June 12, 1916 Influence of Arsenic upon Soil Organisms 



399 



stances may be toxic when used in water culture, but that the same 

 quantities when placed in the soil may act as stimulants. 



The results reported for A. mnelandii, when considered in connec- 

 tion with those obtained for the soil, make very problematic the part 

 played by Azotobacter, especially A. vinelandii, in these soils. The 

 exact mode of action of the arsenic also remains a question. For these 

 reasons the soil used in the first series was plated and the main nitrogen- 

 fixing organisms isolated. Three types of Azotobacter were obtained. 

 These have been designated Azotobacter A, Azotobacter B, and Azoto- 

 bacter C. Azotobacter A has a nitrogen-fixing power of 6.86 mgm. of 

 nitrogen per gram of mannite in Ashby solution, Azotobacter B a nitro- 

 gen-fixing power of 5.00 mgm., and Azotobacter C a nitrogen-fixing power 

 of 6.44 mgm. of nitrogen. 



The preceding results have shown that little information of value can 

 be obtained by the solution method. Therefore another series was 

 planned in which loo-gm. portions of the soil used in the first series were 

 weighed into covered sterile tumblers and autoclaved at a temperature 

 of 120° C. for 30 minutes, cooled, and the moisture content made up to 

 18 per cent. The soil was then inoculated with the various organisms 

 which had been isolated. The soil portions were incubated for 18 days, 

 the moisture content kept constant, and then the total nitrogen deter- 

 mined. Sterile blanks were incubated and analyzed as checks. Each 

 reported result is the average of four or more closely agreeing determina- 

 tions, so that the analytical error has been reduced to a minimum. The 

 results are given in Table VII. 



Table VII. — Quantity of nitrogen (in milligrams) fixed in 100 gm. of soil wiik and 

 -without arsenic and inoculated uiih various nitrogenfixing organisms 



The results reported above show for each organism a fixation much 

 higher in the soil than was found in the solution. The results without 

 arsenic, but with mannite, are as high as are reported in Table I with 

 both mannite and arsenic combined, a fact which would seem to indicate 

 that arsenic acts upon injurious species. This, however, does not account 

 for the entire phenomenon, for we find in this series a very small fixation 

 of nitrogen in the absence of mannite and presence of arsenic, while in 

 37768°— 16 2 



