In dioecious species, production of fruits/seeds and pollen is spatially separated and knowing the sex of individual plants is not always feasible. In this study, we tested the feasibility of using the potential difference in pollen concentration in soil underneath the crowns of individual plants to identify the sex of plant overhead in a dioecious species. We hypothesized that the pollen concentration in soil beneath male plants would be significantly higher than beneath female plants because only males produce pollen and pollen deposition should accumulate in the soil underneath the male plants over time. We collected soil samples under both females and males of an insect-pollinated dioecious shrub, Aucuba japonica (Garryaceae). Pollen concentration under males was significantly higher than under females. Pollen concentrations beneath males were positively correlated with the number of male flower buds, potentially reflecting greater pollen production. Considering the small plant size and relatively low pollen production of A. japonica, the method may hold promise for studying other species. This method may also be useful for assessing long-term productivity of males. Answering any questions about dioecious species that requires knowing the sex of individuals will be facilitated by our time-insensitive, relatively low-cost method using soil pollen beneath individual plants.