Bioassays with grain sorghum (Sorghum bicolorL. Moench) cultivar NK222 were conducted in a glasshouse. Thirty-four natural soils were used in each of two experiments with alachlor 2-chloro-2#x2032;,6#x2032;-diethyl-N- (methoxymethyl) acetanilide and metolachlor 2-chloro-6#x2032;-ethyl-N-(2-methoxy-1-methylethyl)acet-otoluidide respectively. Linear regression analyses were performed to determine the importance of certain soil properties in the prediction of herbicide bioactivity. The order of importance for alachlor bioactivity was as follows: C clay #x2265; CEC P reversion #x2265; soil pH; and for metolachlor: C clay 3 CEC soil pH P reversion. Inclusion of either clay content or P reversion with C in multiple regression equations significantly increased the predictability of alachlor and metolachlor bioactivity. Organic matter content affected alachlor and metolachlor phytotoxicity, despite the relatively low organic matter levels generally found in South African soils.
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