Ifll4] Lipman-Burgess: Ammonification and Nitrification in Soils 135 



A question more difficult of solution is that involving the 

 cause or causes of the stimulating effects of CuSO^ on the 

 nitrifying flora or the production of nitrates in soils. • The 

 experiments of Fred have included such as aimed at the dis- 

 covery of the reply to the question just raised. The results of 

 such experiments, however, would seem to throw no positive 

 light on its solution. Two factors do seem, nevertheless, to have 

 been eliminated. But little support was found for the idea that 

 poisons like the salts above employed destroy soil toxins which 

 might interfere with bacterial efficiency, as well as for the further 

 idea that the effect of such poisons in the destruction of amoebae 

 might manifest itself by a stimulation for the soil organisms. 

 It seems, to use Fred's expression, to be a ''Reizwirkung" on 

 the part of the salts and other poisons added to the nitrifying 

 flora, but the question as to how this "Reizwirkung" is accom- 

 plished still remains unanswered. This question is further dis- 

 cussed below. 



ZnSO, 



Zinc evidently exercises relatively the »same influence on the 

 nitrifying flora as copper. Like the latter, it stimulates but very 

 slightly at a concentration of 0.0125 per cent ZnSO^, does so 

 very definitely at a concentration of 0.025 per cent, very 

 markedly at 0.05 per cent, and still more markedly at concen- 

 trations of 0.075 per cent and 0.10 per cent, at which latter the 

 highest point of stimulation is reached. Concentrations in excess 

 of 0.1 per cent ZnSO^ seems to be much less stimulating to nitrate 

 production than similar amounts of CuSO^. The absolute 

 amounts of nitrate produced by the nitrifying flora under the 

 stimulus of zinc salts seem to be throughout slightly smaller than 

 those produced by similar flora in the presence of CuSO^. 



FeSO, 



With FeSO^ we obtain effects on the nitrifying flora which 

 possess many characteristics different from those obtained with 

 CuSO^ and ZnSO^. In the first place, stimulation is marked 

 with the smallest concentration, 0.0125 per cent FeSO^, while 

 it is either extremely slight or wanting with the other salts at 



