The Secret to Bigger Soybean Yields

Unlocking the Power of Planting Date and Starter Fertilizer

Getting the basics right is often the key to unlocking higher yields.

For soybean growers, two fundamental decisions stand out at the start of every season: when to plant and whether to use starter fertilizer. While these might seem like simple agronomic choices, modern research reveals they are powerful levers that can significantly influence your final yield.

The Foundation: Why Timing is Everything

The timing of soybean planting sets the stage for the entire growing season. The old practice of planting soybeans only after corn is fully in the ground has been radically rethought over the past decade, as research consistently demonstrates the yield advantage of earlier planting 8 .

Yield Impact of Delayed Planting

Research from Michigan estimates a 7% yield reduction when planting occurs on May 31 compared to May 1, with the majority of that decline happening after mid-May 6 .

Soybean Planting Date Impact on Yield
Planting Period Yield Potential Key Characteristics Primary Risks
Mid-April to Mid-May Highest Maximizes light interception, longer seed fill period Late spring frost, cool soil diseases
Late May to Early June Declining 0.35-0.51 bu/acre/day reduction after May 10 8 Compression of reproductive period, early fall frost
Mid-June & Later Lowest Requires higher seeding rates 6 Early frost, incomplete maturity
Daily Yield Reduction After May 10

Data shows yield reductions of 0.35 to 0.51 bushels per acre per day when planting is delayed beyond May 10 8 .

The Planting Date Paradox

Contrary to what one might assume, the very earliest planted soybeans aren't always the tallest. Research from North Carolina has revealed an unexpected quadratic relationship between planting date and plant height 3 .

Optimal Height Period

The tallest soybeans consistently emerged from mid-April to mid-May plantings, while soybeans planted in mid-March to early April were often shorter 3 .

Height-Yield Relationship

Research has established a positive correlation between plant height and yield up to about 36 inches, after which the relationship flattens 3 .

Starter Fertilizer: Separating Fact from Fiction

The question of whether soybeans benefit from starter fertilizer—particularly nitrogen—has been debated for decades. Soybeans have high nitrogen requirements, needing approximately 4.5 pounds of N per bushel of yield, with about 3.5 pounds of that removed with the grain 2 .

Nitrogen Requirements

An 80-bushel crop therefore needs to accumulate about 360 pounds of N per acre 2 .

360 lbs
Nitrogen per acre

Natural Nitrogen Sources

Historically, soybeans meet these substantial needs through a combination of:

Biological Nitrogen Fixation

By symbiotic bacteria in root nodules (50-60% of total N) 2

Mineralization

Of nitrogen from soil organic matter (the remainder) 2

What Multi-Year Research Reveals

A comprehensive multi-year study conducted across Illinois from 2014-2017 provides crucial insights into soybean response to fertilizer nitrogen 2 .

Soybean Yield Response to Nitrogen Fertilizer Across Environments
Location & Soil Type Yield Response Pattern Most Effective Application Timing Key Takeaway
Chillicothe: Coarse-textured, Irrigated, Lower OM Strong positive response (35-38% increase) 2 Planting Responsive environment due to lower natural N supply
Brownstown: Claypan Soil No significant increase 2 Not applicable Even with yield near 61 bu/acre, no benefit to added N
Urbana/Monmouth: Deep Prairie Soils, Higher OM Small, inconsistent response 2 Multiple applications (modest benefit) High natural N supply limits response to fertilizer
Nitrogen Response by Soil Type

An In-Depth Look: The Illinois Nitrogen Fertilization Study

To understand the real-world interaction between planting date and starter fertilizer, we can examine the methodology and results from the multi-year Illinois study that provided the data above 2 .

Methodology: A Step-by-Step Approach

The research team established nine field trials between 2014-2017 across key soybean-growing regions in Illinois 2 .

Site Selection

Locations included university research farms near Urbana, Monmouth, and Brownstown, plus a farmer's field near Chillicothe with pivot irrigation 2 . This provided a range of soil types from coarse-textured soils with <2% organic matter to productive fine-textured soils with >3.5% organic matter 2 .

Treatment Structure

Six nitrogen fertilizer treatments were evaluated:

  • Untreated control (no N fertilizer)
  • Single applications of inhibitor-treated urea (100 lb N/acre) surface-applied at planting
  • Single applications at R1 growth stage
  • Single applications at R3 growth stage
  • Single applications at R5 growth stage
  • A combined treatment with all four application timings (total 400 lb N/acre) 2
Management Practices

Soybeans were planted in May with regionally adapted varieties and managed using standard agronomic practices. All plots followed corn, with appropriate weed and disease control. Fungicide seed treatment was used where needed 2 .

Results and Analysis: Key Findings

Brownstown Results

Despite challenging conditions, the untreated control averaged 61.1 bu/acre. Applications of 100 lb N/acre at any single timing did not produce statistically higher yields compared to the control 2 .

Chillicothe Results

Applying N at planting increased yields by 22.4 bu/acre in 2015 and 19.7 bu/acre in 2016 compared to the control 2 . The four-application treatment provided no additional yield benefit.

Urbana/Monmouth Results

Yield responses were minimal. Only one of five site-years showed a significant response, which was actually a yield decrease of 5.2 bu/acre from planting-time N 2 .

Economic Considerations for Starter Fertilizer Use in Soybeans
Factor Consideration Practical Implication
Soil Type Responsive on coarse-textured, low-OM soils; less responsive on high-OM soils 2 Reserve N applications for specific soil conditions
Yield Level Negative correlation between yield potential and N response 2 Highest-yielding fields least likely to benefit
Application Cost Multiple applications rarely economical 2 Focus on single well-timed applications if used
Commodity Price Lower soybean prices reduce economic viability 2 Strict ROI calculation essential

Practical Applications for Growers

Translating this research into practical management requires understanding both the opportunities and limitations:

Planting Date Decisions

The evidence strongly supports moving toward earlier planting when field conditions allow. The yield benefit is well-established, though risks can be mitigated using several key practices:

  • Use high-quality seed with optimum vigor and germination to enhance stand establishment 8
  • Implement comprehensive seed treatment packages to protect against early-season diseases and insects 8
  • Select appropriate maturity groups for your environment and planting date 8
Starter Fertilizer Decisions

Based on current research, a more nuanced approach is warranted:

  • Most productive soils are unlikely to benefit economically from nitrogen fertilizer applications 2
  • Lighter-textured, irrigated soils with lower organic matter may show responses to planting-time nitrogen applications 2
  • Foliar fertilizer applications at reproductive stages generally provide no economic benefit when no nutrient deficiency symptoms are present 5
Important Note

When nutrient deficiencies are visually confirmed, corrective applications may be warranted, but deficiency symptoms don't always translate to yield loss 5 .

Conclusion: Back to the Basics

The research on planting date and starter fertilizer reveals a compelling story: sometimes the most powerful management practices are the fundamental ones.

While inputs like foliar fertilizers and in-season nitrogen applications can seem technologically advanced, the evidence suggests they rarely provide consistent economic returns.

The real yield gains come from optimizing the basic agronomic factors—planting soybeans early in good soil conditions, selecting adapted varieties, and ensuring soil fertility fundamentals are addressed. As research continues to refine our understanding of these interactions, the principle remains clear: success in soybean production comes not from silver bullets, but from consistently executing the fundamentals based on sound scientific evidence.

For growers looking to maximize yields in the coming season, the message is clear: focus first on getting your planting timing right, assess your specific soil conditions, and let science—rather than tradition or anecdote—guide your fertilizer decisions.

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