Do High-Phosphorus Soils Really Need More Fertilizer?

A recent study from Kansas led by Dr. Megan Bourns looked at a practical question: if a soil already has plenty of phosphorus, is it worth continuing to apply more?

The researchers compared phosphorus maintenance with a drawdown approach in a no-till corn-soybean system. In the drawdown treatment, no additional P fertilizer was applied, allowing crops to use phosphorus already stored in the soil.

Why It Matters

Many fields have built up soil phosphorus after years of fertilizer application. That P is not necessarily lost or unavailable. Some of it can continue to support crop growth.

Continuing to apply fertilizer when soil-test P is already high can therefore increase costs without always producing enough extra yield to justify the expense.

What the Study Found

The P drawdown treatment without cover crops had the highest net return during the study.

There was also a water-quality benefit. By the third year, total P loss was 38% lower and dissolved reactive P loss was 50% lower than in the P maintenance treatment without cover crops.

The cover crop results were more mixed. Rye reduced sediment losses, but it also reduced corn yield and did not consistently reduce dissolved P losses.

Key Takeaway

When soil-test phosphorus is already high, more fertilizer may not mean greater profit.

Strategically drawing down accumulated soil P could allow producers to reduce fertilizer costs while lowering phosphorus losses to water. However, drawdown is not a permanent replacement for P fertilization. Soil testing remains essential for determining when existing soil P is sufficient and when fertilizer applications should resume.

Reference: Bourns, M., Yeager, E., Nelson, N., Kluitenberg, G., Roozeboom, K., Hettiarachchi, G., … Presley, D. (2026). Phosphorus drawdown improves net return and water quality in a no-till corn–soybean system. Journal of Soil and Water Conservation, 81(3), 222–235. https://doi.org/10.1080/00224561.2026.2651052