218 Bulletin 80 



by the acids of plant roots than chalcocite or, probably, chryso- 

 colla. Under field conditions, copper in tailings is originally 

 mostly in the form of sulphides, chiefly chalcocite, which oxidizes 

 only slowly to sulphate in presence of water and air. Chalcocite, 

 3.2 grams, shaken up with 600 c.c. of water, and air, for twenty- 

 eight days, yielded only 16 mg. of soluble copper. The soluble 

 sulphate in contact with silicates and carbonates of the soil is 

 converted to insoluble forms. The process is gradual and the 

 amount of soluble copper present at any one time is small. 



The tilth of the soil is significant. A pot culture very 

 thoroughly mixed with 0.1 per cent of copper as carbonate re- 

 sulted in badly poisoned plants containing about four times as 

 much copper in root systems as in a lumpy mixture of soil con- 

 taining the same amount of copper. The heavy tailings clay, 

 with which copper is chiefly associated in the district studied, 

 tends to remain in lumps and masses, thus minimizing toxic 

 effects of contained copper compounds. 



In water cultures toxic effects of copper salts are lessened 

 by salts contained in well-water or in nutrient solutions. This 

 is due, in part, to the presence of other ions, the effect of which 

 is to decrease the ionization of copper salts, with consequent 

 decrease in toxicity. This observation applies to soil-water 

 solutions which contain considerable amounts of alkali salts. It 

 is of interest in this connection to note that certain combinations 

 of salts representing complete mineral nutrients exert maximum 

 antitoxic action to copper salts ;^* and that therefore a fertile 

 soil containing maximum amounts of plant nutrients will tend to 

 minimize toxic effects of copper. 



Antagonistic solutions, so called, involving copper, may also 

 account for a decrease in toxicity. By reason of a property of 

 the semipermeable membranes of root systems, ions may be either 

 more readily or less readily allowed to penetrate. When pene- 

 tration is decreased through the addition of ions of other soluble 

 salts this salt is said to be antagonistic in character. Copper 

 is thus antagonized by sodium and potassium salts, of which the 

 soluble salt content of the soil is chiefly composed. ^^ 



24 A. Le Eenard, Essai sur la valeur antitoxique de 1 'aliment complet 

 et incomplet. Abstracted in Science n. s. vol. 28, no. 712, p. 236, 1908. 



25 See Bibliograpliy, p. 237, references 3.5-44, 52. 



