Phosphorus Sources, Application Rate, Timing, and Methods
Phosphorus (P) source, application rate, application timing, and application method have a critical impact on P solubility and fate in agricultural soils. In addition, these management practices may impact the potential for P losses in agricultural runoff or leaching events.
To allow for the land application of inorganic and organic P sources, while protecting water bodies from excess P loadings. These practices should help improve crop P uptake efficiency.
How Does This Practice Work?
Commercial inorganic fertilizers typically contain soluble P forms that can be immediately taken up by plants or fixed by soil minerals containing aluminum (Al), iron (Fe), or calcium (Ca).
Organic residuals like manures, biosolids, or food wastes also contain P; however, total and soluble P concentrations in organic amendments are highly variable. For example, total P concentrations in manures are directly influenced by animal type, animal diet, and manure storage methods. Similarly, total P concentrations in biosolids are directly influenced by wastewater influent characteristics and total P concentrations tend to be higher than in other organic residuals (e.g., dairy manure, swine slurry, poultry litter).
Phosphorus solubility also varies widely between organic residual types, especially when these materials are composted or undergo biological or chemical treatment. For example, chemical treatment of manures or biosolids with multivalent metal salts (e.g., Ca, Fe, or Al salts) results in the precipitation of relatively insoluble P compounds, and thus have lower water extractable P (WEP) concentrations than raw manures or digested, composted, or biologically-treated manures and biosolids. Materials with lower WEP concentrations will contain less plant available P. Researchers also report lower risk for dissolved P losses in runoff or leachate from chemically treated manures or biosolids than other organic P sources.
Phosphorus Application Rates
Inorganic P fertilizers applications should follow land-grant university recommendations to meet crop P demands. Most states use the sufficiency level or build and maintain philosophies when making P recommendations, while a few states present both options or use a hybrid approach. Phosphorus application rates that are based on the build and maintain approach are designed to build soil test P concentrations and will typically be higher than rates based on the sufficiency level approach. Avoid applying P when a soil test suggests little to no probability of crop response to added P to reduce the risk of dissolved or particulate soil P losses.
The nutrient content of organic P sources should be determined by laboratory analysis for each material because nutrient concentrations can be highly variable. Typically, N:P ratios in organic residuals are not balanced with crop needs. As such, application rates of organic residuals are typically based on crop N or P requirements. When organic residuals are applied at N-based rates, the amount of P applied often exceeds crop needs. Only apply N-based rates to fields with a low risk for P losses based on an approved P risk assessment tool (e.g., P Index). The potential for P losses is greatest at the time of application and increases with increasing WEP concentrations. Soil test P concentrations should be monitored to ensure that N-based application rates are not resulting in a buildup of soil test P to concentrations that would increase the risk of P losses. Soil test P concentrations should remain relatively stable if organic residuals are applied at P-based rates.
Federal Concentrated Animal Feeding Operation (CAFO) regulations allow for single applications of organic residuals to meet several years of crop P requirements. This multi-year application rate is permissible provided the application rate does not exceed the N-based agronomic rate and the applied P will be removed from the field via harvest/crop removal. Applying a single application to account for multiple crop P needs eliminates practical difficulties in applying organic residuals at P-based rates with standard manure spreading equipment. In addition, the multi-year approach allows producers to select the crop with highest N needs in their rotation to receive the organic P application (e.g., corn in a corn-soybean rotation).
Phosphorus Application Timing
Regardless of source, P applications should be made at times when significant runoff events are not expected. Monitor weather forecasts closely and delay application if significant precipitation events are expected within a day or two of a planned application. Avoid P applications to frozen, snow covered, or saturated ground as these conditions encourage rapid runoff and increase the potential for P losses.
Phosphorus Application Methods
Surface applications of any P source presents the highest risk for P losses in runoff. Phosphorus applications should be incorporated or applied below the soil surface when possible.
Banded applications of inorganic P fertilizers (e.g., 2 inches below and 2 inches over from the seed) are preferred over broadcast applications for several reasons. Banding decreases fertilizer to soil contact, reduces fixation by soil minerals, increases plant P availability at rates up to 50% less than those recommended for broadcast applications, and reduces the risk for P losses in runoff.
