0141732 



crops, including wheat, barley and alfalfa, at or below this 

 level . 



An argument can be made to revise both the phytotoxic and 

 tolerable extractable zinc levels upward to 125 ppm and 40 ppm 

 respectively. The 60 ppm phytotoxic hazard level was selected 

 based on two phytotoxic occurrences noted above (Table 43). 

 Significant yield reductions for most crops were rare at DTPA 

 extractable zinc concentrations less than 146 ppm. The first 

 significant yield reductions for wheat and alfalfa were reported 

 at DTPA extractable soil zinc concentrations of 146 ppm and 195 

 ppm, respectively (Boawn and Rasmussen 1971). Some yield reduc- 

 tions may occur in barley at DTPA extractable soil zinc concentra- 

 tions less than 125 ppm but the level appears more appropriate for 

 wheat, alfalfa and clover which are grown extensively in the 

 Helena Valley. 



No significant yield reductions were noted in the reviewed 

 literature for any crops at DTPA extractable soil zinc concentra- 

 tions less than 40 ppm. The maximum background extractable (IN 

 HC1) zinc concentration found in the reviewed literature was 26 

 ppm (Dudas and Pawluk 1977) and Walsh et al . (1972) noted a yield 

 increase for corn grain at a 29 ppm 0.1 NHC1 extractable soil zinc 

 concentration. The maximum yield of rye was noted at 40 ppm 0.1N 

 MgSC>4 extractable zinc (Chapman 1966) . 



3.4.3 Zinc in plants 



There is a wide range of zinc phytotoxic levels reported 

 among some plant species, different plant types and for different 

 parts of plants (Table 44). Reported phytotoxic zinc levels range 

 from 60 ppm for wheat plants (Takkar and Mann 1978) to values 

 greater than 800 ppm for swiss chard (Boawn 1971) (Table 44). 

 Most values for crops of concern (cereal grains and forages) fall 

 within the range of 189 ppm to 560 ppm (35 and 20 percent yield 

 reductions, respectively) found by Mitchell et al. (1978) and 

 Boawn and Rasmussen (1971). Boawn and Rasmussen (1971) reported 

 20 percent yield reductions for barley, wheat and alfalfa at above 

 ground plant tissue levels of 540 ppm, 560 ppm and 295 ppm, 



132 



