Persistence of 2,4-Dichlorophenoxyacetic Acid in Soil in Relation to Content of Water, Organic Matter, and Lime

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Abstract
Partially decomposed leaf mold, water, and lime were mixed with heavy black-loam soil from northern Illinois in such fashion as to yield 6 lots: air-dry soil; air-dry soil + leaf mold; moist soil; moist soil + leaf mold; moist soil + lime and moist soil + lime + leaf mold. To 4 equal aliquots of each of the 6 lots, 2,4-dichlorophenoxyacetic acid (2,4-D) was mixed on Apr. 7 at the rate of 0, 2, 20, and 200 ppm., respectively. All 24 lots were placed in storage. Portions of each lot were placed in pots on greenhouse benches on Apr. 14, Apr. 21, May 5, July 17, and Aug. 15. Each pot was planted with Red Kidney bean, mustard, African marigold, and barley to test, by the emergence and later development of these plants, the degree and persistence of toxicity from 2,4-D in the different lots of stored soils. The soil originally planted on April. 14 was replanted on May 12, June 2, and July 15 to test for persistence or loss of toxicity in the various treatments under cropping. Soils initially planted on Apr. 21 and July 17 were replanted on May 19 and Aug. 16, respectively. The capacity to induce toxic symptoms was strongly persistent in soil in dry storage as measured by all initial test plantings. There was some diminution of toxicity in some lots in moist storage, although others, especially the limed ones, retained strong toxicity for 18 weeks. In general, as shown by 3 different replantings, 4 weeks of cropping in the greenhouse sufficed to eliminate toxicity to all test spp. from all soil except that which had been limed or to which 2,4-D had been added at the rate of 200 ppm. 7 weeks of cropping resulted in the loss of toxicity to all test spp. from all except the limed soil at the 200-ppm. level. Such long persistence of 2,4-D in this treatment suggests caution in its use in calcareous or newly limed soils. In all replantings the presence of large amts. of leaf mold strikingly reduced the degree and persistence of toxicity to one or more test species in the limed soil. 13 weeks of cropping resulted in complete loss of toxicity from all pots. In some replantings there was more vigorous growth of seedlings in soil treated with 2,4-D than in the controls. Such stimulation was found in one or more test species in one or more replantings for 12 of the 18 lots treated with 2,4-D. Sensitivity to 2,4-D in the soil increased in the order barley, bean, mustard, and African marigold. In the initial plantings on soil containing 2 and 20 ppm. of 2,4-D the first trifoliate-leaves of bean characteristically showed abnormalities involving nondivergence of the leaflets or their multiplication to numbers as high as ten.