June 12. 1916 Influence of Arsenic upon Soil Organisms 



409 



also bring out the fact that the addition of arsenic and the filtering of 

 the soil only shift for the time the equilibrium within the soil, and later 

 it tends to regain its old equilibrium. This is a condition which coincides 

 well with what one would expect if the limiting element were some other 

 microscopic forms of life. The filter would not separate them quanti- 

 tatively, and it is possible that the arsenic has only a selective influence. 

 Later, many of the organisms become accustomed to its presence; or, 

 what is more likely, the arsenic becomes fixed (McGeorge, 191 5) within 

 the soil. 



That this limiting factor is a thermolabile body is brought out more 

 clearly in figure 3, which is made from the data reported in Table XL 



/oo% 



&59 



Fig. 3. — Graph showing the effect of heat on the nitrogen-fixing power of soil treated and not treated 



with arsenic. 



The quantity of nitrogen fixed by the unheated soil receiving no arsenic 

 has been taken as 100 per cent, and the heated soil with and without 

 arsenic is compared with this. 



The heating of the soil extract to 50° C. for 15 minutes has exactly 

 the same influence measured in terms of nitrogen fixed as does 0.0728 

 gram of lead arsenate. The stimulating influence of heat is noted even 

 in the presence of arsenic and reaches its maximum effect in the absence 

 of arsenic at 60°, while in the presence of arsenic at 65° above these 

 temperatures there is a decline in the nitrogen fixed. But even the soil 

 inoculated with solutions which had been heated to a temperature of 85° 

 fixed nitrogen ; or at least there is more nitrogen accumulated in such soil 

 than in that inoculated with the untreated soil solution. The results indi- 

 cate that many of the organisms which take part in the gathering of nitro- 



