Environmental Impacts and Benefits of Manure: Phosphorous and Surface Water Protection

Managing manure nutrients in an environmentally and economically responsible manner is not a mutually exclusive endeavor. This article discusses phosphorus and its potential impacts on water quality.

Phosphorus and Water Quality

Phosphorous (P) is one of the major bio-available nutrients in manure. In aquatic ecosystems, P is typically the most limiting nutrient. When P is introduced into an aquatic ecosystem there is a marked increase in aquatic plant biomass production and increased algal blooms. The increased aquatic plant production and algal blooms can have a negative effect on the aquatic ecosystem such as tying up other nutrients and decreasing the amount of light infiltration.

At the end of the aquatic plant and algae growing cycles, there is a large release of excess nutrients into the ecosystem overwhelming the natural nutrient cycle, tying up oxygen during its degradation leading to fish kills and reducing surface water aesthetic qualities with the accumulation of rotting plant material on the water surface and offensive odors.

How Does Phosphorus Travel to Water?

In cropping systems, providing a sufficient level of P for plant uptake is as important as providing the proper levels of nitrogen (N) and potassium (K). Unlike N and K, P is bound to soil particles and is at low risk of leaching through the soil profile. The greatest risk of P loss from soils is with overland flow of runoff carrying P-enriched soil sediment or manure particles. Research has shown that soils testing high in P have a greater contribution effect for P loss than soils testing low in P.

However, there is a fraction of total P in runoff that is in the dissolved form. The sediment attached P and dissolved P have slightly different impacts in aquatic ecosystems. The sediment attached P contributes to long term P additions to the system whereas the dissolved P is readily available for a high rate of assimilation by aquatic plants and algae.

There are also reported cases of soils with extremely high levels of soil test P that are at risk of P leaching. Typically, soil P is bound tightly to soil particles and has a low risk of leaching. However, in some soils with extremely high soil test P levels, the exchange sites are at maximum capacity, leading to the risk of P leaching.

Blue-green algae bloom in nutrient impaired water. Source: Ron Wiederholt, NDSU Extension


Management Practices to Reduce Environmental Risks from Phosphorus

Cropping system practices that lead to reduced soil erosion are the most effective means of decreasing the risk of off-site movement of P. Besides soil erosion, there are other factors that need to be identified when reducing the risk of P loss from fields.

These factors include but are not limited to:

  • distance to surface water
  • slope of the landscape
  • soil erosivity index
  • soil test P level

Many states have adopted a process of ranking the risk of P loss from agricultural fields using a P-index. The USDA Natural Resources Conservation Service (NRCS) has been the lead agency in developing most of the state-by-state P-indexes. A P-index scores the factors important for off-site movement of P and by using the combined score of these factors a land manager can decide what options are best for managing P application levels to fields when using manure or commercial fertilizer.

However, the use of a P-index is only one of the tools available to nutrient managers. When there has been a long history of P mis-management and soil test P levels are extremely high, a P-index or other tools are not as effective. In these cases, a long term approach looking at the whole cropping and livestock system needs to be adopted.

Livestock rations must be closely monitored to ensure there is no P overfeeding (see the LPELC topic, Feed Management), manure may have to be sold or bartered to other land managers, or some type of intensive manure processing system will have to be adopted that will allow for more affordable long distance hauling of the manure (see the LPE Learning Center topic Manure Treatment Technology).

Recommended Reading On Phosphorus and Surface Water

Page Managers: Ron Wiederholt, North Dakota State University and Marsha Mathews, University of California-Davis