Phosphorus Loss in Surface Water

Feb 15, 2024 | Latest news

Originally published in the Mid America CropLife Association (MACA) November Water Update – November 15, 2023.

I had the opportunity to attend the Agricultural Nutrient Policy Council (ANPC) meeting in Columbus, Ohio a few weeks ago and learned a great deal about what is happening with water quality issues on the production agriculture front. One of the most interesting presentations came from Dr. Andrew Margenot, Department of Crop Sciences at the University of Illinois Urbana-Champaign. He brought insight and developments to the group regarding phosphorus (P) loss in surface water and where it is coming from with the data to back it up.

Dr. Margenot suggested that it is necessary to understand and quantify the contribution of all sources of total phosphorus (TP) loading to surface waters to monitor and meet water quality objectives. There are often fingers pointed at non-point source or point-source, but rarely is discussion of streambank erosion included, leaving this source of P unaccounted for.

Point source contributions of nutrients in the water can easily be attributed to a municipal and industrial facilities. Non-point source contributors are much more difficult to assess for nutrient loading, leading to an overestimation in P contributions from agriculture. This can greatly skew assessments and policies for production agriculture. When many stakeholders suggest the solution being as simple as decreasing the use of fertilizers and manure, Dr. Margenot strongly suggested otherwise.

Three main challenges were outlined in being able to rightly attribute nutrients in surface water:

  • Detecting the signal of nutrient loading and how best management practices (BMPs) can impact positively while accounting for weather events.
  • Attributing nutrient loads from the correct source as natural events such as mineralization can make large impacts on magnitude of nutrients in surface water.
  • Lag times in appearance of nutrients. Measured N or P may not reflect when the nutrient load was lost from fields or when it entered waterways.


Legacy Phosphorus in U.S. Waterways

Addressing these three challenges led to a question of, “if the United States is so efficient at phosphorus use (~60% compared to 16% efficiency globally), why are P levels so high in U.S. surface waters?” Residual phosphorus, also known as legacy phosphorus, can account for 26-48% of all P for decades after application. Where does all the residual phosphorus come from? Enter the moldboard plow and the over-application of P through phosphate rock.

In the 1908 version of The Fertility of Illinois Soils, it was recommended to apply 1000 pounds per acre of raw phosphate rock once every five or six years as the most “practice and profitable” method of maintaining P in the soil. Because of historical P inputs not removed with crop harvest, the result was a positive P balance in past decades leading to a continued positive now. This excess of phosphorus can take decades to get to the field’s edge and then to the streambank, eventually eroding into the nearby waterway.

With research conducted by Dr. Margenot in addition to research conducted on the Baltic Sea Basin, data has pointed to a lack of decrease in total P loads even with decreased P agricultural inputs. Data from the Baltic Sea Basin shows total P loading as follows: 33% from natural sources (decomposition of native vegetation and exposed rock), 45% residual phosphorus (over application and plowing decades ago), 14% runoff from recent agricultural application, and 8% from point source (municipal and industry).

With this information now available and backed with data, there is no short-term solution. Reducing agricultural P is not going to provide a solution due to the P cycle in waterways and decades of residual P currently located on eroding streambanks. It is likely there will be more attention given to developing cost-effective methods for streambank stabilization. The future for maintaining P is unsure, but it is certain that no quick fix is available. Moving forward, the hope is for environmental groups and agencies to be aware of residual phosphorus when setting policy and nutrient reduction benchmarks.


Read the Paper

For more information, see the linked paper written by Dr. Andrew Margenot: Streambank erosion and phosphorus loading to surface waters: Knowns, unknowns, and implications for nutrient loss reduction research and policy

If you have any questions, please reach out to Andrea at or by calling (573) 636-6130.


Each month Steve Taylor or Andrea Rice provide an update on a current water issue. Both are with the Missouri Agribusiness Association which serves as MACA’s Water Consultant. Please share their information with others in your company. ~ Bonnie McCarvel, MACA Executive Director.