400 



Journal of Agricultural Research 



Vol. VI, No. II 



the ordinary soil with its mixed flora as great a fixation was obtained in 

 the presence of arsenic as in the presence of only mannite. This probably 

 indicates that some of the stimulation is due either to the fact that the 

 arsenic acts upon allied species which are gathering carbon that can be 

 used by the Azotobacter, or else to the fact that some species, possibly 

 the cellulose ferments, are stimulated so that they render available to the 

 Azotobacter the carbon-carrying compounds of the soil faster in the pres- 

 ence of arsenic than in its absence. Only one of the organisms isolated, 

 Azotobacter B, is directly stimulated by arsenic. The stimulation, 

 however, is very large in this case. It also fixes large quantities of nitro- 

 gen in the presence of arsenic and absence of mannite. These results 

 are complicated by the carbonaceous material which occurs in the soil. 

 For this reason a series similar to the above was incubated, using silica 

 sand in place of the soil. The silica used was devoid of organic matter 

 and had the following composition : 



Per cent. 



Silicon dioxid (SiOj) 97.5 



Ferrous oxid (FeO) i 



Aluminum oxid (AI2O3) 1.7 



Calcium oxid (CaO) 2 



One-hundred gm. portions of this were sterilized in covered tumblers, 

 and to each was added i gm. of calcium carbonate and 18 c. c. of sterile 

 distilled water containing 0.02 gm. of potassium phosphate, 0.02 gm. of 

 magnesium sulphate, and 0.002 gm. of calcium chlorid. The tumblers 

 were inoculated with the various nitrogen-fixing organisms incubated 

 with a constant moisture content at 28° C. for 18 days, and then the 

 nitrogen determined as in the previous series. They were all compared 

 with sterile blanks. The results are given in Table VIII as milligrams 

 of nitrogen fixed in 100 gm. of sand. Each reported result is the aver- 

 age of six or more closely agreeing determinations. 



Table VIII. — Quantity of nitrogen {in milligrams) fixed in 100 gm,. of quartz sand with 



and without arsenic 



Qualitatively, the above results are the same as those obtained with 

 the soil. Azotobacter B was the only one of the three organisms stimu- 

 lated by the arsenic. Where the mixed flora were used, the stimulation 

 was very marked, but the fixation in the absence of arsenic where either 

 Azotobacter A or Azotobacter C was used is about the same as that 



