by Margaret Smith, PhD, Forage Agronomist & Matt Leavitt, Organic Lead/Agronomist
Good weather conditions this spring have encouraged questions from a number of organic producers: “If conditions remain dry and warmer than average, can I plant my organic soybeans before organic corn?”
The short answer is ‘probably not’, unless you are very confident in your organic weed control, and are willing to risk a reduced stand..
Soybean Planting Date and Organic Weed Control
The number one issue that reduces soybean yields in organic production systems is weed competition. Soybeans are notoriously difficult to keep clean, even with an optimum stand and perfect planting conditions.
Planting soybeans later in the season (after May 15 in southern Minnesota) has the advantage of offering windows for full width, pre-plant weed control passes. These early passes catch the weeds when they’re small and allow control of multiple flushes of seedlings. Cold, wet soil conditions after planting prevents tractor traffic and early weed control passes and can render most weed control equipment ineffective (other than a flame weeder). Anything that delays soybean emergence will put the beans at a growth disadvantage vs. surrounding weeds.
Soil Temperatures for Optimum Organic Soybean Emergence
Soybeans are a warm-season legume and need higher soil temperatures to germinate than corn. Soybeans planted at 50°F soil temperatures will emerge, but extremely slowly. Soybeans germinate and emerge fairly well at soil temperatures of 55-60°F but even more rapidly at 60-65°F.
Soybean take more days at the same soil and air temperature to emerge than corn does. The University of Wisconsin documented 90% soybean emergence after 155 Growing Degree Days (GDD-Base 50) accumulation compared to 100-120 GDD for corn.
The longer soybean seed remains in the ground, the greater the chance for poor stands due to fungal diseases and/or seed feeding insects.
Cold Impacts Soybean Seed
Soybeans are a vigorous plant but the seed is sensitive to cold, wet weather.
After planting, soybean seeds start taking up water almost immediately. Taking up cold water, particularly <45°F, can cause chilling injury to the seed. The critical period is the 12- to 24-hour range right after planting. Planting into cold soils or if a planted field receives a cold rain right after planting are conditions for poor seed performance. Symptoms of seed chilling include slow and uneven emergence and dead tissue on the outside of the cotyledons. A cold rain or drop in temperatures two or more days after planting causes much less damage to the seed but will still slow emergence.
Optimum Organic Soybean Planting Dates
The earliest recommended conventional soybean planting date for Minnesota and northern Iowa is May 1, but many experienced organic growers plant soybeans well into the later part of May and even early June. Data from conventional experiments demonstrate that soybean yields are not reduced much by planting later into May. University of Minnesota data indicates that you have the potential to capture 94 percent of total yield potential when planting on May 20.
When and How to Plant Soybeans, University of Minnesota
The bottom line: Soil temperature and weather forecasts are better guides than calendar dates for soybean planting, but you won’t sacrifice much yield by later planting.
Does Soybean Planting Date Affect Final Population?
Soybeans are less sensitive to plant population loss than corn, as plants can branch extensively to compensate for lost stand. University of Minnesota research illustrates that you can capture optimum yield potential with only a 75% stand (down to 100,000 to 118,000 plants/A), which is well below the 160,000 to 180,000 seeds/acre that most organic farmers routinely plant.
All organic soybean growers expect to lose some stand from early season weed passes and cultivation.
Soybean Damage and Replanting: How plant population affects soybean yield, University of Minnesota
Final Answer: Should You Plant Organic Soybeans Before Organic Corn?
No, not unless you have a good reason – and bragging rights aren’t worth the risk. If you are experimenting, select a field with low weed pressure and one that has a favorable soil type for early season drying. You also need to be confident in your ability to control weeds.
Organic Soybean Planting Best Practices
We recommend these best practices for planting organic soybeans:
- Plant an appropriate maturity for your region.
- Plant soybeans at 1.5’’ deep into a moderately firmed seedbed with warm soil (>60°F).
- Plant 160,000-185,000 seeds/A.
- In southern Minnesota, plant after May 7 at the absolute earliest; after May 20 is likely better in most years.
- Ideally, use soil temperatures and weather forecasts to home in on the best time to plant.
- Avoid planting into saturated soils or when cold, wet weather is in the forecast before and after planting.
What If You Don’t Have a Soil Thermometer?
You can find soil thermometers for about $30 at your local garden center or online, but you also can use quick-read, kitchen thermometers to measure soil temperatures. Check the scale. In general, meat thermometers, due to their calibration for relatively high temperatures, won’t work for this application. Leave the thermometer in the soil for at least three minutes, or until the temperature reading has stabilized.
Meat thermometers. Scale not adequate to capture 40°F to 60°F temperatures.
Quick-read kitchen thermometer. Will work in the absence of a soil thermometer.
- Chilling injury in Corn and Soybeans, University of Nebraska, Lincoln
- Dynamics of Imbibition by Soybean Embryos
- Effect of Temperature on Soybean Germination
- How Cold Stress Affects Soybeans, University of Nebraska, Lincoln
- Identifying Optimal Soybean Planting Dates across the U.S.
- Just the Facts, Jack: Soybean Planting Date, Seeding Rate and Seed Treatment Recommendations, University of Wisconsin-Madison
- Mitigating the Risks of Very Early Soybean Planting, Iowa State University
- Predicting Soybean Emergence, University of Wisconsin-Madison
- Soybean Planting Date, Iowa State University
- Soybean Damage and Replanting: How plant population affects soybean yield, University of Minnesota
- Quantifying and Validating Soybean Seed Emergence Model as a Function of Temperature
- When and How to Plant Soybeans, University of Minnesota
- Variable Germination and Emergence in Soybean: Which Seeds Are Still Viable? University of Wisconsin-Madison