1917] Forbes: Irrigation Effects of Copper Compounds Upon Crops 409 



Table III is of interest because it reveals quantities of 

 dissolved copper in irrigating and in ground waters sufficient, 

 under proper conditions, in water cultures, to produce toxic 

 effects upon plants.* It is noteworthy, however, that, following 

 the order of the court, effective May 1, 1908, prohibiting the in- 

 troduction of tailings into the water-supply, the amount of dis- 

 solved copper in Montezuma canal waters greatly decreased, due 

 to the decrease in quantity of sulphides whose oxidation affords 

 the supply of dissolved copper. Other water-supplies also are 

 found to contain similar amounts of copper, as the Calumet and 

 Arizona mine waters, used for irrigation below Bisbee. As stated 

 above, however, in the soil itself the toxic action of such copper 

 solutions is enormously decreased. Naturally, the question arises 

 as to the possibility of toxic effects in using such waters upon 

 cultivated soils. This is discussed on subsequent pages. The 

 proportions of copper (0.003 to 0.53 parts in 1,000.000 of water) 

 found in the drainage beneath this irrigated district indicate 

 that not all of the copper applied in irrigation remains in the soil. 

 University well water at Tucson was observed to be free from 

 this element. 



Soils Nos. 3435, 3434, and 3436 show maximum amounts of 

 copper, inasmuch as they are composed to a considerable extent 

 of tailings. The soils in place beneath these sediments, Nos. 3501 

 and 3502, contain much less, yet noticeable amounts of copper, 

 most of which is retained where it first comes in contact with 

 the top soil. It is of interest to note that the surface sediments 

 and the deep soils of the Experiment Station farm near Phoenix, 

 Arizona, irrigated from an entirely different watershed, also con- 

 tain small but weighable amounts of copper. This was probably 

 derived from mines at Globe and Jerome, Arizona, whose wastes 

 have found their way into the drainage which svipplies irrigation 

 for Salt River Valley. The quantities observed, however, three 

 parts copper per million of soil, are negligible. Other soils from 

 Salt River Valley also show traces of copper. 



4 See Bibliography, p. 487, references 5, 18. 



