1913] J- E. Greaves 9 



of considerable theoretical and practical importance. We note a 

 marked stimulating effect with each of the substances used, which 

 varied greatly with the different Compounds. For instance, there 

 was great Stimulation with lead arscnate in the lowest concen- 

 tration tested. At this concentration the quantity of resultant nitric 

 nitrogen was nearly twice that produced in the untreated soil. As 

 the concentration of the arsenic increased above240 parts per million 

 there was a sHght decrease in the production of nitric nitrogen, but a 

 concentration of even 1,120 parts of arsenic in the form of lead 

 arsenate caused the production of nearly as much nitric nitrogen as 

 was formed in the untreated soil. It is rather hard to decide, f rom 

 these results, whether the observed Stimulation was due to the chem- 

 ical applied or to a change in the physical condition of the soil. 

 Probably both are concerned but only a small portion can be attrib- 

 uted to a purely physical change in the soil. 



Examining the results obtained where the Paris green was used, 

 we find a stimulating effect with the lower concentrations of arsenic, 

 which reached its maximum when 320 parts per million of arsenic 

 were present. Above this concentration it decreased and at a con- 

 centration of 760 parts per million of arsenic the effect commenced 

 to be toxic. The toxicity increased with the concentration until, 

 at the highest concentrations, the production of nitric nitrogen was 

 reduced to 3.4 mg. of nitric nitrogen per 100 gm. of soil. 



Zinc arsenite exerted no apparent influence on the formation of 

 nitric nitrogen in the soil until the concentration was 480 parts per 

 million; after which a notable stimulating influence was observed. 

 This Stimulation continued until the concentration was 680 parts 

 per million. Above this concentration there was a marked retarding 

 influence upon nitrification. 



Arsenic tristdfide stimulated the formation of nitric nitrogen 

 slightly until the concentration of the arsenic trisulfide exceeded 

 280 parts per million ; after which there was a marked falling off in 

 the production of nitric nitrogen and at the highest concentration 

 tested (1,120 parts per million), the formation of nitric nitrogen 

 practically ceased. 



Paris green, zinc arsenite and arsenic trisulfide after their first 

 slight stimulating influence, which varied in intensity with the dif- 



