STIMULATING INFLUENCE OF ARSENIC UPON THE 

 NITROGEN-FIXING ORGANISMS OF THE SOIL 



By J. E. Greaves, 

 Bacteriologist, Utah Agricultural Experiment Station 



INTRODUCTION 



Arsenic, when applied to a soil, has been found to stimulate the 

 ammonifying (Greaves, 1913c)* and especially the nitrifying organisms 

 of that soil. The stimulation varied greatly with the form, quantity, 

 and method of applying the arsenic. Furthermore it was found that 

 very large quantities of arsenic had to be applied to a soil before its 

 toxic effect became marked. This toxic effect became pronounced 

 only when quantities of arsenic which far exceeded those found in any 

 of the cultivated soils (Greaves, 1913b) had been applied. Therefore 

 it was desirable to determine its influence and mode of action upon the 

 nitrogen-fixing powers of the soil. For, even though arsenic does not 

 inhibit the action of the ammonifiers or nitrifiers, if it stops or materially 

 retards the nitrogen-fixing organism, it can not be said that arsenic is not 

 injurious to the soil flora. To determine this point the following study 

 has been made. 



EXPERIMENTAL WORK 



The soil used in the first part of this work was the same as that used 

 by the author in the previous series. It is a typical bench soil, a 

 sandy loam fairly high in calcium and iron content and supplied with 

 an abundance of all the essential elements of plant food with the excep- 

 tion of nitrogen, which was low, a characteristic of arid soils. 



The determination of the nitrogen-fixing powers of the soil was made 

 as follows: Tumblers covered with Petri dishes were sterilized, and into 

 these were weighed loo-gm. portions of the air-dried soil and 2 gm. of 

 mannite, which were then carefully mixed. Sodium arsenate was added 

 from a standard solution with the proper proportion of sterile distilled 

 water and the mixture thoroughly stirred with a sterile spatula. The 

 other arsenical compounds were added in the dry state and then care- 

 fully mixed. Sufficient sterile distilled water was added to make the 

 moisture content of the soil 18 per cent. The tumblers and contents 

 were weighed and the moisture content made up weekly to the initial 

 concentration. 



' Bibliographic citations in parentheses refer to " Literature cited," p. 414-416. 



Journal of Agricultural Research, Vol. VI, No. 11 



Dept. of Agriculture, Washington, D. C. June 12, 1916 



dw Utah— a 



(389) 



