Effects of Soil upon Toxicity 175 



Only at very high dilutions (one part of copper to from 

 10,000,000 to 100,000,000 of water) are accelerations of root 

 growth observed. These occur with both corn and beans, in well 

 water. In distilled water stimulation was observed only at the 

 highest dilution — 1 :100.000,000. In well water stimulation was 

 observed at from 1:100,000,000 to 1 : 10,000,000— consistently 

 with the well known fact that in presence of other soluble salts 

 the effects of copper are lessened. 



EFFECTS OF SOIL UPON TOXICITY OF COPPER 



SOLUTIONS 



Of prime importance in connection with possible toxic effects 

 of copper in soils are the various reactions (1) converting in- 

 soluble into soluble compounds, (2) reconverting these again into 

 insoluble combinations, and (3) modifying the toxic effects of 

 copper salts in solution. 



As shown in the table of solubilities, both basic carbonate of 

 copper and chrysocolla are soluble in carbon dioxide, forming 

 solutions which in water cultures are highly toxic in character. 

 Sulphides of copper are first oxidized to the sulphate, which is 

 easily soluble : 



Cu^S + 50 = CuSO, + CuO 

 For instance, 100 grams of chalcocite ore containing 3.2 per cent 

 copper were shaken in a flask with 600 c.c. of water, frequently, 

 during twenty-eight days. At the end of that time 500 c.c. of 

 solution contained 0.0132 grams of copper. 



Copper sulphate then reacts in the soil to form various 

 insoluble compounds with consequent lessening of toxic action. 

 With calcium carbonate the following represents a reaction which 

 may occur : 



2 CuSO, + 2 CaCOg + H,0 = Cu(OH),.CuCOp, + 

 2 CaSO, + CO^ 



For instance, two grams of precipitated carbonate of lime were 

 added to an excess of ten grams of copper sulphate in one liter 



6 "University of Arizona well water" contains 250 p.p.m. of soluble 

 solids, mainly sodium sulphate. 



