June 12, 1916 Influence of Arsenic upon Soil Organisms 407 



has noted a stimulation with copper sulphate when used in dilute solu- 

 tions. This, however, may have been due to the anion and not to the 

 cation, as sulphates do stimulate plants by their action on insoluble 

 constituents of the soil (Greaves, 1910, p. 298). The same interpreta- 

 tion could be placed upon the results obtained by Lipman and Wilson 

 (191 3) and also those reported by Voelcker (19 13), in which they noted a 

 stimulation with copper salts. Clark and Gage (1906) have found that 

 very dilute solutions of copper have an invigorating influence upon bac- 

 terial activity. In order that the stimulation may be noted the copper 

 must be present in small quantities. Jackson (1905) found that i part of 

 copper sulphate in 50,000 parts of water killed Bacillus coli and B. 

 typhosus. Kellerman and Beckwith (1907) found that the common 

 saprophytic bacteria are more resistant to copper than is B. coli. There 

 is considerable evidence (Lipman and Burgess, 1914; Greaves, 1913a, p. 

 8) that copper stimulates the ammonifying and nitrifying organisms 

 of the soil, but these results show the nitrogen-fixing organisms of the 

 soil to be very sensitive to copper, and if it does act as a stimulant it 

 must be in extremely dilute solutions. The toxicity of the copper in the 

 Paris green is great enough in the dilution of 10 parts in 1,000,000 to 

 offset the great stimulating influence of the arsenic in combination with it. 



The very marked stimulating influence noted where the arsenic 

 trisulphid is used is very probably due to both the arsenic and the 

 sulphur. Demolon (191 3) attributed much of the fertilizing action of 

 sulphur to its action upon bacteria, and Vogel (1914) found that sulphur 

 decidedly increased the activity of the nitrogen-fixing organisms. The 

 results which Russell and Hutchinson (1913, p. 173) obtained with calcium 

 sulphid are interesting in this connection. They found that after 30 

 days there were five times as many organisms in the soil to which calcium 

 sulphid had been added as in the untreated soil, and the yield of ammonia 

 and nitrates in this time was one-third greater in the treated soil than 

 in the untreated soil. This, in turn, reacts upon the crop harvested, as 

 shown by Shedd (1914, p. 595). 



The first part of the curve (fig. i) for the zinc arsenite nearly coincides 

 with that of the sodium arsenate, but the zinc arsenite stimulates in 

 greater concentrations than does the sodium arsenate. This is partly 

 due to the difference in solubility of the two compounds, but there is 

 another factor which enters, and that is that the zinc also acts as a stimu- 

 lant. Latham (1909) found that small quantities of zinc stimulated 

 algge. The same results have been obtained by Silberberg (1909) in 

 working with higher plants. Ehrenberg (1910) concludes that zinc salts 

 are always toxic when the action is simply on the plant, but that they 

 may lead to increased growth through some indirect action on the soil. 

 He found that zinc stimulated plant growth in soils, but when the soil 

 was sterilized the zinc became toxic. Lipman and Burgess (1914, p. 133) 



