How is Phosphorus Loss from Soil to Water?

Phosphorus loss from soil to water primarily occurs through three main pathways: surface runoff, leaching, and subsurface flow, leading to environmental issues like water pollution. This movement can involve phosphorus in both dissolved and particulate forms, depending on the pathway and environmental conditions.

The movement of phosphorus from soil to water bodies is a critical environmental concern, contributing to nutrient enrichment and impacting aquatic ecosystems. Understanding these pathways is essential for developing effective strategies to mitigate phosphorus pollution.

Key Pathways of Phosphorus Movement to Water

Phosphorus, an essential nutrient for plant growth, can unfortunately become a pollutant when it moves from agricultural fields and other land uses into waterways. This transfer happens through several distinct processes:

1. Surface Runoff

Surface runoff is one of the most significant pathways for phosphorus loss. It involves the water-assisted downhill transport of phosphorus from the soil surface directly into streams, rivers, and lakes. This process is particularly active when rainfall or irrigation exceeds the soil's infiltration capacity, causing water to flow over the land.

Dissolved Phosphorus: This form of phosphorus is soluble in water and moves readily with the runoff water. It often originates from recently applied fertilizers, decaying organic matter, or animal waste on the soil surface.
Particulate Phosphorus: This refers to phosphorus attached to soil particles (sediment), organic matter, or other debris that is carried away by the moving water. Erosion plays a major role in the transport of particulate phosphorus. The top 2 inches of soil are particularly susceptible to losing phosphorus through this mechanism due to direct exposure to rainfall and runoff.

To learn more about surface runoff, visit the USGS Water Science School.

2. Leaching

Leaching describes the downward movement of dissolved phosphorus through the soil profile. While phosphorus is generally less mobile in soil compared to nitrogen, it can still leach, especially in certain soil types or under specific conditions.

Sandy Soils: These soils have larger pore spaces and lower phosphorus retention capabilities, making them more prone to leaching.
High Phosphorus Saturation: When soils are oversaturated with phosphorus, their capacity to bind and retain it diminishes, increasing the likelihood of downward movement.
Heavy Rainfall/Irrigation: Excessive water can push dissolved phosphorus deeper into the soil profile, potentially reaching groundwater or drainage systems.

Discover more about the process of nutrient leaching from ScienceDirect.

3. Subsurface Flow

Subsurface flow, also known as interflow or lateral flow, involves the movement of water and dissolved phosphorus laterally beneath the soil surface but above the water table. This pathway is particularly relevant in soils with restrictive layers, such as a clay pan, which can impede downward percolation and redirect water horizontally.

Water infiltrates the topsoil and moves through macropores (e.g., wormholes, root channels) or permeable layers until it encounters a less permeable layer.
It then flows horizontally along this layer, eventually discharging into ditches, streams, or other surface water bodies.
This pathway can transport phosphorus from deeper soil horizons that might not be directly affected by surface runoff.

Factors Influencing Phosphorus Loss

Several factors dictate the extent of phosphorus loss from soil to water:

Soil Type and Properties: Clay soils generally have a higher capacity to bind phosphorus than sandy soils, but very fine particles can also be easily transported via erosion. Soil pH also affects phosphorus availability and mobility.
Rainfall Intensity and Duration: Heavy and prolonged rainfall events increase both surface runoff and the potential for leaching.
Land Use and Management Practices: Agricultural practices (e.g., tillage, fertilizer application, manure management), urban development, and forestry significantly impact phosphorus loss.
Topography: Steep slopes enhance runoff velocity, increasing erosion and particulate phosphorus transport.
Vegetation Cover: Bare soils are much more susceptible to erosion and runoff than those with dense plant cover.
Soil Phosphorus Levels: Soils with very high concentrations of phosphorus, often due to historical over-application of fertilizers or manure, have a greater potential for loss through all pathways.

Impact of Phosphorus Loss

The primary environmental impact of phosphorus loss to water bodies is eutrophication. This process involves:

Algal Blooms: Excess phosphorus acts as a nutrient, promoting rapid growth of algae and aquatic plants.
Oxygen Depletion: When these organisms die and decompose, bacteria consume large amounts of oxygen, creating "dead zones" where fish and other aquatic life cannot survive.
Reduced Water Quality: Eutrophication impairs drinking water quality, recreational activities, and ecosystem health.

Mitigating Phosphorus Loss

Effective management strategies are crucial to minimize phosphorus movement from soil to water:

Soil Testing: Regularly test soil to determine actual phosphorus needs and avoid over-application of fertilizers and manure.
Precision Nutrient Management: Apply phosphorus fertilizers at the right rate, time, and in the right place to maximize plant uptake and minimize loss.
Buffer Strips: Plant vegetation (grasses, trees) along field edges and waterways to filter runoff, trap sediment, and absorb phosphorus before it reaches water bodies.
Cover Crops: Utilize cover crops during fallow periods to reduce soil erosion, improve soil structure, and absorb residual phosphorus.
Conservation Tillage: Practices like no-till or reduced-till farming minimize soil disturbance, preserving soil structure and reducing erosion.
Manure Management: Store and apply manure responsibly, considering nutrient content, soil capacity, and weather conditions.
Erosion Control: Implement measures like contour plowing, terracing, and sediment basins to reduce soil erosion.
Drainage Water Management: For tiled fields, controlled drainage can help retain water and nutrients in the soil profile.

Summary of Phosphorus Loss Pathways

Pathway	Description	Primary Form of Phosphorus	Key Influencing Factors
Surface Runoff	Water flowing over the land surface, carrying phosphorus either dissolved in water or attached to eroded soil particles (top 2 inches).	Dissolved & Particulate	Rainfall intensity, slope, soil cover, erosion, fertilizer application.
Leaching	Downward movement of dissolved phosphorus through the soil profile, potentially reaching groundwater or drainage systems.	Dissolved	Soil type (sandy), high soil P saturation, heavy rainfall/irrigation.
Subsurface Flow	Lateral movement of dissolved phosphorus through shallow soil layers, eventually discharging into surface water bodies.	Dissolved	Soil layering, presence of restrictive layers, macropores.