1917] Lipman-Gericlce : Smelter Wastes and Barley Growth 541 



be overwhelmingly in favor of the stimulating effects of zinc to 

 plant growth in soils, several instances of stimulation are quali- 

 fied to hold for certain plants only or at very low concentrations 

 of the metal. Therefore the data submitted are not as strong in 

 favor of the stimulating effect of zinc salts to plants as one would 

 suppose from the review above given. Nevertheless, it is strong 

 enough in our opinion to satisfy even the critical that zinc can 

 be a stimulant to plant growth in certain rather considerable con- 

 centrations. Besides that, its toxic effects are nowhere to be 

 regarded as very serious if small quantities of the salt are 

 present. Our results indicate, in addition to all this, that ZnSO^, 

 for example, may be stimulating to barley growth at consider- 

 able concentrations, but that the after-effects on the soil in the 

 third season or crop may be injurious. Such injury, however, is 

 relatively speaking, not verj- great unless very high concentra- 

 tions of ZnSO^ are employed. Even in the third season of 

 cropping in the case of the same soil, it appears that ZnSO^ con- 

 tinues to be stimulating to barley at a concentration of 200 

 p. p. m. of that salt as referred to the dry weight of the soil in 

 question. Moreover, it is not unlikely that the reversal from a 

 toxic to a stimulating condition occurring in the manganese 

 series between the second and third crop might occur in the zinc 

 series between the third and fourth crop. This possibility would 

 seem to find some support from the fact that the third crop 

 in the zinc series corresponds to the second crop of the man- 

 ganese series, since two treatments — one before the first, and one 

 before the second crop — were given to the zinc-treated pots. 



Iron Sulfate 

 Results obtained in experimental trials with FeSO^ in cul- 

 tures of the higher plants have been perhaps more contradictory 

 than those noted in the cases of CuSO^ and ZnSO^ which are 

 reviewed above. This is particularly manifest in the extensive 

 bibliography prepared by Horton'° dealing with the use of 

 sulfate of iron in agriculture. While the latter emphasizes 

 primarily the results obtained with FeS04 in combating weeds. 



'■o A Contribution to the bibliography of the use of sulfate of iron 

 in agriculture, Chicago, 1906. 



