Persistence of Benzoic and Phenylacetic Acids in Soils

In a greenhouse experiment, dosages of 2-methoxy-3,6-dichlorobenzoic acid [dicamba] and 2-methoxy-3,5,6-trichlorobenzoic acid [tricamba] required initially to reduce fresh weights of snap beans (Phaseolus vulgaris L.) 50% (hereinafter referred to as ED50 values) increased as clay and organic matter increased in five soils. In all soils, 2,3,6-trichlorobenzoic acid [2,3,6-TBA] was about equally toxic, but ED50 values for 2,3,6-trichlorophenylacetic acid [fenac] and 2-methoxy-3,6-dichlorophenylacetic acid [hereinafter referred to as methoxy fenac] decreased as clay and organic matter increased. Under experimental conditions of alternately moist and dry soil, phytotoxicity of dicamba, tricamba, and methoxy fenac decreased with time, but that of 2,3,6-TBA and fenac remained approximately the same for about 22 months. When soils were maintained moist throughout the incubation period, 3-amino-2,5-dichlorobenzoic acid [amiben] was least persistent, dicamba intermediate, and 2,3,6-TBA and fenac most; under these conditions, phytotoxic effects of 2,3,6-TBA and fenac decreased with time. In soil-enrichment studies, fenac, methoxy fenac, dicamba, and 2,3,6-TBA, in comparison with 2,4-dichlorophenoxyacetic acid [2,4-D], appeared very resistant to microbial degradation.