The presence of Palmer amaranth (AMAPA) residues in the soil reduced fresh weight accumulation in onions and carrots and markedly decreased seedling field establishment of carrots. Solid-phase separation techniques were used to isolate fractions containing water-soluble organic compounds from AMAPA residues and soil amended with such residues. At concentrations of 20-100 mg/liter most of the organic solids thus extracted were inactive in seed germination assays using onion, carrot, AMAPA, and tomato seeds. Extracts from the roots of AMAPA increased 72-hr germination percentages in carrot, AMAPA, and tomato. A time-study of AMAPA residue decomposition in soil showed an increase in extractable inhibitors of onion germination after 62 days but no other significant changes in the activity. The most active allelochemicals from AMAPA proved to be volatile compounds. Volatiles emitted by soil containing AMAPA residues and by the dried and partially rehydrated leaf and flower residues themselves reduced carrot and tomato seed germination to less than 7%. Freshly harvested aerial AMAPA inhibited only carrot seed. Germination of AMAPA and carrot seeds was retarded by exposure to volatiles from dried AMAPA residues. Residues from AMAPA grown in Texas and Louisiana exhibited comparable inhibitory activity after air-drying two weeks. Onion seeds were also inhibited by volatiles from AMAPA residues.