Mitchell et al. 1978) and very large yield reductions for field 

 beans, peas, carrots and wheat grain were noted in the 27 to 40 

 ppm range (Table 38). Davis et al . (1978) found barley shoot 

 cadmium concentrations of 14 to 16 ppm to be phytotoxic. These 

 authors noted that 15 ppm cadmium in barley shoots was associated 

 with 10 percent yield reduction. It is clear that the 50 ppm 

 phytotoxic hazard level for cadmium concentrations in plant tissue 

 will be associated with phytotoxi ci ty in nearly all cases and that 

 phy totoxici ty may occur in many species at notably lower concen- 

 trations. All of the above cadmium concentrations far exceed 

 recommended levels for forage and will likely increase the 

 probability of high levels of cadmium entering the food chain. 

 A tolerable plant tissue cadmium level of 10 ppm was sug- 

 gested based on the generally low yield reductions that were noted 

 in the literature below this concentration (Table 38). The 

 alfalfa study of Taylor and Allinson (1981) was of particular 

 importance in that these authors reported several cases of 

 increased production up to the 10 ppm cadmium concentration in 

 alfalfa tops. Again, the 10 ppm tolerable level selected for the 

 Helena Valley will allow much higher cadmium concentrations in 

 forages than the maximum recommended level (0.5 ppm) (NRC 1980). 



3.3 Lead in soils and plants 



3.3.1 Lead literature review 



Mean values for total lead concentration in soil range from 

 10 to 67 ppm, while common levels in plants range from 0.5 to 4 

 ppm (Kabata-Pendias and Pendias 1984). Meyer et al. (1982) found 

 that background soil lead levels ranged from 3 to 23 ppm (mean of 

 12 ppm) for 290 locations in the United States. In urban areas 

 soil lead values may be considerably higher due to contamination 

 from automobile exhaust and industrial activity. Lead is not an 

 essential plant element, and is apparently taken up passively from 

 the soil. While plant toxicity to lead has been noted, it is 

 extremely rare even when excessive amounts of lead are added to 

 the soil (Cannon 1976). This is because lead is one of the least 



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