LINCOLN, Neb. — As of April 12, 2023, the Nebraska Drought Monitor shows 98% of Nebraska in moderate to exceptional drought (D1-D4). With planting being critical for everything else that happens during the growing season, the dry conditions have led to a variety of questions this planting season. This article will work through the questions we’ve received.
This is perhaps the most critical component to planting because soil conditions set the stage for the decisions we make regarding planter adjustments, which crop is planted and planting depth. The winter allowed for freeze/thaw and the mellowing of soils in some areas. One difference going into the 2023 growing season compared to the 2022 growing season is that in 2022, much of the state had more subsoil moisture from some fall rains. In 2023, we have had several small precipitation events since the first of the year, but little to fully recharge the subsoil moisture. Thus, the surface soil should be better to plant into, depending on how winds impact those conditions. When planting into drier conditions, increased down pressure is most likely necessary; however, be careful of causing any sidewall compaction.
Soil Moisture, Temperature and Planting Depth
Corn needs to absorb 35% of its weight in moisture to complete the imbibition (water-uptake) phase of germination. When adequate soil moisture is available, this typically occurs within 48 hours. Soybean needs to absorb 50% of its weight in moisture to complete the imbibition process. When adequate soil moisture is available, recent research has shown imbibition can occur anywhere from eight to 24 hours. Once imbibition has occurred, soybean seeds enter an osmotic phase and are quite tolerant of soil temps as low as 35-40°F, although extended low soil temperatures can be expected to lengthen the germination to emergence timeframe.
In general, we’d recommend planting into soil temperatures as close to 50°F as possible and when the forecast is calling for warm temperatures the next few days, as that would also help increase the soil temperature. Avoid planting prior to a cold snap (cold rain/snow) within eight to 24 hours for soybean and 48 hours for corn. You can take soil temperatures using a thermometer or you can view soil temperatures on CropWatch. Also be aware that soil moisture can help buffer soil temperatures, reducing larger swings.
How does soil moisture impact planting depth? Research has consistently shown the need for corn to be planted at least 1.5 to two inches deep. UNL showed best soybean yields were obtained by planting at a 1.75-inch depth. So, the short answer — aiming for close to two inches is a good consideration for both corn and soybean. Planting shallower than 1.5 inches leads to more impacts on root development in corn, such as rootless corn syndrome. Proper seeding depth for soybean helps keep that seed in buffered soil moisture and temperature when planted early. It also aids that seedling from emerging too early.
Ensuring a good soil-seed contact is key to emergence success. When seeds fall into an air pocket or rows are not well closed, the imbibition phase can be interrupted or never occur. Digging up some seeds when starting planting to check for proper soil-seed contact and moisture around the seed and regulating the planter accordingly is advised.
Uniform emergence is more important for corn than soybean. Because of this, Rob Nielsen, emeritus extension agronomist at Purdue shares, “When seedbed conditions are dry, make sure you choose a seeding depth that ensures uniformly adequate soil moisture for (corn) germination and emergence. Even though a 1.5- to two-inch seeding depth is a good choice for many conditions, do not hesitate to increase seeding depth to 2.5 or three inches if that is the depth where uniform soil moisture is located.” When it comes to soybean, we wouldn’t recommend planting much deeper than 2.5 inches.
For corn seeding rates, it’s best to check with your local seed dealer as all our research shows that optimal corn population varies by hybrid. For those concerned about moisture in non-irrigated or limited-irrigated situations, one may consider using a hybrid with higher flex at a reduced seeding rate.
For soybean, our recommendation after 16 years of on-farm research studies (2006-2022) in heavier textured soils and 30-inch rows continues to be: plant 120,000 seeds/acre, aim for a final plant stand of 100,000 plants/acre and you’ll save money without reducing yields. If that’s too scary, try reducing your rate to 140,000 seeds/acre or try testing it for yourself via on-farm research!
PPO inhibitors are an effective chemistry often used for pre-emergence application in soybean. The different PPO-inhibiting active ingredients, products and timing restrictions can be reviewed in this article. Soybean injury can occur in situations where the seed/germinating seed comes in contact with the PPO-inhibiting herbicide and via water splash onto the emerged soybean hypocotyl and cotyledons.
Some considerations to reduce injury to soybean include:
- Do NOT apply a PPO inhibitor herbicide if the seed vee is not closed, the soil is cracking along the seed trench or the soybean is beginning to emerge or has already emerged. Another option if one is concerned about PPO inhibitor injury is to consider using a Group 15 herbicide (such as acetochlor, pyroxasulfone, etc.) for residual in a post-emergence application once the soybean has emerged. Remember that residual herbicides applied post-emergence do not have foliar activity, so they will not control emerged weeds but would prevent the emergence of new weeds.
- Do not plant soybean shallow (less than 1.5 inches), particularly if the soil at seeding depth is dry. If a PPO inhibitor is applied in these conditions, it can allow the soybean to imbibe water with the herbicide in it, particularly if a rain and/or irrigation event occurs after application.
