Key Takeaways

• Planting early during soil-warming windows maximizes yield potential and return on investment in crop inputs.

• Minimum soil temperature thresholds for germination: corn and soybean are safe to plant at 50°F or greater soil temperature, while sorghum requires 55–60°F or greater. 

• Soil temperature above the thresholds during the first 24 hours after planting is critical for adequate water uptake (imbibition phase) to ensure good germination. After this imbibition phase, corn and soybean seeds can tolerate temperatures below 50°F without affecting germination as long as the seed is in proper soil moisture.

• Start planting corn and soybean when soil temperatures at 2 inches are above 50°F and the weather forecast shows 24 to 48 hours of rising temperatures or similar weather as of planting day.

• Adequate soil moisture is essential for rapid seed imbibition, making it even more important to time planting with both temperature and moisture conditions to support strong emergence.

Why do we want to plant early? The planting date management is not about when you plant your first field, but when you plant your last field. Planting early is a key factor in achieving high yields, especially in irrigated fields. Early planting is more critical for soybean than for corn. An eight-day advance in the Nebraska irrigated soybean 50% planting progress date was associated with a historic 4 bu/ac increase in irrigated soybean fields and a 2 bu/ac increase in rainfed fields. Delays in the Nebraska irrigated corn 50% planting progress date did not have much impact on corn yields. Indeed, there is a planting window between mid-April and mid-May in which corn yield does not seem to change much. 

If you have only one planter and always plant corn first, planting corn earlier can also shift soybean planting earlier and increase their yields while maintaining or slightly increasing corn yields. 

Another option is to plant soybean first, followed by corn, as many farmers are beginning to do. For dryland corn in the west-central region, some research has shown that planting early increases yields because the crop has a greater chance of rain during the critical period for yield determination, as rainfall declines consistently after May. Additionally, corn development is largely driven by temperature. Therefore, later planting leads to later harvest, and a higher probability of harvesting with high moisture and late in the fall. 

Extensive research shows that early planting can maximize yield potential and improve returns on crop input investments. Progressive, high-yielding farmers often plant under “cold conditions”. Exploring the potential advantages of adjusting the planting schedule in west-central Nebraska could offer new strategies to optimize yields and return on investments in the region. (See also, “Impact of Hybrid Selection, Planting Date and Seeding Rates on Dryland Corn in Western Nebraska”.)

Based on USDA Risk Management Agency (RMA) crop insurance standards, the earliest corn planting date was April 10 for corn and April 20-25 for soybean in west-central Nebraska, depending on the county. In addition to insurance coverage, proper soil temperature and moisture are important for good germination. Our data show that planting windows have already occurred in west-central Nebraska for corn and soybean during the last 10 days. Therefore, it is time to plant.

Traditionally, farmers in the west-central Nebraska region wait until the risk of severe frost has passed and the soil has warmed sufficiently to ensure fast germination. Average soil temperatures that support proper germination are 50°F for corn and soybean, and between 55-60 °F for sorghum. 

The key is successfully moving through the imbibition phase of the germination process with good temperature and moisture. The imbibition phase of the germination process is the rapid uptake of water by the seed, which typically occurs within 24 hours of planting. After the imbibition phase comes the osmotic phase. During the osmotic phase, water uptake slows, and seedlings in this phase are quite tolerant of soil temperatures as low as 35-40°F.

With this objective in mind, the Cropping Systems Lab at the West Central Research, Extension and Education Center (WCREEC) in North Platte installed soil temperature sensors in early April across the region (North Platte, Brule, Grant, and McCook) to track daily soil temperature changes at a depth of 2 inches. 

Each sensor was connected to a datalogger, providing online access to hourly soil temperature changes on a phone or laptop. See soil temperature trends by location in Figures 1 through 4. 

For other locations, you can check the daily 4-inch-deep soil temperature on the CropWatch soil temperature page, which will serve as a proxy for soil temperature for planting.

Temperature Records Interpretation

Overall, there have been between two and nine days, depending on the location, with soil temperatures at 2 inches at 50°F or more. Fields with soybean residue are consistently warmer than those with corn residue. In these windows, corn planting was possible and the WCREEC fields started to be planted. 

Although soybean could have also been planted during these planting windows above 50°F of soil temperature, the insurance does not begin until April 20 or 25, depending on the county. Therefore, this week, April 20 to April 24 has been a good week to get started planting soybean. However, planting corn and soybean after Thursday afternoon of April 23 this year may be at risk of chilling injury as minimum air temperatures are forecasted to be considerably below 50°F for at least four days starting on Saturday, with possibilities of rain/snow. Planting corn or soybean before Friday remains a viable option. Then, it will be advisable to check soil temperatures to see if they remain at 50°F over the weekend, as some lower air temperatures are forecast, and to check whether the weather forecast will be favorable after planting.

Soil temperature sensors in dryland systems in the McCook area indicate that soil temperatures have been above 50°F for about nine days. Therefore, planting corn in dryland systems has had the longest planting window in the area so far. Even during the past weekend, when temperatures dropped in the area, wheat residue areas remained warmer. One consideration is that some dryland systems may lack sufficient moisture for the seed to germinate due to the dry conditions. Nonetheless, on April 15, we planted a corn field on wheat stubble in the dryland farm of WCREEC, where seeds were placed at approximately 2.75 inches deep to get moisture. Figure 5 shows that these seeds have started to germinate, despite the slightly cold weather we had over the past weekend.

In Brule, there is a spike in soil temperature on April 9(blue line) in the observations, and since then, soil temperature in the corn residue has been high and more fluctuating. The field was strip-tilled that day. Based on the locations and observations above, the corn residue in Brule is the only line that represents strip-till conditions in the area. All other observations are no-till conditions with no residue removal or grazing.

Final Considerations

Minimum air temperature and frost forecasts are often used as a reference for starting to plant, rather than soil temperature at the seed-planting depth. This approach can delay planting and reduce yields. While aboveground freezing temperatures can occur even after planting, this does not directly affect the underground planted seeds. Seed emergence typically take 12-23 days for corn and 13-25 days for soybean when planted in late April to early May, based on our planting-date experiment in North Platte over the last two years at WCREEC. 

The most critical time for chilling injury is the first 24 hours after planting. During the remaining time until emergence, the seeds can tolerate temperatures below 50°F and exposure to aboveground frost. If you haven’t started yet, it’s time to plant. Check soil temperature, soil moisture and the weather forecast, and get the planter to the field. It looks like this week, until Thursday, it is good to plant corn and soybean. Don’t miss the opportunity to achieve higher yields and improving the return on investment of every other input used in the season.