High Soil Phosphorus Levels Offer No Yield Gain but Increase Water Risk

By Andi Anderson

Recent research is reinforcing the need for balanced phosphorus management in crop production. High soil test phosphorus (STP) fields, although only a small share of the agricultural landscape, lose more phosphorus per acre than lower-testing fields.

This loss can contribute to water quality problems such as algal blooms in Lake Erie, where agriculture remains the leading phosphorus source.

A 2025 study in the Western Lake Erie Basin found that fields testing above 75 parts per million for phosphorus contributed up to 34% of dissolved reactive phosphorus loads while representing just 10% of the land.

These high-STP fields are not the main source of phosphorus loss overall, but their per-acre losses are significantly higher.

The process of lowering phosphorus levels, known as soil test phosphorus drawdown, is slow. Research indicates it can take up to 16 years to reduce high phosphorus levels by half through crop removal without additional phosphorus applications.

From an economic standpoint, another 2025 study found no yield or profit advantage in maintaining high to very high STP levels for corn and soybeans. Instead, the best return on investment comes from fertilizing when STP is in the low to medium range.

Maintaining phosphorus at or slightly above the critical level supports crop productivity while reducing the risk of nutrient runoff.

Experts recommend following the Tri-State Fertilizer Guidelines, which balance yield goals with environmental protection. For high-testing fields, the focus should be on gradual drawdown and avoiding unnecessary phosphorus applications.

For low to medium testing fields, applying fertilizer according to crop needs will maintain yields and protect water quality.

Farmers are urged to work with a soil fertility advisor to develop phosphorus management plans tailored to their specific field conditions. This approach helps safeguard farm profitability while protecting valuable water resources from nutrient pollution.

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