In this study, soil frost depth during winter was controlled to enhance onion productivity via suppression of nitrogen leaching and improvements to physical soil properties in onion fields in the Okhotsk area of Hokkaido, Japan. In 2016 and 2017, soil frost depths under natural snow conditions measured <11 cm. We compared the conditions in controlled plots including soil frost depths of 30–40 cm with those in uncontrolled plots. The controlled plots contained higher amounts of residual inorganic nitrogen in the 0–40 cm soil layer after snowmelt and higher porosity (volume of gaseous phase at pF3.0) and saturated hydraulic conductivity in the 30–40 cm soil layer compared with the respective values in the uncontrolled plots. In addition, the average weight of onions in the controlled plot was higher than that in the uncontrolled plots, and the yield was ～10％ higher in the controlled plot. Thus, improvements in productivity were seemingly induced by the combined effects of increased nitrogen supply and improved soil physical properties, such as air permeability, water retention, and water permeability, in the main root zone (0–40 cm) of onion plants. Onion yields were increased when the soil frost depth in the study fields was 23–37 cm. Considering a possible error of several centimeters in soil frost depth control, we recommend a target frost depth of 30 cm to improve the productivity of onion fields. However, in fields with high nitrogen fertility, soil frost depth control may result in excessive nitrogen supply. In such cases, growth suppression due to salt injury and damage from diseases, such as dry and soft rot, may occur, leading to lower onion yield. Therefore, proper nitrogen management might be required in fields in which soil frost depth is controlled.