4IO Journal of Agricultural Research voi. vi, no. h 



gen in this soil are very resistant to heat. It is also significant that the 

 greatest stimulating influence is exerted in soil which had been inoculated 

 with solutions heated just above what Cunningham and Lohnis (1914) 

 found to be the thermal death point of soil protozoa. 



The data presented in this paper, together with these presented in 

 former publications, make it possible to compare the sensitiveness of the 

 ammonifying, nitrifying, and nitrogen-fixing organisms toward the 

 various arsenical compounds. Figure 4 represents the percentage of 

 activity of the various classes of organisms in the presence of 400 

 p. p. m, of arsenic in the form of the various arsenical compounds. The 

 untreated soil has been taken in every case as 100. The ammonifying 

 organisms are retarded more by the lead arsenate than the nitrogen- 

 fixing or nitrifying organisms. The latter two are influenced in nearly 

 the same way by this concentration of lead arsenate. All three types 

 of organisms are influenced in the same order by the arsenic trisulphid, 

 while with the zinc arsenite the nitrogen-fixing and nitrifying organ- 

 isms act about normally in concentrations of 400 p. p. m. of arsenic, 

 but the ammonifiers are greatly depressed. Paris green stimulates the 

 nitrifiers, but greatly depresses the other types of organisms. The 

 results, with the exception of copper, show that the nitrifying and 

 nitrogen-fixing organisms are very similar. 



In figure 5 are shown graphically the quantities of arsenic in the form 

 of various arsenicals which are required by the different organisms to 

 give the greatest stimulation. 



It has been shown that stimulation within a specific group of organisms 

 varies with the quantity of water-soluble arsenic and the stimulating 

 influence of the electropositive ion associated with the arsenic. But 

 when we examine stimulation by these substances with diff'erent groups 

 of organisms, we find a marked difference which can not be attributed 

 to solubility but must be due to a physiological difference existing 

 in the various organisms; for instance, the nitrogen-fixing organisms 

 require 200 p. p. m. of arsenic in the form of lead arsenate for the 

 greatest stimulation, w^hile the nitrifiers and ammonifiers require much 

 smaller quantities. For maximum stimulation with arsenic trisulphid 

 the nitrogen-fixing organisms require the greatest concentration, fol- 

 owed by the nitrifying and ammonifying organisms in the order given. 

 Zinc arsenite, on the other hand, has to be present in large quantities 

 for a maximum stimulation of the nitrifying organisms, while very small 

 quantities give a maximum stimulation with the other two groups of 

 organisms. Practically the same order is followed by the organisms in 

 the presence of sodium arsenate and Paris green, there being, however, 

 this significant difference, that neither the ammonifiers nor the nitrogen- 

 fixing organisms are stimulated in any concentration by the presence of 

 copper, and it is quite possible that the same holds for the nitrifying 



