1917] Lipman-Gericke : Smelter Wastes and Barley Growth 547 



will be remembered, was grown one year after the second and last 

 copper application had been made, the increases in the nitrifying 

 powers of the treated soils were from 33 per cent to 100 per cent 

 greater than in the control soils receiving no copper. In a gen- 

 eral way the highest concentrations of CUSO4 gave the largest 

 increases in nitrifying power in the second crop, but in the third 

 crop there was more or less irregularity in that regard and the 

 smaller concentrations appeared to be as effective as the larger. 

 In the case of the zinc series, determinations of the nitrifying 

 powers of the different soils were made after the third crop only. 

 In that case also, the nitrifying power was increased by appli- 

 cations of ZnS04 equivalent to 200, 600, and 1000 p. p. m. The 

 increases, however, were much smaller than in the case of the 

 CUSO4 and varied from 3 per cent to 16 per cent at the different 

 concentrations, the most favorable concentration being 600 p. 

 p. m. An important difference exists between the CUSO4 and 

 the ZnS04 series in that all the concentrations of the former, 

 which were employed increased the nitrifying powers of the 

 soil in the third crop to some extent, while only the concentra- 

 tions just given were instrumental in imparting such a stimulus 

 in the case of the latter salt. Iron behaved very similarly to zinc 

 in most respects so far as the soil's nitrifying powers were con- 

 cerned, and 0.2 per cent, 0.4 per cent, 0.6 per cent, and 0.8 per 

 cent were the range of concentration of FeSOi corresponding 

 to those named for ZnS04 above. One difference between iron 

 and zinc in their influences ou nitrification in the greenhouse 

 soil is that the former does not seem to have been appreciably 

 toxic in any concentration, even though as much as 2 per cent 

 FeS04 was employed, whereas the latter, as we have already 

 seen, markedly depressed the soil's nitrifying power when used 

 in excess of 0.1 per cent of the dry weight of the soil. Like 

 ZnS04 and FeS04, PbS04 was tested as to its effect on nitrifica- 

 tion after the third crop only. Under those conditions it gave, 

 however, very different results from the other salts, since no 

 stimulation to nitrification was noted at all, no matter what 

 amounts of PbSOi were employed. On the other hand, while 

 PbS04 was throughout slightly toxic to nitrification under the 



