Supersize Me
Managing P and K the American Way
Summer Agronomy Conference 2025
Key Takeaways
- Moist vs. Dry Soil Testing: Drying soil samples can artificially inflate or deflate potassium levels depending on clay content, leading to misleading recommendations.
- Critical Soil Test Levels: Across states, the critical P level is consistently between 15–25 ppm; above 40 ppm, phosphorus loss risk increases.
- Economic Impact: Switching from a standard recommendation to a precision-based approach saved up to $88/A in some examples—over $8,000 per field.
- Buffer Capacity: Different soils require different amounts of fertilizer to change soil test levels, yet most states assume uniform buffer capacity.
- ROI vs. Yield Response: Even with a 10% chance of yield increase, applying fertilizer can be economically justified—especially when crop prices are high.
What to Expect
Tune it as Brad Joern, the lead soil scientist and research agronomist for Radicle Agronomics, challenges farmers and agronomists to rethink how they manage phosphorus (P) and potassium (K) in their fertility programs. With decades of experience in soil fertility and nutrient management, Brad dives deep into the science behind fertilizer recommendations, soil testing variability, and the economic implications of different management strategies.
As a farmer and agronomist, you must take ownership of your fertilizer dollar. Too often, growers follow blanket recommendations without tailoring them to their specific soil conditions, crop needs, or management style. Brad encourages a shift toward precision fertility - targeting soil test levels that optimize return on investment (ROI) rather than blindly following buildup and maintenance approaches.
Through informative charts and real-world examples, Brad explains how soil test levels, sampling methods, and drying techniques can dramatically affect fertilizer recommendations. See how his idea of “field mice” - skinny, fit, and fat mice representing low, optimal, and excessive nutrient levels – can help you think of your own soil fertility status.
Insights
- Soil Testing Matters: Moist soil samples provide more accurate potassium readings than dried samples due to clay mineral interactions.
- Field Variability is Real: Uniform fertilizer rates across variable fields lead to inefficiencies and wasted dollars.
- Precision Fertility Pays: Targeting specific soil test levels based on field conditions can save thousands per field annually.
- Recommendation Systems Vary: States use different models—buildup & maintenance, sufficiency, or hybrid—each with unique cost and yield implications.
- Nutrient Banking is Risky: Over-fertilizing with the hope of future benefit often leads to locked-up nutrients and poor ROI.
- Crop Response Trials Are Key: On-farm trials help determine the true value of fertilizer applications and guide smarter decisions.
