548 University of California Publications in Agricultural Sciences [Vol. 1 



conditions named, the toxicity seemed to be about the same with 

 the larger as with the smaller concentrations of PbS04 employed. 



The manganese salts were tested in the tirst crop only, in 

 connection with their powers to affect nitrification. The follow- 

 ing were the resnlts : MnSO^ was not toxic under the conditions 

 named in any of tlie concentrations in which it was employed, 0.6 

 per cent being the highest. It appeared to be very slightly stim- 

 ulating at all concentrations. In the case of MnCl, we find 

 marked toxicity to nitrification at concentrations in excess of 0.4 

 per cent, and ver.y distinctly toxic effects at concentrations 

 in excess of 0.15 per cent. On the other hand, we also note 

 that nitrification was stimulated by the following concentra- 

 tions: 0.05 per cent, 0.1 per cent, and 0.15 per cent. The stimu- 

 lation was very marked only in the case of the latter two con- 

 centrations and was very much in excess of that induced by 

 MnSO^ at any concentration. 



The nitrifying powers of the Oakley blow sand employed in 

 one copjDer series, which is described above, were also deter- 

 mined. Marked stimulation to the nitrifying power of the soil 

 was noted at concentrations of CuSO^ equivalent to 100 p. p. m., 

 200 p. p. m., and 300 p. p. m., the first two being most marked. 

 Ammonium sulfate was employed as the nitrifiable material. 

 Amounts of CuSO^ in excess of 300 p. p. m. were decidedly toxic, 

 and very little or no nitrification occurred in the soil containing 

 more than 700 p. p. m. CuSO^. 



While there is considerable discrepancy in the correlation of 

 the effects of the different salts on barley growth and on the 

 nitrifying bacteria, there appears to be a general relation, at 

 least, between the stimulating effect exerted by a salt on the 

 nitrifying flora and its effect on the barley plant. The serious 

 irregularities which seem to militate at present against the 

 definite establishment of such a relationship based on our data 

 can undoubtedly be explained on the basis of certain factors 

 like the residual nitrate supply in soils and the differences in its 

 distribution throughout the soil mass which of course must exist. 

 Wliile therefore we make no attempt to assert that the stimu- 

 lating effects and perhaps the toxic effects to barley exhibited by 



