June 12, 1916 Influence of Arsenic upon Soil Organisms 



403 



It probably would have been better if in every case untreated soil 

 could have been incubated with the variously treated soil, but this so 

 greatly increased the number of determinations that it was not thought 

 advisable. Furthermore, all the work has been done on the high-humus 

 soil, A, without the addition of any carbohydrate, and repeated deter- 

 minations have shown that the arsenic more than doubles the nitrogen 

 fixed in the soil in 20 days, so that the absence of the stimulation can be 

 safely attributed to the treatment. In the above results, it is readily 

 seen that the soil extract on passing through filter paper loses to a very 

 great extent its power of being stimulated by arsenic. Hence, it is safe 

 to conclude that the main stimulating influence of arsenic upon nitrogen 

 fixation is due to its suppressing something which is found in the soil and 

 which is removed by the filter paper. 



That this factor is to a great extent the same as is removed by heat is 

 shown by the results reported in Table XI. The arrangement of this 

 series of experiments was as follows: loo-gm. portions of the soil were 

 weighed into covered tumblers. To one-half of the set was added 

 arsenic — 0.0728 gm. to each 100 gm. of soil. The tumblers were all 

 carefully sterilized and half of them were placed in the incubator in the 

 sterile condition. To the others was added a soil extract prepared by 

 shaking one part of soil with two parts of sterile distilled water for three 

 minutes. After standing for about five minutes the liquid was decanted 

 and 10 c. c. portions of this were used to inoculate the soil. Before 

 inoculating, this extract was placed in thin-walled test tubes in 10 c. c. 

 portions and then held at the required temperature for exactly 15 min- 

 utes before adding to the soil. The moisture content was made up to 18 

 per cent and the whole incubated for 20 days. Each reported result is 

 the average of six closely agreeing determinations. 



Table XI. 



-Quantity of nitrogen {in milligrams) fixed in 100 gm. of soil, v:ith and 

 •without arsenic, inoculated with soil extract 



Temperature of soil extract (°C.). 



0.0728 gm. 



of lead arsenate 



added. 



No arsenic 

 added. 



Room 



50.... 



55---- 

 60.... 



65.... 

 70.... 



75--- 

 80.... 



85.... 



8.77 



9. 24 



14. 28 



12. 60 



13-85 

 12. 18 

 12.88 

 13- 44 

 II- 54 



5- II 

 9. 00 



14. 14 

 16.38 

 14.42 

 13.02 



11-34 

 12. 66 

 10.36 



The heating of the soil extract to a temperature of 55° C. for 15 minutes 

 changes the soil so that it is no longer stimulated by arsenic. The 

 heating of the soil extract to a higher temperature stimulates its nitrogen- 



