148 Bulletin 80 



originate in the ores of the district and are, as in the case of the 

 carbonates, directly soluble to a slight extent in drainage waters, 

 especially in the presence of carbon dioxide. In other cases, the 

 original ores are changed through the action of air into soluble 

 substances which then escape downstream. Sulphide ores are 

 thus oxidized in the presence of air into soluble copper sulphate. 

 Inasmuch as it is well known that minute amounts of copper in 

 solution are extremely toxic to plant roots directly exposed to 

 them, some apprehension naturally existed as to the eifects of 

 these small amounts of copper salts escaping into the water- 

 supply of an irrigated district. 



In some respects, conditions were especially favorable here 

 to the successful prosecution of a study of the foregoing ques- 

 tion. The irrigated lands are at a distance of twenty miles or 

 more from the smelters, so that injurious gases could not com- 

 plicate effects upon irrigated crops. There are, also, only traces 

 of other toxic metals to be found within the district — more par- 

 ticularly, arsenic, antimony, and zinc. Injurious effects due to 

 the possible toxic action of compounds originating in the mines 

 are therefore limited to copper. 



Scientific study relating to toxic effects of copper upon plants 

 under varying conditions has thoroughly established not only 

 the fact that copper compounds are extremely toxic to plants 

 when they obtain entry to their tissues, but also that various 

 agencies standing between these poisonous salts and the living 

 plant tend to prevent injury.^ Soluble copper compounds, for 

 instance, react with carbonate of lime, commonly abundant in 

 soils of the arid region, to form the solid carbonates of copper. ^ 

 The partly decomposed silicates of these soils also precipitate 

 soluble compounds of copper and mask their toxic character. 

 Organic matter in the soil likewise holds large quantities of 

 copper in comparatively harmless combinations. Through phj-si- 

 cal attraction or adsorption, soluble copper compounds enter into 

 weak combination with fine soil particles and toxic effects are 

 thereby greatly lessened. In the presence, also, of other soluble 

 salts, such as the various forms of "alkali" commonly found in 



1 See Bibliography, pp. 236-237, references 1, 8, 14, 15, 16, 19, 34, 51. 



