390 



Journal of Agricultural Research 



Vol. VI, No. II 



The samples were incubated at 28° to 30° C. for 18 days and the total 

 nitrogen determined. The tumblers and contents at the end of this 

 time were placed in an electric incubator and kept at 95° C. until dry. 

 The soil was then ground in a mortar, after which 20-gm. portions were 

 weighed and placed in Kjeldahl JBasks. The nitrogen was then deter- 

 mined according to the Lipman and Sharp (191 2) method. The deter- 

 minations were all made in duplicate and compared with sterile blanks, 

 so that each result reported is the average of two or more closely agree- 

 ing determinations. The compounds used were sodium arsenate, lead 

 arsenate, cupric aceto-arsenite (Paris green), arsenic trisulphid, and zinc 

 arsenite. In each case the quantity of the compound added was such 

 as to give equivalent quantities of arsenic. The results reported as 

 milligrams of nitrogen per 100 gm. of soil are given in Table I. 



Table I. — Quantity of nitrogen {milligrams) fixed in loo gin. of soil during 18 days -with 

 varying amoxmts and different forms of arsenic 



P. p. m. 



O 



20 



40 



80 



120 



160 



200 



240 



280 



320 



360 



400 



O 



Zinc 

 arsenite. 



9. I 

 II. 9 



9-7 

 9.6 

 10.5 

 9-7 

 8.4 

 8.4 

 8.4 

 9.0 

 9.1 

 9.1 

 9.1 



In this series the concentration of the arsenic was not carried above 

 400 p. p. m., for previous work had shown that the main stimulation 

 occurs below this concentration. Furthermore the arsenic occurring in 

 agricultural soils seldom exceeds 150 p. p. m., so it is likely that in agri- 

 cultural soils it will never be found to exceed the quantity used in this 

 work. 



The results reported in the above table bring out some very inter- 

 esting facts and show that the nitrogen-fixing organisms are very similar 

 to the nitrifying organisms in so far as their relations to arsenic are 

 concerned. The addition of 20 p. p. m. of sodium arsenate stimulates 

 their action and 40 p. p. m. or more have a toxic influence. When the 

 concentration of arsenic reaches 280 p. p. m., it stops all nitrogen-fixing 

 activity. The toxic influence which becomes so very prominent above 

 this concentration must be due entirely to the arsenic and not to the 

 sodium ion, as Lipman and Sharp (191 2) have added many times this 



