Velvetbean (Mucuna pruriens) has been reported to release 3-(3′,4′-dihydroxyphenyl)- L-alanine (L-DOPA) as an allelochemical that inhibits the growth of other plants, although the inhibitory activity depends on the soil type. To clarify the effects of soils on the concentration, chemical structure, and plant growth inhibitory activity of L-DOPA, L-DOPA solution was reacted with three soils (volcanic ash, calcareous, and alluvial soils), and the resultant solution was subjected to quantitative analysis with HPLC, spectral analyses with NMR and UV-VIS spectrometers, and plant growth inhibitory activity tests with lettuce (Lactuca sativa) seedlings. In the presence of soils, L-DOPA disappeared with the reaction time, and the disappearance was associated with the following three reactions: adsorption reaction (characterized by fast and sudden disappearance of L-DOPA within the initial 8 h period), catalytic transformation reaction (constant L-DOPA disappearance throughout the reaction period), and biotransformation caused by microbial activity (accelerated L-DOPA disappearance observed after 72 h of reaction time). The amount of L-DOPA adsorbed was largest in the presence of volcanic ash soil among the three soil types. It is likely that the mechanism of L-DOPA adsorption includes a ligand exchange reaction. The L-DOPA transformation was observed at equilibrium pH values higher than 4 and it was faster at higher pH values. In the absence of soil, however, L-DOPA transformation did not occur at an equilibrium pH value lower than 6.0, indicating that L-DOPA transformation was accelerated by the presence of soil. Similar transformation occurred when light was irradiated. At pH values higher than 4.0, the transformed products from L-DOPA consisted of humic substances-like heterogeneous components, whereas specific components with low molecular weight were included when L-DOPA was transformed at a pH value of 9.7 or higher. The rate of L-DOPA transformation mediated by soils at constant equilibrium pH value was in the following order: alluvial soil > calcareous soil > volcanic ash soil. The plant growth inhibitory activity of L-DOPA was extremely weakened when L-DOPA was adsorbed on or transformed (polymerized) by soils. The adsorption and transformation reactions were caused by the catechol structure (o-dihydroxybenzene moiety) of L-DOPA. Based on the reduction effect of soils on the plant growth inhibitory activity of L-DOPA (without pH-adjustment), calcareous soil ranked first, followed by volcanic ash soil, then alluvial soil. This was because the calcareous soil showed the highest soil pH value (7.8), and thereby the L-DOPA transformation reaction was accelerated. Therefore, in soils with high abilities of adsorption and transformation of L-DOPA such as calcareous soils, it was likely that the L-DOPA concentration in the soil solution decreased quickly by adsorption and transformation reactions and the allelopathic activity of L-DOPA was lost.