junei2, i9i6 Influence of Arsenic up07i Soil Organisms 391 



quantity of sodium in the form of sulphates, chlorids, and carbonates to 

 the soil without retarding its nitrogen-fixing power. 



The lead arsenate at the lower concentrations has no influence upon 

 the nitrogen-fixing powers of the soil, but when the concentration 

 reaches 80 p. p. m. a stimulating influence becomes quite perceptible. 

 This continues until the concentration exceeds 200 p. p. m. Above this 

 concentration the nitrogen fixed, within experimental error, is the same 

 as that fixed in the untreated soil. It is interesting to note that the 

 compound does not become toxic, even when the quantity added reaches 

 400 parts of arsenic per million parts of soil. This series shows a very 

 close similarity to the nitrification series previously reported, and it is 

 quite likely that part of the stimulating influence is due to the lead ion. 



Paris green is toxic even in the lowest concentration used, and the 

 toxicity increases as the quantity of Paris green added increases. This 

 toxicity is due mainly to the copper ion. However, as was shown in the 

 ammonification and nitrification work, the quantity of soluble arsenic 

 present would be much higher where the Paris green was added than 

 where the other compounds were used. The fact that no stimulation 

 occurs in the Paris-green series points to the conclusion that the toxicity 

 of the copper must increase much more rapidly than the stimulating 

 influence of the arsenic. Yet it is quite possible that if a lower con- 

 centration of the substance had been taken a stimulation would have 

 been noted. 



Arsenic trisulphid stimulates in the lowest concentration tested and 

 increases in stimulating influence until a concentration of 160 p. p. m. 

 is reached. In concentrations above this its stimulating influence de- 

 creases. In concentration above 320 p. p. m. there is fixed no more 

 nitrogen in the presence than in the absence of arsenic. But even at 

 the highest concentration tested (400 p. p. m.) this compound exerts no 

 tonic influence on the nitrogen fixers. 



Zinc arsenite probably stimulates slightly in low concentrations, but 

 aside from this it has little apparent influence on the nitrogen-gathering 

 organisms. Had fresh soil been used in this series, greater stimulation 

 would have been noted, as was found by later work. 



The amount of nitrogen fixed in the untreated soil of the above series 

 shows a marked variation. This is probably due to various factors, chief 

 among which is the fact that the nitrogen-fixing powers of the soil with 

 sodium arsenate, lead arsenate, and Paris green were made in the order 

 named on the air-dried soil soon after it had been brought to the labora- 

 tory. In the case of the arsenic trisulphid and zinc arsenite the soil had 

 been in the laboratory in an air-dried condition for about two months 

 before the determinations were made, but each set of samples within each 

 series was handled in exactly the same manner, and the samples are 

 directly comparable within each set, as has been the case in the previous 



