1914] Lip man-Burg ess: Ammonification and Nit)-ification in Soils 137 



behaves like a similar concentration of FeS04 to the nitrifying 

 flora and becomes about as toxic as 0.0125 per cent was shown 

 to be above. Then at 0.0125 per cent PbS04 the latter again 

 behaves like FeS04 in that definite stimulation follows the tox- 

 icity of the previous concentration, but while the stimulation of 

 the FeSO^ at a similar concentration is very marked it is only 

 slight in the case of PbS04. Then, again, at the further con- 

 centration of 0.15 per cent PbS04, the latter acts entirely differ- 

 ently from all the other salts in that it becomes markedly toxic 

 again. 



The idiosyncracies of PbS04 as regards the nitrifying flora 

 of a soil are even more numerous than those of FeS04. How- 

 ever, it must be borne in mind that PbS04 is a very insoluble 

 salt and while the other salts are capable of being added in 

 solution to the cultures. PbS04 has to be mixed with the soil as 

 thoroughly as possible in its dry state. This fact would account 

 for a poor distribution of the salt through the soil mass and 

 thus indicate one possible cause of the extremely erratic behavior 

 of PbSOi in nitrification cultures. The fact, however, that 

 FeS04 aiid PbS04 behave alike at a concentration of 0.1 per 

 cent would seem to be more than a mere coincidence, and some 

 of the speculations made above with respect to FeSOi and its 

 behavior may have some cogency here. The most striking fact, 

 however, gleaned from the series of cultures with PbS04 is that 

 the latter possesses a stimulating power toward the nitrifying 

 flora. Considering the well-known toxic powers of lead with 

 respect to other organisms, this fact assumes considerable im- 

 portance. In all cases, of course, the stimulating powers of the 

 other salts are much greater than that of PbS04. 



General Discussion 



The experimental data above submitted constitute the first 

 series of systematic experiments dealing with the effects of copper, 

 zinc, iron and lead on the ammonifying and nitrifying flora of 

 the same soil. Once more we are confronted by the striking 

 differences between the physiological characteristics of the 

 ammonifying and the nitrifying flora of soils. Other similar 



