How Phosphorus is Lost from Farmland

Introduction

While phosphorus (P) is an essential nutrient for plant growth, phosphorus runoff from the landscape can also lead to degradation of surface waters. Enrichment of streams, lakes and rivers with phosphorus leads to algae growth and subsequent decay, which depletes the water of oxygen necessary for fish and other aquatic organisms. Not only does this process, known as “eutrophication,” result in degraded aquatic habitats, it makes surface waters unfit for recreation or use as a drinking water source. 

Common sources of runoff phosphorus are eroded soil and surface applied manure or fertilizer, but it can also originate from fields without these obvious phosphorus sources. In general, soils having high phosphorus levels also produce runoff that contains high concentrations of phosphorus. These high phosphorus soils are typically the result of field or farm level nutrient imbalances; these imbalances often extend to the regional scale. Phosphorus that is mined in Florida and used as fertilizer in the Midwest is commonly shipped to Eastern farms in the form of livestock feed (Figure 1). Only a fraction of the phosphorus in the livestock feed leaves the farm in the form of a food, fuel or fiber products. The majority of the phosphorus is distributed on the land in the form of manure, often at rates selected to meet the nitrogen need of the crop and exceeding the phosphorus need of the crop. Phosphorus accumulates in the soil until the soil cannot absorb anymore; runoff phosphorus concentrations tend to increase as this accumulation occurs.  

Figure 1. Current nutrient cycle for phosphorus (P) in United States. †

Phosphorus Loss Pathways

Phosphorus leaving a field can either be completely dissolved in water, attached to eroding soil, or contained in fertilizer or manure that is being carried by runoff. Erosion always occurs on the soil surface, but the other phosphorus-loss pathways can take place both on the surface and through the subsurface (Figure 2).

Figure 2. Factors affecting phosphorus transport to surface waters in agricultural ecosystems. †

Surface

Tilled fields and overgrazed pastures are especially susceptible to the phosphorus loss that is associated with erosion. Phosphorus binds very strongly to soil particles, and the erosion of these soil particles can be a significant source of phosphorus loss, even if the soil phosphorus level is low. 

Dissolved phosphorus can also be carried in surface runoff from tilled or untilled fields. While different soils can absorb different amounts of phosphorus, for a given soil, the higher the soil phosphorus level is in the field, the higher the phosphorus concentration generally is in the runoff. The soil phosphorus level is highest in the uppermost soil layer, especially in undisturbed soils, like no-till, pasture and hayland. This is also the layer that runoff interacts with the most as it travels through a field. This is the dominant phosphorus loss pathway from pastures. 

If present, manure and fertilizer containing phosphorus can be readily washed off the soil surface by runoff from rainfall or snowmelt. Application rate and timing are large factors in how much phosphorus is lost through this pathway. A couple runoff events following manure applications can represent the bulk of the surface runoff phosphorus for the entire year.

Subsurface

Dissolved phosphorus can move from upper soil layers into shallow groundwater with percolating water. This is more common in sandy and gravelly soils, but it can be enhanced by large pores (e.g., wormholes) in other soils. Once phosphorus is in shallow groundwater beneath a field, it can flow to a bordering stream or ditch. Drainage tiles can increase the magnitude of this phosphorus loss pathway. 

A little-recognized phosphorus loss pathway is the movement of manure or fertilizer phosphorus from the surface directly into drainage tiles through large soil pores or cracks. This process has been observed and demonstrated in many agricultural areas, but it is more probable when applying liquid manure sources. 

Management to Limit P Loss

Risk of phosphorus loss can be separated into two categories: 1) source factors and 2) transport factors (Table 1). Source factors are directly related to the amount of phosphorus that water interacts with as it passes through a field and can often be controlled with management efforts. Transport factors are related to the volume of water leaving a field and the likelihood of that water reaching a stream once it has interacted with a phosphorus source. Various management methods and other conservation practices are also helpful for limiting transport factors. 

Table 1. Summarized phosphorus transport and source factors.†

Phosphorus source control is achieved primarily through managing the rate, timing and method of phosphorus applications to fields. The degree of phosphorus “saturation” is also an important factor to consider for source control. 

The rate of phosphorus additions to a field should be based on the agronomic need of the crop being fertilized. When phosphorus is applied at rates greater than the agronomic need of the crop, phosphorus will accumulate in the soil. As phosphorus accumulates in the soil, the risk of phosphorus loss increases. Annual soil testing allows for informed and economical use of phosphorus to meet crop needs. 

