Rethinking Nitrogen: What the Soil Really Tells Us - Lessons from Dr. Richard Mulvaney | Crop Cast Episode 42
In a deep-dive episode of Crop Cast, host Sean Nettleton sat down with Dr. Richard Mulvaney of the University of Illinois to challenge long-held assumptions about nitrogen (N) management. If you're still basing fertilizer decisions on yield goals alone, it's time to rethink how you're managing one of the most critical and expensive crop inputs.
The Nitrogen Paradox: What We “Know” Ain’t So
Dr. Mulvaney opened with a fitting quote from Will Rogers: “It’s not what we don’t know that gives us trouble, it’s what we know that ain’t so.” That sentiment perfectly captures the central problem with nitrogen recommendations today, many are built on flawed assumptions.
Key takeaway? The soil, not fertilizer, is the primary source of nitrogen for your crop. Years of isotopically labeled nitrogen studies (using N-15) consistently show that soil N contributes more to plant uptake than applied fertilizer, regardless of application rate.
The Microbial Engine of Soil Nitrogen
So how does soil deliver all this N? It’s all about mineralization, a microbial process where nitrogen is released from organic matter and made plant-available. The soil microbiome is essentially a fierce, competitive world of decomposition and nutrient cycling. Understanding and measuring this dynamic microbial activity is the key to smarter N management.
Why Yield-Based Recommendations Miss the Mark
One of the biggest myths Dr. Mulvaney tackled is the "1.2x yield goal" formula still used in many N recommendations. This method assumes:
- Two-thirds of crop N comes from fertilizer (wrong).
- Higher yields always require more fertilizer (not necessarily).
- Fertilizer efficiency is consistent across soils (definitely not true).
In reality, soils vary dramatically in their N-supplying capacity. Studies show that the fertilizer N needed for optimal yield is not strongly correlated with yield itself because different soils contribute varying amounts of N naturally. Poorer soils require more supplemental N, while healthier soils require less.
The Game-Changer: Illinois Soil Nitrogen Test (ISNT)
To bridge this knowledge gap, Dr. Mulvaney and his team developed the Illinois Soil Nitrogen Test, a tool designed to quantify the soil’s N-supplying power. Unlike organic matter content or other basic indicators, ISNT measures a specific organic N fraction (primarily amino sugar-N) that correlates with crop responsiveness to added N.
Here’s how ISNT helps:
- Distinguishes responsive vs. non-responsive sites.
- Supports variable rate N applications based on soil biology.
- Reduces over-fertilization while maintaining yield.
Calibrating ISNT: It's Not One-Size-Fits-All
Dr. Mulvaney emphasizes that interpreting the ISNT requires context. Soil pH, subsoil fertility, drainage, residue levels, and plant population all influence how much N a crop will need—even on soils testing high for ISNT.
For instance:
- Soils with low pH or poor drainage may still show a fertilizer response despite high ISNT levels.
- High-residue systems or high plant populations may require higher ISNT thresholds for the same response.
Putting ISNT to Work: Real-World Results
Dr. Mulvaney highlighted case studies from Crop Smith and growers using variable-rate N management with ISNT. In one field, N rates varied from 125 to 175 lbs/acre based on ISNT levels with higher yields often coming from lower N zones. On average, growers reduced N applications by 25–60 lbs/acre with no yield penalty.
Final Thoughts
Nitrogen is one of the most misunderstood, and misapplied, nutrients in modern agriculture. Dr. Mulvaney’s research reveals that trusting the soil, understanding microbial mineralization, and using better diagnostics like the ISNT can help growers save money, reduce environmental loss, and maintain high yields.
Bottom line: Stop guessing and start testing. The soil knows more than we think—it’s time we listened.