0141717 



lead (MacLean et al . 1969). These authors noted a 71.4 percent 

 reduction in alfalfa yield at this level but stated that the 

 observed yield reduction may have been due to excess chloride 

 rather than high lead in the soil pots. MacLean et al. (1969) 

 reported IN NH4OAC extractable soil lead levels were in accord 

 with concentrations found in plants which suggested extractable 

 soil lead concentrations reflected soil characteristics. The 200 

 ppm tolerable extractable lead level has been selected based on 

 data reported by Lagerwerff et al . (1973) who found no significant 

 yield reductions for corn and alfalfa at a concentration of 212 

 ppm IN HC1 extractable soil lead. Only one occurrence of a yield 

 reduction was noted at levels less than 200 ppm extractable soil 

 lead (3.8 percent for alfalfa at a concentration of 124 ppm IN 

 NH4OAC extractable soil lead (Table 40). 



3.3.3 Lead in plants 



There is a wide range of values, 4 to 300 ppm, reported for 

 the phytotoxic level of lead in plant tissues (Table 41) . Plant 

 tissues vary considerably in their tendency to accumulate lead. 

 High lead levels were observed in the roots of many plants. 

 Alloway (1968) noted 500 ppm lead in the roots of apparently 

 healthy radish plants, and Keaton (1937) reported 808 ppm lead in 

 the roots of barley plants which contained only 3.08 ppm lead in 

 plant tops. Alfalfa plants, grown in pots with 1000 ppm total 

 soil lead and amended with lime and phosphate, were shown to 

 accumulate up to 730 ppm in plant top tissue without apparent 

 phytotoxici ty (MacLean et al . 1969). Taylor and Allinson (1981) 

 noted 65 ppm lead in alfalfa plant tissues without yield reduc- 

 tions. Davis et al . (1978) found the critical level (10 percent 

 yield reduction) of lead in barley shoots was 35 ppm. The 

 tolerable level of 25 ppm lead in vegetative tissue was selected 

 based on two factors: 1) it was within the range which Davis et 

 al. (1978) noted the "onset of growth reduction" in barley 

 seedlings (20 to 35 ppm) and 2) it was below the 35 ppm concentra- 

 tion these authors found to be associated with a 10 percent yield 

 reduction . 



117 



