0141703 



of cadmium that may enter the food chain at either 100 or 50 ppm 

 total soil cadmium concentration. 



The total soil cadmium tolerable concentration of 4 ppm was 

 selected for the Helena Valley based on the generally small or 

 nonsignificant yield reductions reported below this level, 

 compared to the higher yield reductions (up to 46.8% for corn 

 shoots) noted at the 5 ppm total soil cadmium level. 



3.2.2.2 Extractable soil cadmium 



The DTPA extractable soil cadmium phytotoxic and tolerable 

 concentrations selected for the Helena Valley were 30 and 2 ppm, 

 respectively (Table 37). All extractable cadmium concentrations, 

 found in the reviewed literature, that were in excess of 30 ppm 

 were phytotoxic. The hazard level was based on the 25 percent 

 yield reductions that were noted for wheat grain and white clover 

 at concentrations of 30 and 29 ppm, respectively (Bingham et al. 

 1975). Numerous occurrences of phytotoxici ty were noted for a 

 number of species in the 4.8 to 30 ppm extractable cadmium range 

 (Table 37) . Of particular interest were the 22 and 25 percent 

 yield reductions for alfalfa and wheat grain at extractable soil 

 cadmium levels of 22 and 23 ppm respectively (Bingham et al . 1976, 

 Mitchell et al. 1978). Extractable soil cadmium concentrations 

 between 2 and 4.8 ppm were associated with both yield increases 

 and yield decreases. Concentrations less than the suggested 2 ppm 

 tolerable level were not generally significantly phytotoxic except 

 under specific experimental conditions (Table 37). 



3.2.3 Cadmium in plants 



The phytotoxic concentration of cadmium in plant tissues (50 

 ppm) selected for the Helena Valley was based on the literature in 

 which most concentrations greater than 50 ppm were associated with 

 phytotoxicity . The only exceptions were slight yield increases 

 noted for lettuce and alfalfa at levels of 51.1 and 57.6 ppm, 

 respectively (Table 38). Large yield reductions in ryegrass and 

 wheat grain (50 and 42 percent, respectively) were reported at 

 tissue cadmium levels at or near 40 ppm, (Dijkshoorn et al. 1979, 



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