The timing of phosphorus application can have a large influence on the amount of phosphorus in runoff. Phosphorus sources should not be applied to frozen or saturated soils. Probability of runoff in West Virginia is greatest in the winter with snowmelt and in the spring with rain on saturated soils. Large rain events in the summer can also produce significant runoff. Weather predictions can be used to select application times when the forecast is relatively dry. 

The method of phosphorus application can be on the surface, on the surface followed by incorporation, or injected into the subsurface. Injecting or incorporating phosphorus sources can reduce phosphorus loss if it can be done without increasing the risk of erosion. Injection and incorporation also forces phosphorus into deeper soils and reduces phosphorus-enrichment of surface soils that interact with runoff.

Soils can only retain a certain amount of phosphorus, before they become “saturated.” As a soil retains more and more phosphorus and gets closer to the saturation point, the amount of phosphorus in runoff from that soil increases. It is important to understand the degree of phosphorus saturation of a soil, and to factor that into the evaluation of risk of phosphorus loss from a field (e.g., using tools like the P-Index). The phosphorus saturation ratio (PSR) is an established indicator of phosphorus saturation and is defined as the ratio between the amount of phosphorus present in the soil and total capacity of that soil to retain phosphorus. The ability of phosphorus to be bound in the soil is primarily a function of the aluminum and iron content found in that soil. A Mehlich 3 extraction (i.e., a specific soil testing method) allows aluminum and iron levels to be determined and then a simple ratio of phosphorus to aluminum and iron is reported. This ratio indicates how much iron and aluminum are currently holding phosphorus and how much is left to hold additional applied phosphorous. Research has shown that there is a PSR threshold level above which a soil that is a sink for phosphorus switches to become a source for phosphorus loss to the environment. 

Phosphorus transport control relies on a number of management and conservation practices that serve to reduce the risk of runoff phosphorus from reaching a water body. Even with proper phosphorus transport control measures, some fields and soils inherently have a greater risk of phosphorus loss. In these fields, source control measures become more important. 

Soils that are more prone to producing large amounts of runoff have an increased risk of phosphorus loss. Several management practices can be implemented in these fields to help reduce runoff volumes. Cover crops, reduced or no-till and contour farming all help infiltrate precipitation and reduce runoff. 

Concentrated flow paths (ditches, natural swales, etc.) increase the risk of phosphorus loss by greatly reducing the opportunity for runoff to infiltrate into down-slope soils or for sediment-bound phosphorus to settle out or be trapped by vegetation. If concentrated flow paths do exist in a field and cannot be avoided, making sure they are well vegetated and are not eroding will help to reduce phosphorus loss. 

As the distance from a field to a stream becomes shorter, the risk of phosphorus loss increases. This risk can be reduced by leaving forest or grass buffers between the field and the stream. These buffers trap sediment-bound phosphorus in runoff and help prevent stream bank erosion, another potential source of P loss. In general, the wider the buffer is, the more effective it is at trapping phosphorus. 

Erosion control methods are also very effective at reducing phosphorus transport. Avoiding overgrazing, maintaining vegetative cover, and reduced or no-till practices limit the amount of sediment-bound phosphorus that can be moved with runoff.

Phosphorus transport through the subsurface is difficult to control. Excessively drained soils increase the risk of transporting phosphorus through the subsurface; drainage tiles can also enhance this risk in less well-drained soils. If possible, wetlands constructed and positioned at tile outlets slow runoff and help reduce phosphorus loss through adsorption, sedimentation and plant uptake. 

P-Index

The Phosphorus Index is a tool that was introduced in the 1990s and is now a widespread method of evaluating risk of phosphorus loss from agricultural fields. The P-Index rates the source and transport factors of a particular field, with the understanding that the risk of phosphorus loss is highest when there are interactions between phosphorus sources and transport pathways. Conservation practices and management are considered when evaluating risk of phosphorus loss. Each state has developed their own version of the P-Index based on their local soils, climate and landscape. Most versions are straightforward and are designed to help nutrient management planners and producers evaluate the effects of alternative management practices on phosphorus loss. 

Authors: Tom Basden, Retired WVU Extension Nutrient Management Specialist, and Joshua Faulkner, Research Assistant Professor and Farming and Climate Change Program Coordinator, University of Vermont (former WVU Extension Agricultural Engineering Specialist)

Last Reviewed: January 2023

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