain some light on this subject, determinations were 

 made of the water-soluble arsenic in soil to which the various forms 

 of arsenic were added. These determinations were made as fol- 

 lows : Quantities of lead arsenate, Paris green, zinc arsenite, and 

 arsenic trisulfide were added to loo-gm. portions of soil in quanti- 

 ties sufiicient to give 1,120 parts of arsenic per million of soil. The 

 soil and arsenic, together with 2 gm. of dry blood, were placed in 

 sterile tumblers; the water content was made up to 18 percent and 

 then incubated at 28° C. for three weeks. At the end of this time the 

 soil was transferred by means of 1,000 c.c. of carbon dioxide-free, 

 distilled water to large acid bottles. The mixture was left in these 

 bottles with occasional shaking for eight days, then filtered, and 

 the arsenic determined^ in an aliquot part. In another set the vari- 

 ous forms of arsenic were mixed with lOO-gm. portions of soil 



* Greaves : Jour. Amcr. Chem. Soc, 1913, xxxv, p. 150. 