Solid organic residuals should be incorporated as soon after application as possible. Incorporation increases soil contact and reduces the potential for soluble P losses in runoff events. Incorporating solid organic residuals has the added benefit of reducing N volatilization losses, thus improving the N:P ratio. Surface applications of solid organic residuals is the only option in forage or pasture systems or on operations in continuous no-till. When surface applications are necessary, applications should be scheduled to avoid peak runoff times.
Injection or subsurface application is the preferred application method for liquids or slurries. There are several equipment options available to facilitate subsurface application of liquids and slurries, even in permanent pasture or no-till systems. Liquids may also be applied through irrigation systems. Applications through irrigations should be applied at a rate that allows for adequate infiltration to reduce ponding and runoff.
Where This Practice Applies and Its Limitations
State and local regulations may require specific application rates, methods, and timing for various P sources. In addition, selection of the appropriate P source, rate, application method, and timing will be site-specific due to variability of soils, climate, and management scenarios.
Federal and state regulations may require setbacks for P applications, especially in the case of manures and biosolids. For example, the Environmental Protection Agency (EPA) requires a 100-foot setback from surface waters, open tile line intake structures, wellheads, or other conduits to surface waters; no manure, litter, or wastewater can be applied in these areas. Similarly, the USEPA federal biosolids regulations (40 CFR Part 503) dictate that biosolids that do not meet stringent pathogen reduction and metals concentration requirements may not be applied within 10 m of any U.S. waters (e.g., intermittent streams, lakes, rivers, etc.). State or local regulations may require greater setbacks. In addition, applications of organic residuals to steep fields with low infiltration rates should be avoided if possible. Use of an approved P risk assessment tool (i.e., P Index) to determine the most suitable locations for P application is always recommended.
Cost of Implementing the Practice
Phosphorus-based applications of organic residuals may increase application costs (compared to application of an N-based rate) as P-based application may require a larger land base, resulting in increased labor and transportation costs. However, many of the practices described in this factsheet should not result in significantly increased costs if they occur during planned P applications.
Operation and Maintenance
The P management decisions described in this factsheet are not one-time decisions that can be applied indefinitely. Rather, growers must take an adaptive and iterative approach to P management. Management decisions should be evaluated and reconsidered every time that P is applied to land with consideration of economics and site specific soil and climatic conditions.
Brandt, R.C., H.A. Elliott, and G.A. O’Connor. 2004 Water-extractable phosphorus in biosolids: Implications for land-based recycling. Water Environ. Res. 76:121-129.
Code of Federal Regulations, 40 Part 412. 2023. Concentrated animal feeding operations (CAFO) point source category. https://www.ecfr.gov/current/title-40/chapter-I/subchapter-N/part-412
Ippolito, J.A., Barbarick, K.A. and Norvell, K.L. (2007), Biosolids Impact Soil Phosphorus Accountability, Fractionation, and Potential Environmental Risk. J. Environ. Qual., 36: 764-772. https://doi.org/10.2134/jeq2006.0308
Sharpley, A.N., Daniel, T.C. and Edwards, D.R. (1993), Phosphorus Movement in the Landscape. Journal of Production Agriculture, 6: 492-500. https://doi.org/10.2134/jpa1993.0492
Shober, A.L., and J.T. Sims. 2003. Phosphorus restrictions for land application of biosolids: Current status and future trends. J. Environ. Qual. 32:1955-1964.
USEPA. 1994. A Plain English Guide to the EPA Part 503 Biosolids Rule. Office of Wastewater Management. Washington, D.C.
USEPA. 2004. Managing manure nutrients at concentrated animal feeding operations. https://www.epa.gov/sites/default/files/2015-08/documents/cafo_manure_guidance.pdf
USDA-NRCS. 2011. National Agronomy Manual. https://www.nrcs.usda.gov/sites/default/files/2022-10/National-Agronomy-Manual.pdf
For Further Information
Contact your local soil and water conservation district, USDA-NRCS or Cooperative Extension Service office. To find your local USDA Service Center, visit https://www.nrcs.usda.gov/contact/find-a-service-center.
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Shober, A. and J. Ipollito. 2023. Phosphorus Sources, Application Timing and Methods. SERA17 Phosphorus Conservation Practices Fact Sheets. https://sera17.wordpress.ncsu.edu/phosphorus-sources-application-rate-timing-and-methods/