June 12, 1916 



Influence of Arsenic upon Soil Organisyns 



395 



gathering organism use more economically its usual source of carbon, 

 which in the absence of mannite is probably the plant debris which has 

 been slowly added to the soil. The belief that this is the case is strength- 

 ened by the fact that soil rich in organic matter (soil A) acts practically 

 the same in the absence of mannite and presence of arsenic as it does 

 when both arsenic and mannite are added to the soil. The clay soil 

 (C), which is low in organic matter, acts about the same in the absence of 

 arsenic as in the absence of mannite. It is interesting to note that in 

 soils B and C the total fixation in the soil containing mannite plus that 

 fixed by the soil containing arsenic approximates the total fixation in the 

 series in which both arsenic and mannite are present. 



Table III. — Quantity of nitrogen {in milligrams) fixed in 100 gm. of soil with and -with- 

 out arsenic 



LEAD ARSENATE 



ARSENIC TRISULPHID 



SODIUM ARSENATE 



In all of the tests so far reported the incubation period has been 18 

 days. Longer periods of incubation may give results very different 

 from those so far obtained, for the stimulating influence of arsenic may 

 be of short duration, and we may find later a slowing up of the reaction, 

 or, inasmuch as we are dealing with the algebraic sum of many reac- 

 tions which are taking place in the soil, we may find it to be negative. 

 An attempt was made to determine this by the following experiment : 

 loo-gm. portions of the high-humus soil (A) were mixed with 0.0728 



