There are many risk factors for phosphorus loss from soil that can lead to water quality problems. It is important to consider both source and transport when using an index to identify these potential losses.
Phosphorus (P) is one of the most important plant nutrients and is essential for most physiological processes in plants, such as photosynthesis, energy transfer, genetic regulation of cell division and growth, and the production of seeds and fruits. Profitable crop production requires plants be provided with an adequate supply of P, either from the soil or from soil amendments such as commercial fertilizers or manures.
However, P is also important from a water quality standpoint, as excess P in freshwater systems leads to eutrophication, defined as “… an increase in the fertility status of natural waters that causes accelerated growth of algae or water plants.”
In most fresh surface waters (lakes, ponds, rivers) and some estuaries, the growth of algae or aquatic plants is limited by inadequate levels of P. Point sources of P, such as municipal wastewater systems, can contribute to eutrophication. In addition, inputs of P to surface waters from non-point sources via erosion, runoff or sub-surface groundwater flow can contribute to surface water eutrophication.
Therefore, P in agriculture has been a target for regulation as P losses from non-point sources are a significant contributor to water quality problems.
In many agricultural regions, particularly those with an intensive livestock or poultry industry, soil test P levels can be greater than needed for crop production.
Soils in these categories have little, if any, need for fertilizer or manure P. Because in some situations high-P soils contribute to non-point source pollution of surface waters, P management strategies that maintain both agricultural profitability and environmental quality are needed. Historically, there have been two main regulatory tools used by states to address non-point P losses from agriculture, namely soil test P thresholds or P site indexes.
By implementing a soil test P threshold, some states have regulated the application of P (as manure and fertilizer) based on the amount of available P in the soil. Once a soil surpasses a set soil test P threshold, the application of P is no longer of agronomic value, and excess P not utilized by plants can be lost and contribute to surface water pollution.
Therefore, by setting a threshold and limiting P applications above this threshold, less P will be left in fields, thereby reducing the potential for P losses from these fields. However, using a soil test threshold only focuses on one aspect of non-point source P losses, namely the P source.
Scientists have argued the risk of P losses from agricultural fields that could contribute to surface water pollution is not adequately addressed by only looking at the source of P (soil test). Risk assessment also needs to assess the potential transport of P from fields to surface waters. Several states opted to adopt a P site index, which addresses both the source and transport factors related to non-point source losses of P.
With a P site index, the potential sources of P utilized in crop production are accounted for, including soil test P as well as P applied as either fertilizer or manure. Most indexes also take into account the fertilizer or manure application method and timing, both of which can affect how susceptible the source is to being lost from the field.
The soil test P, P application rate, method and timing of P application are combined into a “source” factor describing the amount of P available on a field that could potentially be lost to surface waters.
Most indexes then assess the potential transport pathways that could move P from agricultural fields to surface waters. This is typically accomplished by looking at the fields’ potential for soil erosion, surface runoff, leaching, sub-surface drainage and how far the field is from a water body or its connectivity to surface water.
Conservation practices are also accounted for in determining potential erosion and runoff losses from a given field. A “transport” factor is calculated which describes the potential for losing P from the field to a surface water body.
These two categories, source and transport, are then combined to identify those fields that are a high risk for P loss. For example, a field may have a low P source factor but high transport factor – and therefore may pose a high risk for non-point P losses.
In this instance, conservation practices to reduce P movement from the field to surface waters may be a priority. Alternatively, a field with a high source potential but low transport potential might not be as great a risk for P losses.
It is important to keep in mind many P site indexes were designed to provide a systematic assessment of the risks of P application to soils, not to provide an estimate of the actual quantity of P lost in runoff.
This method of risk assessment allows producers, managers and regulators to identify or design management practices to reduce agricultural P losses to surface waters and to better prioritize the locations within a watershed where these practices will have the greatest water quality benefits. It also gives producers greater flexibility in identifying and implementing practices on-farm that will not only protect water quality but allow the producer to maintain profitability. 
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April Leytem
- Research Soil Chemist
- USDA Agricultural Research Service
- Email April Leytem
