134 University of Calif ornia PuMications in Agricultural Sciences [Vol. 1 



a most striking stimulating effect on the nitrifying flora, which 

 results in the production of nearly twice as much nitrate nitrogen 

 as is yielded in the normal soil receiving no copper at all. But 

 the stimulation of the nitrifying flora does not cease at a con- 

 centration of CuSO^ equivalent to 0.05 per cent of the dry 

 weight of the soil. Even at a concentration of 0.075 per cent of 

 CuSO^ we obtain stimulation and one-third again as much nitrate 

 as is produced when copper is present at the preceding concen- 

 tration (0.05 per cent CuSOJ. At 0.1 per cent CuSO^ about 

 the same amount of nitrate is produced as at a concentration of 

 0.075 per cent, or perhaps slightly less, and at the highest con- 

 centration of CiiSO^ employed, namely 0.15 per cent, we obtain 

 an increase of about 100 per cent of nitrate over the normal soil 

 to which no copper is added. That such remarkable stimulation 

 should be exercised by CUSO4 for the nitrifying flora and at such 

 high concentrations of that supposedly toxic salt is a new and 

 interesting fact of great significance. 



While our results agree with those of Fred above cited as 

 regards the lower concentration of CuSO^ which he employed, 

 namely 0.01 per cent, they are wholly at variance with his results 

 as regards his only other concentration. 0.1 per cent CuSO^. 

 The latter gave Fred no stimulation for the nitrifying flora what- 

 ever, but it yields us nearly the maximum point of stimulation 

 of CuSO^ for the flora in question, and in a series of results in 

 which the existence of stimulation is throughout remarkable. 

 To what circumstance these differences between our work and 

 that of Fred are attributable is not readily determined. It can 

 scarcely be due to the difference in the strain or vigor of the 

 nitrifving flora, for we have obtained similar results to those 

 above described with widely different soils which were employed 

 in corresponding vegetation experiments. That the nitrifiable 

 materials were wholly different may. however, be a factor of con- 

 siderable significance. Not only because of its totally dift'erent 

 physical nature, but because of possible antagonistic eft'ects which 

 it might induce. (XH^)oSOi might well be expected to yield" 

 different results in nitrification work from those obtained with 

 dried blood or similar forms of nitrogen, especially when salt 

 effects are studied. 



