The allelopathic potential of a plant has been evaluated on the basis of twoindicators: specific activity, which is the specific concentration of theallelochemical to exert a half-maximum effect on a receiver plant (EC50), and total activity in a plant, which is the ratio ofthe concentration of an allelochemical in the producing plant to its EC50. In the present study, a new indicator, total activity ina soil, which takes into account the effects of a soil on the allelopathyactivity, is proposed because allelopathic activity is affected by thepresence of soils. The total activity in a soil was calculated bymultiplying the “total activity in a plant” with a “soil factor.” In thiscalculation, we assumed simplified cases for comparison, such that theallelopathic plant materials are evenly incorporated in the soils and theallelochemicals are released from the plant materials to the soils at aconstant rate. We conducted bioassay experiments in the presence and absenceof soils and cited some published data to calculate the specific activitiesand total activities in a plant and in a soil. The results indicated thatthe allelopathies of buckwheat caused by (+)-catechin, Leucaena leucocephala by L-mimosine, Xanthium occidentale by trans-cinnamic acid, and Brassica parachinensis by cis-cinnamicacid were not significant in a volcanic ash soil, an alluvial soil, and acalcareous soil, but the allelopathy of sweet vernalgrass caused by coumarinand Spiraea thunbergii by cis-cinnamoylglucosides was highly effective in those soils. The allelopathies of Juglans species caused by juglone plus jugloneprecursors and Mucuna pruriens by L-DOPA would dependhighly on the soil types. Although some limitations exist for this approach,the total activity approach would allow for a better quantitative estimationof the allelopathic potential of plant materials in soils.