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Potash Corp. Director of Agronomy Robert Mullen discusses management techniques for phosphorus (P) and potassium (K) in soybean production. Mullen says it’s important to have a holistic plan that factors in all macronutrients.

Key Takeaways

• Understanding P and K Chemistry

o   What happens in soil?

• 99.9% of P and K in the soil is not able to be utilized
• Soil tests focus on Exchangeable K to measure adequate potassium
• Soil tests focus on Absorbed P to measure adequate phosphorus

o   How does soil testing work?

• Lab tests flood samples with ammonium to show an estimate of Exchangeable K
• Dissolved P increases with P loading of soil and has dramatic environmental consequences

• Rate Decisions

o   Translate soil test levels to fertilizer rates

• Depending on the soil test ppm, that determines the P/K recommendation
• 200 ppm of K in a soil test is the ideal rate to prevent yield loss
• 30-40 ppm of P in a soil test is the ideal rate to prevent yield loss

o   How do economic models impact rate decisions?

• Figure in potash cost with soybean prices and yield potential in order to figure the return $/acre
• Return on investment of fertilizer is a function of yield

• Manage Loss Pathways (Timing and Placement)

o   Managing particulate P is best mitigated by reduced-till or no-till

o   Dissolved P requires desorption to provide solution P

o   Rainfall is one of the biggest loss pathways

o   Chisel plowing, no-till and ridge-till impact how P is incorporated