juneij, i9i6 Influence of Arsenic upon Soil Organisms 



393 



together with 2 gm. of mannite, were placed in sterile tumblers, the water 

 content made up to 18 per cent, and then incubated at 28° C. for 18 days. 

 At the end of this period the soil was transferred by means of i ,000 c. c. of 

 carbon-dioxid-free distilled water to large acid bottles. The mixture 

 was left in these bottles, with occasional shaking, for 8 days, then filtered 

 and the arsenic determined in an aliquot part (Greaves, 1913d). In 

 another set the various forms of arsenic were mixed with loo-gm. portions 

 of soil and 2 gm. of mannite and the water-soluble arsenic determined as 

 above without incubation. 



The results are given in Table II as milligrams of water-soluble arsenic 

 occurring in 100 gm. of the soil both before and after the three weeks' 

 incubation. Each reported result is the average of three or more closely 

 agreeing determinations. 



Table II. — Quantity of water-soluble arsenic (in milligrams) in 100 gm. of soil before 

 and after three vjeeks' incubation 



The arsenic in each case became more soluble as bacterial activity 

 progressed. This is especially marked in the soil containing arsenic 

 trisulphid, which yielded 10 times the water-soluble arsenic after incuba- 

 tion that it did before. A remarkably close agreement is found to exist 

 among the results obtained for water-soluble arsenic at the close of the 

 incubation period, which shows that the maximum stimulating influ- 

 ence is obtained when soil contains between 10 and 15 p. p. m. of 

 water-soluble arsenic. This is a quantity that exceeds that found in 

 agricultural soil (Greaves, 1913b); hence, the influence of the arsenic 

 occurring in soil must be to increase and not to retard nitrogen fixation. 

 The maximum fixation varies with the form of arsenic applied. This is 

 undoubtedly due, as will be pointed out later, to the elements accom- 

 panying the arsenic, which may have either a retarding or an accelerat- 

 ing influence upon the bacterial activity. 



The finding of this marked stimulating influence of arsenic upon the 

 nitrogen-fixing powers of soil raises a number of very interesting and 

 important questions. Some of these are: (i) Does this stimulating 

 influence exist in other soil or is there something inherent within this 

 particular soil which makes its bacterial flora susceptible to the influence 

 of arsenic? (2) Is the stimulating influence brought about by the retard- 

 ing of injurious species or is it a direct stimulant to the soil organisms? 



