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.
- 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