When sufficient soil moisture is available, soybean seeds take up water within the first hours after planting (imbibition period). Initial water uptake by the seed is usually completed within the first 24 to 30 hours. During this period the soil temperatures, where the seed is placed, should not be cooler than about 45°F. Therefore it is important to avoid planting soybean seed if cold rain or cold temperatures are expected within 24 hours of planting.
After the initial imbibition period the seeds take up water by osmosis. Seeds and seedlings in this phase are tolerant of soil temperature as low as 35-40°F. Temperatures below 50°F will lengthen the period from imbibition to emergence. Seed laying in cool soil for extended periods have an increased risk for soil-borne diseases, especially when the soil is also wet. Fungicide seed treatments can be used to prevent diseases, usually resulting in a higher percent established plants, compared with no treatment. During cold conditions, seed growth and development is delayed until soil temperatures warm up.
A large North Dakota grower study, with over 1,000 reported observations, indicates that there is a reduced yield potential of about 0.3 bushel per acre per day, when planting is delayed between May 1 and May 31. NDSU research indicates an average yield increase of 6.5% with soybean planted during the first 10 days of May or earlier versus planting two weeks later (21 trials). This information about early planting should be interpreted with some caution. The recommendation is that the field soil condition is adequate for planting, the soil temperatures are conducive (around 50°F), the weather forecast is favorable, and the risk of frost after germination is low. It typically takes 10 to 15 days for early planted soybean to germinate (depends on the moisture availability and temperature), so planting can be done before the last predicted frost. Knowing the long-term average last frost day in the spring and the average date of first frost in the fall can help in managing risk. The cost-benefit of potential higher yield for early planting must be weighed against the risk for potential frost damage.