General Discussion 207 



Partll.-GENERAL DISCUSSION 



PRELIMINARY STATEMENT 

 The copper compounds, in solid form and in solution, that 

 result from mining operations in the Clifton-Morenci district, 

 have found their way down the San Francisco and Gila rivers 

 to the underlying irrigated agricultural soils of Graham County 

 in sufficient amounts to raise the question of their toxicity to 

 crops. The largest amounts of copper in these soils are found 

 at the heads of irrigated lands, especially where alfalfa is or 

 has been, at which points old accamulations of tailings, laid 

 down for the most part prior to 1908, are still to be found. 



Accumulations op Copper 



The amounts of copper accumulating in the Gila River valley 

 soils in this way are small, the observed range being from 0.006 

 per cent to 0.111 per cent in surface soils and the average for 

 eighteen soils analyzed being 0.046 per cent of copper. Irrigated 

 soils elsewhere have been observed to contain larger quantities 

 of copper than those above noted, for instance 1.002 per cent on 

 the Deer Lodge River below Anaconda, Montana, with an aver- 

 age of 0.09 per cent for eleven other samples taken in the same 

 locality.^** 



These amounts of copper in a soil may or may not be toxic 

 according to the combination in ^\hich the copper exists, the 

 physical character of the soil and its chemical composition, 

 climatic and moisture conditions, the crop grown, and other con- 

 siderations which may now be discussed in order. 



The small amounts of soluble copper constantly coming down 

 stream from the mines which cannot, like solid tailings, be en- 

 tirely excluded from irrigating water supplies, are of importance 

 because of their tendency to accumulate by reason of the fixing 

 power for copper of silicates, carbonates and organic matter in 

 the soil. The completeness of this fixing power of soil for copper 

 is shown by several experiments in which solutions of copper 



10 U. S. D. A. Bur. Chem., Bull. 113, p. 34, 1907. 



