Eriocaulon heleocharioides is a Japanese annual aquatic herb and extinct in the wild. Decrease of established population size in the reintroduction site was suggested after reintroduction it in the wild in 2009. Original habitat of E. heleocharioides is located in wetland area of a reservoir, soil condition probably changed by years.
    The purposes of this study are, to examine (1) yearly change of E. heleocharioides population and soil conditions in reintroduction site, (2) effect of soil conditions on growth and reproduction of E. heleocharioides using cultivation experiment.
    Field investigation for reintroduced population of the species and soil sampling were conducted from 2011 to 2017. To examine effect of soils, soils from various sampling sites in habitat were used for cultivation experiment.
    Population size was obviously declined from 2011 in the reintroduction. Significant changes of soil particle fractions were found in particle sizes of 2 mm ~ 0.5 mm and < 0.25 mm from 2011 to 2015. The positive effects of amounts of smaller particle size and soil nitrogen on growth and reproduction of E. heleocharioides were shown by cultivation experiment. Limiting factors of growth and reproduction of the species seems to be not only soil particle size but also soil nutrients including nitrogen.
    Furthermore, soils with rich in smaller particle size and nitrogen were distributed relatively deeper site. Improvement of habitats using the soils form deeper site may be effective for successful reintroduction of E. heleocharioides.