In recent years, Mongolia has witnessed an increase in not only wheat fields, which have been present for a long time, but also rapeseed fields. This has led to increasing concerns about soil degradation due to inappropriate cultivation. This study aims to determine the impacts of rapeseed production on soil water storage in Mongolia. The soil water content and matric potential were measured in wheat and rapeseed fields and adjacent steppe rangeland for five years, including crop production and fallow years, and the soil water storages in the fields were compared. The results demonstrated that the matric potential below the root zone in the rapeseed field and both rangelands was drier than the wilting point, whereas the potential in the wheat field was usually almost the same or wetter than this point. The comparison of the amount of soil water storage during the fallow year with that of the adjacent rangeland showed it to be 5–10% higher for the wheat field and almost equal for the rapeseed field. Field management must consider the fact that rapeseed fields use more water than is required by wheat fields and that less water is stored during fallow periods.