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



similar concentrations. While stimulation continues up to and 

 including concentrations of 0.075 per cent, it is not so great as 

 with the smallest concentration, and increases or decreases with 

 the different amounts of FeS04 employed, with no regularity. 

 Then, beyond the concentration of 0.075 per cent FeSO^, a 

 wholly inexplicable sudden toxicity manifests itself, and stranger 

 still, beyond that point with two additional and larger concen- 

 trations employed we obtain the most marked stimulation in the 

 whole series of cultures and about equivalent to the stimulation 

 effected by CuSO^ at similar concentrations. 



The causes of this erratic behavior of FeS04 are a mystery 

 as yet. We suggest that the rapidity of oxidation of the FeSO^ 

 may vary and so the ferrous and ferric salts may yield different 

 effects, though this explanation does not appear wholly satis- 

 factory. The distribution of the salt in the soil culture may also 

 be a factor. What appears to us as another promising explana- 

 tion for the behavior of the FeSO^ under discussion is its effect 

 on the physical condition of the soil and the modification of the 

 latter 's absorptive power for moisture and gases. Whether or 

 not the effects noted may be related to transformations in the 

 amounts of available bacterial foods present cannot be answered 

 from the data in hand. 



PbSO, 



In this series of cultures we are confronted with effects which 

 are in certain definite ways different from either those of CuSO^ 

 and ZnSO^, on the one hand, and FeSO^ on the other hand. 

 Lead is the only one of the metals used above which exercises an 

 uncpiestionable and marked toxicity on the nitrifying flora at 

 even the lowest concentration (0.0125 per cent PbS04). But 

 while that is so, larger amounts of PbSO^ make conditions in the 

 soil more favorable for nitrification as the lead salt increases 

 in concentration from 0.0125 per cent to 0.05 per cent, at which 

 point nitrification is about as active as in the normal soil. When, 

 however, the PbS04 is still further increased we obtain what 

 appears to be a definite stimulation to nitrification. Oddly 

 enough, however, at a concentration of 0.1 per cent, PbSO^ 



