Tha analytical method and accompanying interpretive guide was 

 developed by N.R. Benson (Benson and Reisenauer 1951, Benson 1968) 

 primarily through many years of field experience in diagnosing 

 arsenic toxicity problems in orchard vegetation in central and 

 eastern Washington (A.R. Halvorson, personal communication 1985). 

 Soil arsenic is extracted with concentrated HC1 (12. 3M) in a 1:5 

 soil to acid ratio for a period of one hour, and standard instru- 

 mentation methods are used to determine actual concentrations. 

 Interpretation of the results of the analysis in terms of poten- 

 tial phytotoxicity can be made by refering to Table 34. 



Benson and Reisenauer (1951) rated the relative tolerance of 

 crops to arsenic (Table 35). Crops such as those found in the 

 Helena Valley (e.g. barley, wheat, alfalfa) were considered not 

 tolerant to soil arsenic. The tolerance of wheat to soil arsenic 

 was compared to peach and apricot fruit trees. The interpretation 

 is that grain and forage crops will do poorly when the concen- 

 trated HC1 extractable soil arsenic exceeds 50 ppm (Tables 34 and 

 35) . 



This result compliments other investigations of the effect of 

 soil extractable arsenic on crops (Table 32) . These investigators 

 found significant yield reduction of vegetable crop when extract- 

 able arsenic was in the range of 6 to 48 ppm. 



3.1.2 Arsenic in soils 



3.1.2.1 Total arsenic in soils 



The phytotoxic and tolerable levels of total arsenic in soils 

 of the Helena Valley are 100 and 25 ppm, respectively (Table 30). 

 The 100 ppm concentration has been selected primarily based on 

 data of Woolson et al . (1973) and Steevens et al. (1972) who noted 

 large yield reductions in oats, corn, peas and potatoes at 100 ppm 

 total soil arsenic. All total soil arsenic values equal or 

 greater than 100 ppm in the reviewed literature were associated 

 with phytotoxicity. Soil characteristics, especially texture and 

 organic matter content, strongly influence the relative toxicity 

 of arsenic. Weaver et al . (1984) reported phytotoxicity of 



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