The amounts of 6 phenolic compounds (p-hydroxybenzoic, vanillic, p-coumaric and ferulic acids, p-hydroxybenzaldehyde and vanillin), released by 4 extractants, differed considerably between soils from 14 plots on which 4 graminaceous species [Agropyron repens, Agrostis gigantea, A. stolonifera, Holcus mollis], 8 dicotyledonous species [Aegopodium podagraria, Calystegia sepium, Campanula rapunculoides, Cicerbita macrophylla, Cirsium arvense, Petasites fragrams, Ranunculus repens, Urtica divica] and 2 spp. of the Pteridophytes [E. quisetum arvense, Pteridium aquilinum] had been grown individually for 16 yr. There were also large differences in the amounts of the phenolic compounds extracted from the roots of these species. When expressed in terms of .mu.g phenolic compounds g-1 organic C, differences between the roots were greater than differences between the soils. Also the amounts extracted by water from the roots (after freeze-drying and grinding) were much greater than the amounts extracted from the soils. The amounts extracted by 2 M NaOH were, in general, greater from the roots than from the soils and, with p-coumaric and ferulic acids from the graminaceous species, much greater. In instances where the amounts extracted from the roots by 2 M NaOH were small, e.g., vanillic acid from Pteridium aquilinum, the amounts from the soils, expressed per unit of organic C, were greater than those from the roots. The sum of the 6 phenolic compounds extracted from the soils by 2 M NaOH represented between 0.03-0.33% of the soil organic matter, these values referring to soils under P. aquilinum and Campanula rapunculoides, respectively. In 13 of the soils, p-coumaric acid was present in greater amounts than any of the other compounds. With the possible exception of the soil under Agropyron repens, the amounts of the identified phenolic compounds extracted by water appeared unlikely to exert allelopathic effects. The amounts released by 0.5% Ca(OH)2 were equivalent to concentrations in the soil solution that have been reported to have adverse effects on plant growth under certain conditions, which suggests that such effects might occur in the field as a consequence of liming.