- If soybean is planted into moisture, herbicide can be applied within a few days and watered in as long as the seed vee is closed and no soil cracking of the trench is occurring.
Irrigating Prior to Crop Planting
In general, we would only suggest watering before planting if the planter needs higher soil moisture levels to work well. So, if the soil is too hard, too powdery or cloddy, it may be worth running the pivot. Another situation to consider pre-watering is if greater than 180 lb/ac anhydrous ammonia was applied in a strip with less than two inches of moisture received since application to help reduce ammonia burn to the corn. Otherwise, our recommendation is to run the pivot after you plant if needed.
Irrigation Considerations Early in the Year
Running center pivots early in the season do have some extra challenges. First, keep in mind cold nights. The usual recommendation is not to run a pivot when temperatures are below 40 degrees. Last year, several pivots were operated below 40 degrees without problem, but keep in mind with low dewpoints the pivot can ice up when the actual air temperature is well above 32. So, if you do choose to run in these conditions, keep a close eye out for ice buildup.
Secondly, bare, powdery soils will seal over very easily from rain or irrigation, so keep an eye out for runoff problems even with fairly low application amounts. And thirdly, make sure if you do irrigate that you put on enough to get water down to the moist soil below. This is particularly a problem with tillage or where fertilizer knives have been used and dried the soil out.
Irrigating for Herbicide Activation
It’s important that herbicides are activated with 0.5- to 0.75-inch of rainfall or irrigation, preferentially between five to seven days after herbicide application. If moisture received is less than this amount, some herbicide products have the potential to remain on the soil for up to 14 days without being fully activated. We will have to see how the high winds blowing soil and removing soil particles containing herbicide impact future weed control.
Irrigating to Reduce Urea Loss
For those who applied dry or liquid urea on the soil surface, particularly without the use of an inhibitor, irrigation of 0.5-inch can help with incorporating the urea into the soil and minimize urea loss. If irrigation is not available, an inhibitor was not used and no rainfall has been received within seven days, monitor the corn crop to determine if nitrogen deficiency occurs due to nitrogen loss. The losses can occur early and be significant if there is high surface crop residue, high soil temperature (>70°F), moist soil, high soil pH (>7.0), or sandier soils while the losses can be lower with dry soil, cooler soil temperature (<70°F), low soil pH (<7.0), bare soil or clayey soils.
Irrigating in Strip-till
Regardless of whether anhydrous was applied in the fall or spring, and regardless of amount applied, ammonia burn may occur due to the dry conditions. Sometimes ammonia burn is seen early with seed germination and emergence. Most often, it is seen when roots get to four to eight inches long and hit the ammonia band. Irrigation can help alleviate the impacts of the ammonia injury.
Irrigating and Cold Water Impacts
John Mick, Pioneer agronomist, shared that water from irrigation wells in the southern part of the state often is around 50-53°F, with it slightly less in temperature as one moves north in the state. These temperatures are not a problem to be concerned with regarding any negative impacts to seeds imbibing water.
Each field situation will vary and we hope these considerations will help as you begin or continue this year’s planting season!
Jhala, Amit. April 22, 2020. Timing is Critical for Applying PPO-Inhibiting Pre-emergence Residual Herbicides in Soybean. https://cropwatch.unl.edu/2020/timing-critical-applying-ppo-inhibiting-pre-emergence-residual-herbicides-soybean
Mick, John. April 15, 2022. Pioneer Agronomy Season Update. https://drive.google.com/file/d/1QE7PpVDIe0HTbwRAuyrpH7E8JhHjN6Y1/view
Nielsen, R.L. (Bob). April 2020. Requirements for Uniform Germination and Emergence of Corn. https://www.agry.purdue.edu/ext/corn/news/timeless/GermEmergReq.html
Rees, Jenny, Roger Elmore, Jim Specht, Megan Taylor. April 22, 2020. Corn and Soybean Planting Considerations for 2020. https://cropwatch.unl.edu/2020/corn-and-soybean-planting-considerations-2020
Rees, Jenny and Jim Specht. April 9, 2020. Understanding the Soybean Germination Process for Early Planted Soybean Decisions. https://cropwatch.unl.edu/2020/understanding-soybean-germination-process-early-planted-soybean-decisions
Rees, Jenny and Jim Specht. April 14, 2021. Demonstrating Critical Imbibitional Time for Soybean. https://cropwatch.unl.edu/2021/demonstrating-critical-imbibitional-time-soybean
Specht, Jim, Jenny Rees, Keith Glewen, Patricio Grassini. April 30, 2014. Soybean Planting Depth: Consider Planting Deeper. https://cropwatch.unl.edu/soybean-planting-depth-consider-planting-deeper
— Jenny Rees, Extension Educator; Steve Melvin, Extension Educator Irrigated Cropping Systems; Amit Jhala, Extension Weed Management Specialist; Javed Iqbal, Extension Nutrient Management and Water Quality Specialist; Nicolás Cafaro La Menza, Cropping Systems Specialist
University of Nebraska-Lincoln