Efficacy of Selected Herbicides as Influenced by Soil Properties

Field experiments were conducted on six loam and sandy loam soils to study the influence of various soil parameters on atrazine, cyanazine, alachlor, metolachlor, and pen- dimethalin efficacy. Herbicidal activity was highly correlated to the soil organic content. Humic matter content was equally or better correlated (r = 0.70 to 0.91) with herbicide bioactivity than was organic matter content (r = 0.66 to 0.84). Regression equations were determined which allow herbicide rate recommendations for 80% weed control to be calculated based on soil humic matter or organic matter levels. Nomenclature: Alachlor, 2-chloro-N-(2,6-diethylphenyl)-N- (methoxymethyl)acetamide; atrazine, 6-chloro-N-ethyl-N'-(l-methylethyl)-1,3,5-triazine-2,4-di- amine; cyanazine, 2-((4-chloro-6-(ethylamino)-1,3,5-triazin-2-yl)amino)-2-methylpropanamide; metolachlor, 2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide; pen- dimethalin, N-(l-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine. Additional index words: Atrazine, cyanazine, alachlor, metolachlor, pendimethalin, humic matter, organic matter.