Abstract: We carried out a field soil moisture control experiment in central Jilin Province in 2017 and 2018 to explore the effects of drought on seedling emergence speed, seedling rate and seedling growth during spring sowing to seedling period, and the spring drought index, with the final aim to guide the spring drought assessment and drought control irrigation. Soil moisture was controlled by rainproof sheds and artificial water spraying. There were seven soil moisture levels and one natural rain-fed treatment. We measured soil moisture, emergence date, emergence ratio, plant height, leaf age and biomass. The effects of soil moisture on emergence date, emergence ratio and seedling growth potential were analyzed by regression method, and the evaluation index of soybean spring drought was established accordingly. The results showed that soybean emergence date, emergence rate, plant height, leaf age and aboveground biomass were closely related to soil moisture in the sowing layer. Under the range of experiment treatment (soil moisture was less than 24%), with every 1% decrease of soil moisture, the emergence date was delayed for 1-2 d, the emergence rate was decreased by 1.6%, and the relative leaf age, seedling height and aboveground biomass were decreased by 6.1%, 5.4% and 10.2%, respectively. Suitable soil moisture for soybean emergence ranged between 20% and 23%, and for seedling growth ranged between 21% and 24%. The indicators of light, moderate, severe and extreme drought in spring were as follows: soil humidity in 0-10 cm was 16%-17%, 14%-15%, 12%-13% and <12%, respectively; in 0-20 cm was 17%-18%, 15%-16%, 13%-14% and <13%, respectively.

Key words: spring soybean, soil water stress, seedling growth situation, assessment model, spring drought indicator.