Legacy phosphorus refers to phosphorus that continues to impact soil and water bodies long after it was initially introduced. This includes phosphorus from past fertilization that has accumulated in soils, settled in sediments, or leached into groundwater.

Most legacy phosphorus originates from previous fertilizer applications that exceeded plant needs. Typically, only 10-30% of phosphorus from fertilizer is absorbed by crops in the first year; the rest is either lost to the environment or accumulates in the soil. This buildup often occurs faster than plants can use it, resulting in high soil phosphorus levels for years, even without new applications.

Legacy phosphorus contributes to continued nutrient pollution, which can lead to excessive algae growth and poor water quality in rivers and lakes. It diminishes the effectiveness of modern conservation practices and can affect water runoff for decades if not properly managed.

Not all soil phosphorus is readily available to crops. Initially, phosphorus from fertilizers is in a form that plants can absorb, but it soon becomes bound to soil particles or mixed into organic matter. In acidic soils (pH < 7), phosphorus binds with iron and aluminum, while in alkaline soils (pH > 7), it binds with calcium. This phosphorus only becomes available to plants through natural breakdown processes.

Excess phosphorus can wash into waterways, where it continues to contribute to nutrient problems, such as algal blooms. This ongoing issue can reduce the benefits of modern conservation efforts and impact water quality for many years after on-farm changes have gone into effect.

There are two key strategies to effectively use existing soil phosphorus:

1. Reduce Fertilizer Applications: Cut back on phosphorus fertilizer until soil levels drop to a point where yield loss is a risk. This approach can leverage phosphorus already in the soil, which can support good crop yields for up to ten years or more. Partial reductions are often more practical than complete withdrawal, as they help lower the risk of yield loss.
2. Improve Cropping Systems: Enhance cropping systems to make phosphorus use more efficient. This involves finding ways to help crops thrive even when soil phosphorus levels are lower, thus making better use of the phosphorus already present in the soil.

Simply reducing new phosphorus applications is not enough. To fully utilize existing phosphorus reserves, it's essential to unlock the less accessible forms. This can be achieved through:

• Precision Farming: Tailor phosphorus application based on soil type and use precise application techniques. Incorporating recycled phosphorus sources and manures can also enhance soil microbial activity.
• Plant Breeding: Develop crops with improved root systems or lower phosphorus needs to optimize phosphorus use.
• Fostering Soil Microbes: Adopt practices like no-till farming and diverse cover crops to support beneficial microbes that help plants access more phosphorus.

Challenges include variations in how much phosphorus crops recover and the time it takes for soil phosphorus levels to stabilize. Current methods may not always identify different phosphorus forms, and other nutrients, such as carbon, also influence phosphorus availability.

Current STP methods have several limitations:

• Buffering Capacity: They do not account for the soil’s ability to retain inorganic phosphorus even as it is used by crops.
• Rhizosphere Role: They ignore the importance of the root zone (rhizosphere) in phosphorus acquisition.
• Phosphorus Types: They cannot distinguish between different types of phosphorus in the soil solution.
• Variability: Different STP methods extract varying amounts and forms of phosphorus, leading to inconsistencies.

Phosphorus recommendations were traditionally based on the Bray soil test. Many now use the Mehlich 3 method, but there has been limited correlation and calibration with this newer method, leading to potential inconsistencies in results.

Alternatives include:

• Improved Soil Testing: Methods that better mimic phosphorus acquisition processes or account for soil buffering capacity.
• Anion Exchange Resin Membranes: These simulate root surfaces and remove dissolved phosphorus at the soil's native pH, which can improve predictions of phosphorus bioavailability.

However, they can be labor-intensive, time-consuming, and may not be compatible with commercial labs needing quick results.

Surveys indicate that water pollution from runoff persists even when farmers follow NRCS phosphorus application recommendations. This suggests a need to update the guidelines to consider additional factors, such as ecosystem services, modern agronomic practices, advances in crop genetics, and differences in soil properties.

To reduce phosphorus use, consider the following alternatives:

• Lowering the Optimal STP Threshold: In Iowa, lower STP values are deemed "high" or "very high," which means no additional fertilizer is needed. This approach can help reduce phosphorus buildup and loss.
• Precision Agriculture and Variable Rate Technology: These techniques optimize fertilizer application based on specific yield and soil characteristics, potentially reducing phosphorus waste.

BMPs are strategies tailored to specific agricultural settings designed to reduce phosphorus runoff, keep soil and nutrients on the land, and minimize phosphorus entering waterways. By implementing BMPs, stakeholders can effectively address phosphorus runoff issues, protect watershed health, and improve soil and nutrient management.

• Crop Rotation: Enhances yields, improves nutrient uptake, and manages pests and diseases.
• Perennial Forages: Adding crops like hay reduces nutrient loss and erosion.
• Reduced Tillage: Methods such as no-till and strip-till decrease soil erosion and increase water absorption. Pairing conservation tillage with practices like reducing soil test phosphorus (STP) and subsurface fertilizer application can address surface phosphorus issues.
• Cover Crops: Help reduce soil erosion and retain nutrients but should be combined with other practices to manage dissolved reactive phosphorus (DRP) losses.
• Nutrient Management: Applying phosphorus based on STP recommendations and using the 4R Nutrient Management approach (right source, right time, right rate, right place) enhances crop uptake and reduces nutrient losses.
• Filter or Buffer Strips: Reduce water contamination and soil erosion by providing a barrier between farmland and waterways.
• Wetlands: Improve water quality by absorbing nutrients from runoff and benefit wildlife.
• Prairie Strips: Integrate native perennial vegetation to capture runoff and reduce nutrient loss.
• Water and Sediment Control Basins: Prevent downstream sediment movement and manage runoff to improve water quality.
• Controlled Drainage Systems: Manage water levels in fields to reduce nutrient loss and soil erosion.

Combining a variety of BMPs tailored to the specific needs of the land is the most effective way to improve water quality and manage phosphorus runoff.