496 University of California Puhlications in Agricultural Sciences [Vol. 1 



PAGE 



Additional investigations 546 



Nitrification 546 



Nitrogen content of the grain 550 



Absorption of metals by soil and plant 551 



General and practical considerations 554 



Theoretical considerations 558 



Summary 559 



INTRODUCTION 



lu 1913^ the senior author and F. H. Wilson reported briefly 

 the results of some preliminary investigations on the effects of 

 CUSO4, MnS04, ZnSO^, and H2SO4 on the growth of wheat and 

 of vetch in a humus sand in pots, under greenhouse conditions. 

 One year prior to the appearance of the report just cited, the 

 present authors instituted new and more complete experiments 

 with the objects noted below. These experiments, covering a 

 period of three years, are now complete in several significant 

 phases and we are therefore proceeding to a discussion of them. 



The importance of a study of this subject is attested by the 

 recent appearance of monographic works devoted to it, by the 

 significance of the practical bearings of the physiological studies 

 involved and, in view of tliese, by the conflicting nature of the 

 results thus far obtained, and the evident non-consideration, by 

 investigators, of the nature of the medium of plant growth as 

 a vital determinant of the results. Some of the outstanding 

 early work on the inorganic poisons, particularly copper, as 

 affecting plant growth is either reviewed or cited in the com- 

 munication above referred to. In this paper no historical sketch 

 w\\\ be given, but important investigations which may be rel- 

 evant to our findings will be discussed in connection with the 

 results and meaning of our experiments. 



OBJECTS OF THE EXPERIMENTS 



The objects of our experiments were as follows: (1) To 

 ascertain whether metals like copper, zinc, lead, iron, and man- 

 ganese used in tlie sulfate form in the soil as a medium are toxic 

 in am- quantity to barley. (2) To ascertain whether the sub- 

 stances named can be toxic in soils if found in quantities which 



1 Bot. Gaz., vol. 55, no. 6, p. 409, June, 1913. 



