Phosphorus Loss

Assessing the Potential for Phosphorus Runoff

Land application of manure can be beneficial to crop production but can result in increased risk of P loss to surface waters. When manure is applied to meet crop nitrogen needs, the amount of P applied is typically much more than the P removed in the harvest of one crop. Effective January 1, 2007, operators of large CAFOs in Nebraska need to assess the risk of P delivery to surface waters from each of their designated fields by using a P index before manure can be applied. This assessment needs to be done once every five years and results need to be kept. Phosphorus indexes are tools for the assessment of the potential for P delivery from agricultural lands to surface waters. 

The Phosphorus Index

The P index considers source and transport factors to estimate P loss to surface waters. The source factors allow assessment of the quantity and forms of P present at the site (see table below). The transport factors allow assessment of the potential for transport of P from the site to a water body.

The Nebraska P Index (2012) and the publication, The Nebraska Phosphorus Index (2012): Background and Users Guide, are available at on the manure.unl.edu website.

The P Index was designed to be used on the basis of a whole field or management units within a field. In many fields, the risk of P loss is often considerably greater for part of a field than for the whole field, and it may be economically and environmentally advantageous to do the P loss risk assessment by zones within the fields. The P index risk scores fall into four risk levels.

• Low (0-2). Current practices keep water quality impairment due to agricultural P pollution low. Manure can be applied at rates sufficient to meet crop N needs.
• Medium (2-5). Delivery of agricultural P may cause some water quality impairment and consideration should be given to alternative conservation and P management practices. Manure can be applied at rates sufficient to meet crop N needs if the P index rating of the field or sub-field will not become high^†.
• High (5-15). Phosphorus loss from the field causes much water quality impairment. Remidial action, such as alternative conservation measures or P management practices, is recommended. Manure can be applied at a P removal-based rate to this field or sub-field if recognized P best management practices are adopted to reduce the P index rating to medium within five years^†.
• Very High (>15). Impairment of water quality is extreme and remedial action is urgently recommended. Phosphorus application should be discontinued. Improved conservation measures should be implemented.

^†The bolded parts of the interpretation for Medium and High risk are recommended by Nebraska Extension but are not yet in NDEQ regulations.

Management for Reducing Phosphorus Losses

The most effective management principle is to apply no more phosphorus than the crop needs over a set time period. If manure is to be applied every three years, then the manure phosphorus load should not exceed the three-year removal rate, unless the soils are extremely low in phosphorus at application time. Application of manure on a nitrogen basis every year should be avoided on any sensitive site with a high Phosphorus Index.

The decision of whether or not to incorporate manure is a problem without a perfect solution. Surface-applied manure will allow more soluble phosphorus to run off in a moderate rainfall event. Research has shown that water soluble phosphorus movement will increase without incorporation. However, with no incorporation, the increased crop residue and manure covering will reduce erosion and the movement of phosphorus which is attached to soil particles. Tillage will reduce water soluble phosphorus in runoff but will increase sediment attached phosphorus.

Physical structures to hold water in settling basins and behind dams will decrease soluble phosphorus reaching surface water. A combination of not applying manure near streams and waterways; reduced tillage to decrease sediment loss; and water control will minimize phosphorus reaching surface waters.

In high soil phosphorus situations, choosing a crop such as alfalfa will remove more phosphorus per acre and help draw down reserves. Crops where the whole plant is harvested will remove more nutrients than crops where crop residue is returned. Near sensitive areas, perennial forages and grass filter strips provide more ground cover and reduce erosion potential.

Part I of Phosphorus Dynamics | Part II - Phosphorus Loss | Complete the Quiz