1917] Forbes: Irrigation Effects of Copper Compounds Upon Crops 473 



to May for grain, may be explained by the following consider- 

 ations: The wedge-shaped deposit of tailings indicated in the 

 diagram (fig. 15) at first so obstructed access of water and air 

 to alfalfa root-systems that only stunted development was pos- 

 sible either of roots or tops. With an annual cultivation of this 

 blanket and the incorporation of river sediments and better 

 penetration of irrigating waters, deleterious effects tend to dis- 

 appear and the crop again approaches normal. 



Similar land when plowed for grain contains most of the 

 copper associated with old tailings at the surface of the soil. 

 Young grain, therefore, with shallow and susceptible root sys- 



Fig. 15. — Diagram showing beliavior of root systems under influence 

 of tailings blanket. 



tems, at first, if ever, shows effects of copper in the soil, 

 recovering as root systems penetrate to greater depths wliere 

 they encounter uncontaminated soil. 



Effects of River Sediments 



With reference to the further trend of copper effects upon 

 vegetation in the district, assuming the permanent exclusion of 

 solid tailings but a constant addition of about one part of copper 

 to one million of irrigating water used, it is of interest to take 

 into account the diluting effect of river sediments upon copper 

 compounds in the district. 



In four acre-feet of Gila River water, these sediments will 

 amount to about eighty tons per acre a year,-" of which amount 

 the ten pounds of copper contributed in irrigating waters is ouly 

 0.006 per cent. 



-'0 Forbes, E. H., Ariz. Agr. Exp. Sta. Bull. 53, p. 61. 



