General Discussion 217 



and (in the case of corn) showing characteristic proliferated root 

 tips. The epidermis is thick and rough and the cells in longi- 

 tudinal tangential section contract from the oblong toward the 

 circular form. Here, again, other factors, such as alkali salts in 

 excess, may lead to similar appearances: and these must be 

 eliminated in a diagnosis of copper injury. 



Soil Conditions Relating to Toxic Effects of Copper upon 



Plants 



Certain conditions favor, others oppose the toxic action of 

 copper under field conditions, the general tendency being to 

 modify or do away with toxic effects, where the amounts of copper 

 are not excessive. 



Carhon dioxide in the soil, alone and in conjunction with cer- 

 tain salts (NaCl, NaoSOJ tends to form solutions of basic cop- 

 per carbonate. Carbonates (Na,C03,CaC03) lessen the solubil- 

 ity of basic copper carbonate in carbon dioxide and, therefore, 

 the toxicity of copper compounds in soils containing these 

 carbonates. '-- 



Coarse, sandy soils favor toxicity by permitting free move- 

 ment of solutions and because the withdrawal in them of copper 

 from solution by physical and chemical reactions is minimum.-^ 



The character of the compound of copper to which roots are 

 exposed is important. In pot cultures of precipitated carbonate 

 of copper, of sulphide in the form of chalcocite pulverized to go 

 through a 100-mesh sieve, and of silicate in the form of chryso- 

 colla pulverized to 100-mesh, toxic effects appeared with corn as 

 follows : 



Pot culture of corn; Cu in form of pptd. carbonate— showing toxic 

 effects at .023% Cu in soil 



Pot culture of corn; Cu as chalcocite, 100-mesh— showing toxic effects 

 at .08% Cu in soil 



Pot culture of corn; Cu as chrysocolla, ]00-mesh— showing toxic effects 



at .08% Cu in soil 



The precipitated carbonate is not only more soluble in car- 

 bon dioxide than in chalcocite, but also more easily acted upon 



22 See Bibliography, p. 236, reference 12. 

 -■'■ See Bibliography, reference 18. 



