Weather this past weekend had people thinking about planting corn and soybean. I’ve already heard a rumbling or two about planting soybean during the next warm up. As spring planting approaches, soil temperature becomes one of the most important factors influencing successful corn and soybean planting. While it’s tempting to roll into the field the moment soils look fit, making decisions based on both soil temperature and soil moisture can make the difference between a strong stand and a season-long struggle.
Why Soil Temperatures Matter
For corn and soybean germination, early-season growth and seedling vigor is influenced dramatically by soil temperature. Germination is optimal when soil temperature rises above 50°F. I encourage farmers not to look at soil temperature alone, but to also consider the 3 to 7 day forecast having a warming trend to avoid chilling injury. From late March through mid-April there can be rapid air and soil temperature swings. Residue cover, soil color, and drainage all affect daily soil temperature fluctuations.
Where to Find Current Soil Temperatures in Iowa
ISU Soil Temperature Maps (4-inch depth): The Iowa Environmental Mesonet (IEM) provides daily soil temperature analysis maps using ISU Soil Moisture Network observations combined with bias-corrected NWS forecast model analysis. Trends can be viewed for the past 3 days as well as forecasted into the future using the GFS forecast model outputs.
ISU Soil Moisture Network (live station data): The ISU Soil Moisture Network provides real-time observations at four depths, including dedicated soil temperature plots for the 4-inch depth. This data comes from weather stations that are fully instrumented with air temperature, solar radiation, precipitation, humidity, soil moisture, soil temperature, wind speed, and wind direction.
Key Factors to Watch Before Planting
Soil temperature trend.
The current temperature is important, but it’s also important to consider how the forecast will influence future soil temperatures. We want to plant into warming soils, not cooling soils. Even if the soil reaches 50°F once, a cold front can reverse progress quickly. Furthermore, cold rain shortly after plating increases the risks of imbibitional chilling, cold injury, delayed emergence, and early season seedling diseases. Imbibitional chilling occurs in the 12 to 24 hours right after planting when the seed absorbs water, whereas cold injury occurs after water absorption until emergence. Prioritize planting ahead of warmer, stable patterns.
Soil moisture matters too.
Avoid “mudding in” seeds as this can lead to sidewall compaction, which can be problematic for root development. Check for proper crumble, soil should fracture, not smear. And avoid fields that are only dry on the surface but tacky underneath to avoid creating compaction layers.
Residue levels and soil texture can influence soil planting conditions.
Heavy, high residue fields warm more slowly because the residue act like an insulating blanket. No-till or cover crop systems may require waiting longer for soil temperatures to stabilize. While the extra residue with cover crops can keep soil temperatures cooler, cover crops can help dry up fields as the cover crops take up water from the soil. Cover crop fields may also be ‘fit’ for field traffic earlier because of stabilizing root growth and additional residue on the soil surface.
Black soils without residue or cover crop growth typically warm up fastest. Light-textured soils warm up and dry out quickly, whereas heavy, high organic matter, or poorly drained soils tend to lag behind and remain cool and wet longer.
Planting date versus soil conditions
Decades of research at Iowa State University and other Midwestern universities consistently show that planting into the right soil conditions is more important than rushing to plant early. Waiting for soils to reach appropriate temperatures and avoiding wet planting conditions almost always pays dividends in emergence uniformity and stand quality. Yes, planting dates should be part of the consideration, however, not the sole determining factor.
Early planting should not be prior to April 10 statewide for corn. For soybean, the early planting date is April 10 for the southern third of Iowa and April 15 for the northern two thirds of Iowa. This is the USDA RMA early planting date and takes into consideration risk late spring frosts and freezes.
Ultimately, it is critically important for soybean and corn to be planted before May 15 to May 20. Beyond mid-May, yield losses become more significant. Yield penalties for late planting corn is greater than that of late planting for soybean. Therefore, if number of acres and days suitable for planting do not add up to allow planting completion by mid-May, a priority should be given to getting corn planted.
Take Home Message
Monitoring soil temperatures is easier than ever with high-quality, real-time resources from the Iowa Environmental Mesonet. As we enter another planting season, staying flexible and responsive to soil temperature and moisture conditions will help set the stage for strong, uniform stands, and higher yield potential.
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- Associate Professor
- Extension Cropping Systems Specialist