392 



Journal of Agricultural Research 



Vol. VI, No. II 



discussion. In order to make those containing different forms of arsenic 

 more nearly comparable with each other — that is, the lead arsenate with 

 the arsenic trisulphid, etc. — the nitrogen fixed in the untreated soil has 

 been taken as loo, and from this the ratio has been calculated with each 

 of the concentrations of arsenic. This gives us more nearly comparable 

 results, which are shown in figure i. 



Comparing these results with those obtained for the ammonification 

 and nitrification series (Greaves, 1913c), we find a marked similarity exist- 

 ing between them. In all of the series there is a marked stimulation 

 with all of the compounds except Paris green. The arsenic trisulphid 



O A?0 ^OO 300 ^tTOO 



Fig. I. — Graph showing the action of five compounds of arsenic on nitrogen fixation in dry soil. (Nitrogen 



fixed in untreated soil equals loo.) 



stimulates growth much more in the nitrogen-fixing series than it does 

 in the other series. The arsenic trisulphid has the greatest stimulating 

 influence, followed in the order named by lead arsenate, zinc arsenite, 

 and sodium arsenate. Paris green was the only compound tested which 

 exerted no stimulating influence. It may be seen that the maximum 

 stimulation was not obtained when equivalent quantities of arsenic in 

 the various forms are applied to the soil. Hence, it seems possible that 

 a relationship may exist among the various cases in the water-soluble 

 arsenic found. In order to answer this, determinations were made of 

 the water-soluble arsenic existing in the soil. The soil and arsenic, 



