Legacy Phosphorus: The Enduring Hidden Driver of Tile Drainage Loss
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EmailPhosphorus loss is dominated by legacy P while new P (recently applied fertilizer) has a minor role. We can make a big impact on Lake Erie’s Water Quality only if we tackle both legacy P and new P.
Important takeaways
- Legacy P dominates P loss while new P plays a minor role.
- Soils tested across the USA showed that they have enough legacy P to sustain high P loss for decades.
- A drawdown of soil test P (STP) by crop removal can address the legacy P problem, but this takes a very long time.
- Soils with very high STP levels required 16 years to halve their STP concentration without any fertilizer application. The higher the STP, the slower the drawdown, making it vital to avoid additional accumulation of legacy P through careful nutrient management.
- Nutrient management can address the smaller but still important contribution of new P loss over the short term.
- To make a big impact on Lake Erie’s water quality, we need to address the dominant legacy P in two ways: first, drawdown of the STP to achieve results over the long term, and second, by implementing water management to achieve results over the short term.
Lake Erie continues to have algal blooms in the summer because of excess phosphorus. There are various sources of phosphorus (P) entering Lake Erie, but agriculture is a dominant source. Recent research highlights the dominant role of legacy phosphorus (P) and the need for innovative solutions to protect water quality.
Phosphorus loss is dominated by legacy P while new P has a minor role
The Osterholz et al. 2023 study introduced a novel method to distinguish contributions of "new P" (recently applied fertilizers) from "old P" (legacy soil phosphorus) in subsurface drainage discharge from sites in Ohio and Indiana. The findings revealed that new P accounted for only 0–17% of dissolved reactive phosphorus (DRP) loss across sites, while legacy soil P dominated at 83–100%. These findings highlight the need to emphasize strategies to mitigate legacy P to make a big impact on water quality.
Similar findings emerged from the MSU edge-of-field research project, where legacy P accounted for about 97% of the loss from an inorganic fertilizer-applied on-farm site and 84% from an organic manure-applied on-farm site. This reinforces the critical role of legacy P in water quality.
The water quality problem from legacy P that will endure for a long time
Simpson et al. 2024 analyzed over 600 soil and sediment samples across diverse sites in the USA. They found that these soils have enough legacy P to sustain high P loss for decades. These findings show that without proactive management, legacy P will continue to degrade water quality for the foreseeable future, presenting a significant obstacle to achieving our water quality goal. The legacy P problem can be addressed by the drawdown of the soil test P, but this has its challenges.
Reduce the legacy P with drawdown of soil test P by crop removal to make a big impact, but this takes a very long time
A drawdown of soil test P (STP) by crop removal can address the legacy P problem. Progress will be steady but slow. This is a marathon, not a sprint. A meta-analysis by Gatiboni et al. 2025 of 56 fields under no fertilization showed that STP could be reduced by 4.3–8.2% annually, depending on the initial STP levels. Soils with very high STP levels (116–261 Mehlich-3) required 16 years to halve their STP concentration without any fertilizer application. The higher the STP, the slower the drawdown, making it vital to avoid additional accumulation of legacy P through careful nutrient management.
Nutrient management can address the smaller but still important contribution of new P loss over the short term
While legacy P is the main driver of water quality degradation, new P loss (recently applied fertilizers) also plays a role. The fertilizer source, timing, rate and application methods each affect new P loss. The Osterholz et al. 2024 study analyzed 155 P applications across Ohio and Indiana. The researchers found that surface broadcast of liquid manure posed the highest risk of increased new P loss, while injection and incorporation methods reduced new P loss. These findings emphasize the benefit of nutrient management (4R approach) as a faster way to address the smaller but still important contribution of new P loss. But to significantly improve water quality, we need to tackle the dominant legacy phosphorus (P) in two ways. First, we should focus on reducing the soil's total phosphorus (STP) levels for long-term results. Second, we need to implement effective water management strategies to achieve quick, short-term results.
Water management can address both the dominant legacy P loss and new P loss over the short-term
Since phosphorus moves with water, managing water movement off the farm can improve water quality more quickly. Drainage water recycling integrated with controlled drainage offers advantages (Figure 2). First, it reduces drainage discharge through controlled drainage. Second, it captures and stores the remaining drainage discharge for supplemental irrigation (Figure 3). This practice can improve water quality by retaining nutrients in the reservoir as well as building crop yield resiliency with supplemental irrigation during drought periods, according to Hay et al. 2021. They found yield increases of up to 61% for corn and 31% for soybean. Research in Missouri, Ohio, Iowa, and North Carolina has shown water quality and crop yield benefits from this system. These benefits warrant a thorough investigation of the system’s economics and additional advantages.
Summary
Without proactive management, legacy P will continue to degrade water quality for decades, presenting a significant obstacle to achieving our water quality goal. Nutrient management is a good start, but by itself may not be enough to achieve our water quality goal. We need to tackle both legacy P and new P by integrating nutrient and water management practices. With collaborative efforts, we can protect Lake Erie’s water quality and sustain agricultural production for future generations.
