Allelopathic Effects of Palmer Amaranth (Amaranthus palmeri) and Other Plant Residues in Soil

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Abstract
Several crop and weed species were field grown to maturity and their residues incorporated with soil to determine the effects of the plant residues on the growth of subsequent plantings in a spring-fall planting sequence in 1980 to 83. Growth of Palmer amaranth (Amaranthus palmeriS. Wats. # AMAPA) was studied in 1984 and 1985, and the effects of soil-incorporated Palmer amaranth on the growth of cabbage (Brassica oleraceavar.CapitataL.) and carrot (Daucus carotaL. var.sativa) seedlings were studied in 1985. Under warm, moist, saline soil conditions and intense sunlight following the spring plantings of the 1980 to 83 study, soil-incorporated residues of Palmer amaranth inhibited the growth of carrot 49% and onion (Allium cepaL.) 68%. Johnsongrass (Sorghum halepenseL. Pers. # SORHA) residues inhibited the growth of onion 67%, and in one of two years inhibited the growth of cabbage 26% and common sunflower (Helianthus annuusL. #HELAN) 10%. In 1984 and 1985, Palmer amaranth grown at population densities of 300 plants/m2attained heights of nearly 2 m and biomass of 2 to 3 kg/m2within 10 weeks after planting. Phytotoxicity with these biomasses persisted 11 weeks in cabbage and carrot plantings. In 1985, phytotoxicity was dissipated 16 weeks after soil incorporation of the weed.