June 12, 1916 



Inflti^nce of Arsenic upon Soil Organisms 



397 



Table V. — Quantity of nitrogen {in inilligrams) fixed in 100 gm. of aerated soil with and 

 without the addition of arsenic after different periods of incubation 



Days incubated. 



20 

 44 



Nitrogen 

 fixed in 

 soil con- 

 taining 

 0.0728 gm. 



of lead 

 arsenate. 



5. 8S 

 8.26 



Nitrogen 



fixed in 



tintreated 



soil. 



2.58 

 3-Q2 

 3-78 



Days incubated. 



66. 

 96. 

 162 



Nitrogen 

 fixed in 

 soil con- 

 taining 

 0.0728 gm. 



of lead 

 arsenate. 



9-38 

 4.90 

 2.80 



Nitrogen 



fixed in 



untreated 



soil. 



14.00 

 2. 52 

 4. 20 



These results show conclusively that it was the lack of air in the 

 former series which caused such great losses of nitrogen and that they 

 could in no way be attributed to the arsenic added. This series was 

 stirred but once a week and after the stirring the moisture content was 

 made up to the optimum so that the soil became quite compact. It is 

 quite likely that greater care in the aeration of the soil would have reduced 

 very materially the loss of nitrogen which was observed in this series. 

 In the first stages of the experiment the soil containing arsenic gained 

 the greater quantity of nitrogen, while in the later stages the soils con- 

 taining no arsenic were the highest. If, however, an average of the 

 quantity found in each soil is taken, it will be found to be considerably 

 higher in the soil containing arsenic than in the other. 



It was thought that some of the questions referred to in the first part 

 of this article could be answered more readily with the solution method 

 than with soil. For this reason a series was incubated using a solution 

 of the following composition: 



Dibasic potassium phosphate (K2HPO4) .... 0.2 gm. 



Magnesium sulphate (MgS04) 2 gm. 



Calcium chlorid (CaCla) 02 gm. 



Ferric chlorid (FeaClg) i drop (10 per cent solution). 



This was made up to 1,000 c. c. with tap water and distributed in 100 

 c. c. portions into 750 c. c. Erlenmeyer flasks. One gm. of calcium car- 

 bonate was added to each, and the flasks were then sterilized and inocu- 

 lated. One series was inoculated with Azoiohacter vinclandii. This was 

 done by making a suspension in sterile tap water of the organism and 

 adding 5 c. c. of this suspension to each flask. In the other series the 

 inoculating medium was 10 gm. of soil. The solutions were incubated 

 at 28° to 30° C. for 18 days, and then the nitrogen determined in the 

 manner previously outlined. The results are given in Table VI and are 

 reported as milligrams of nitrogen fixed in 100 c. c. of the solution. Each 

 reported result is the average of three closely agreeing determinations. 



