June 12, 1910 Influence of Arsenic upon Soil Organisms 413 



SUMMARY 



Arsenic, when applied to a soil in the form of lead arsenate, sodium 

 arsenate, arsenic trisulphid, or zinc arsenite, stimulates the nitrogen-fixing 

 powers of the soil. This stimulation is greatest when lead arsenate is 

 applied and least when zinc arsenite is applied. Paris green did not stimu- 

 late in any of the concentrations. This compound becomes very toxic 

 when the concentration reaches 120 p. p. m. The toxicity of this com- 

 pound is due to the copper and not to the arsenic contained in it. 

 Sodium arsenate became toxic when a concentration of 40 p. p. m. 

 of arsenic was added, and when 250 p. p. m. were added it entirely 

 stopped nitrogen fixation. Lead arsenate was not toxic even at a con- 

 centration of 400 p. p. m. of arsenic. The toxicity of arsenic trisulphid 

 and zinc arsenite was very small at this concentration. 



The stimulation noted when arsenic is added to a soil is not due to any 

 inherent peculiarity of the soil used, for soils which vary greatly in 

 physical and chemical properties had their nitrogen-fixing powers greatly 

 increased when arsenic was applied to them. Soils high in organic matter 

 fixed as much nitrogen in the presence of arsenic and in the absence of 

 mannite as they did in the presence of mannite and absence of arsenic. 

 The stimulation is greatest when the water-soluble arsenic content of the 

 soil is about 10 p. p. m. This quantity exceeds that found in most 

 soils, so it is likely that in agricultural practice arsenic will stimulate 

 and not retard bacterial activity in the soil. 



Only one type of Azotobacter was isolated which was stimulated by 

 arsenic, and in this case the stimulation was due to the organism utilizing 

 more economically in the presence of arsenic its source of carbon than it 

 did in the absence of arsenic. The arsenic compounds do not act as a 

 source of energy to the organisms. The main part of the stimulation 

 noted in the soil with its mixed flora is undoubtedly due to the arsenic 

 inhibiting injurious species. 



A quantity of arsenic which acts as a stimulant to bacteria when placed 

 in soil may become very toxic when tested by the Remy-solution method. 



Arsenic can not replace phosphorus in the vital process of the nitrogen- 

 fixing organisms, but it can in some manner liberate the phosphorus from 

 its insoluble compounds. This may be either a direct or an indirect 

 action. 



Arsenic stimulates the cellulose ferments, and these in turn react upon 

 the activity of the nitrogen-fixing organisms. 



The nitrogen-fixing powers of soil extract, of filtered soil extract, and 

 soil dried for some time are only slightly stimulated by arsenic, showing 

 that arsenic acts mainly by the removal of a thermolabile body which 

 occurs in the soil. 



