Agricultural Phosphorus and Eutrophication by Don Pitts Agricultural Engineer & Water Quality Specialist USDA, NRCS Champaign, IL

Phosphorus (P) Is an essential element for plant growth Additions of fertilizer P inputs have been long recognized as necessary for profitable crop production Phosphorus inputs can also increase biological productivity of surface waters and cause eutrophication

What is Eutrophication? It is the loading of dissolved and particulate matter to a water body at rates sufficient to produce high biological production It has been identified as the main cause of impaired surface water in the U.S. (USEPA) (36 Percent of Illinois Lakes are classified as hyper-eutrophic, IEPA, 1999)

Eutrophication Eutrophication is the natural aging process of lakes brought on by nutrient enrichment Eutrophication is generally accelerated by Phosphorus (P) inputs Source: USDA (Sharpley et al., 1999)

Primary Nutrient Inputs for Aquatic Biota Growth Since C and N can be obtained from the atmosphere, P is generally considered the limiting input in fresh water systems.

Relationship Between P and Chlorophyll-a in Fresh Water Systems Oligotrophic Mesotrophic Eutrophic Hyper-eutrophic P concentration (mg/l) Lake Productivity (Chlorophyll a, ppm) (source: Sharpley, 1997) (assuming sunlight and temperature not limiting)

Phosphorus (P) Water Quality Standards and Criteria Total P in streams entering a lake should be less than 0.05 mg/l (Illinois Water Quality Standard) Ambient Water Quality Criteria Recommendations for Central Illinois Ecoregion, Total P = 0.02 mg/l (USEPA, 2000) IEPA criteria for streams and rivers is in development

Phosphorus (P) Sediment-bound or Particle P –it is generally thought that most of the P losses from cropland is in this form (however, in many cases, this may not be true) Dissolved or Soluble P –is defined as P that will pass through a 0.45 micro filter (sometimes equated to orthophosphate) –is very bio-reactive and available for algae production Total P = Particle P + Dissolved P

Several Common Myths about Phosphorus Transport Soils are infinite sinks for P P does not move through the soil Erosion control will stop P losses in runoff

Soils are not Infinite Sinks for P Research has shown that soils cannot indefinitely fix applied P Continued application of P beyond crop requirements will eventually result in soil P saturation

Long-Term Build-up of Soil P from Continual Over-application From Barber, 1979

Dissolved P in Runoff verses Soil P Level Source: Sharpley et al. (1999) and Hoeft (2000)

Phosphorus Does Move Through the Soil P may move through the soil given: –low soil P-fixing capacity –high percolation potential (macro pores) –high soil test P

Dissolved Phosphorus in Tile Water verses Soil P Levels Source: Heckrath et al. (1995) Olsen convert to Bray for tile flow P

Erosion Control May Not Stop P Losses in Runoff Erosion control measures may not reduce dissolved P level in runoff: – high soil test P levels may cause significant dissolved P losses –stratified P (concentrated near the surface) may cause high dissolved P runoff levels

Dissolved P in Runoff verses Soil P Level Source: Hoeft (2000)

Illinois Soil Test Laboratory Reports A survey of agricultural soil test laboratories in Illinois indicated 64% of the soil samples analyzed had high to very high Soil P levels (Fixen, 1998)

Once elevated, it may take many years of no P fertilizer application to bring soil tests P to normal levels It was estimated to take years of corn/soybean production to drop a Portsmouth fine sandy loam from 100 ppm to the agronomic threshold of 20 ppm (McCollum, 1991).

Phosphorus (TP) Concentrations in Surface Water in Illinois (Source: IEPA)

USDA ARS Bulletin Available online