1914] Lipman-Burgess: Ammonification and Nitrification in Soils 139 



plants in the soil in the presence of copper. Why might not a 

 similar effect be exercised by metals as regards the nitrifying 

 bacteria which, physiologically speaking, in every way resemble 

 the higher plants much more than the other soil flora? 



Conclusions 



1. Copper, zinc, iron and lead exercise toxic effects on the 

 ammonifying flora of a sandy soil from Southern California in 

 all concentrations, ranging from 50 to 2500 parts per million of 

 their sulfates. 



2. Such toxicity is relatively small, however, and is more 

 marked at concentrations below 0.1 per cent than above it, in 

 most cases. 



3. The metals named exercise no stimulating effect on the 

 ammonifying flora at any concentration. 



4. Copper, zinc, iron and lead exercise marked stimulating 

 effects on the nitrifying flora of the same soil and i>equently 

 more than double the normal nitrate yield. 



5. The same metals may in very small concentrations exercise 

 toxic effects on the nitrifying flora or no effect at all, but they 

 are markedly stimulating at much higher concentrations. 



6. With the one exception of lead sulfate, the metals named 

 showed very marked stimulation for nitrification even at 0.15 

 per cent, the highest concentration employed. 



7. Comparisons are given of our work with that of one or 

 two other investigators on portions of the same subject. In some 

 cases our results confirm previous work. In other cases they are 

 wholly at variance with it. 



8. A brief discussion is given of possible causes for the effects 

 of the metallic salts under consideration on the soil flora. 



9. Fuller consideration will be given in another paper to the 

 theoretical phases of the subject treated above. 



Transmitted March 6, 1914. 



