No Payoff From High Soil Test Phosphorus: Save Fertilizer and Protect Water Quality

By Ehsan Ghane

Important takeaways 

• High soil test phosphorus (STP) fields lose more phosphorus per acre, even though they make up a small share of the landscape. 
• Most phosphorus loss still comes from fields with soil test phosphorus below 75 mg/kg, so conservation practices are important across all fields, not just the high-testing ones. 
• Adding phosphorus to high or very high soil test phosphorus soils doesn’t increase yield. There’s no agronomic or economic benefit for corn and soybean production. 
• Fertilizing when soil test phosphorus is in the low to medium range gives the best return on investment. 
• Lowering high soil test phosphorus through crop removal takes time. It can take up to 16 years to cut soil test phosphorus in half in high-testing soils. 
• Stick to the tri-state fertility guidelines to maintain crop productivity while reducing phosphorus loss to water. 

Lake Erie continues to experience summer algal blooms due to excess phosphorus. While phosphorus enters the lake from various sources, agriculture remains the dominant contributor. Recent research highlights the significant role of legacy phosphorus—phosphorus that has accumulated in soils over time—in fueling these blooms. If your soil already tests high in phosphorus, adding more won’t improve yields, but it can harm your bottom line and local water quality.