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To review[edit]

Phosphorus Cycle from Wikimedia Commons

Limiting excess phosphorus[edit]

Volume reduction[edit]

In many forms of LID practice, the dominant mechanism reducing the loading of phosphorus, is the significant volume reduction of the outflow achieved.

Chemical control[edit]

For infiltration practices an 'amendment' or chemically reactive 'additive' can help to retain even more phosphorus. Typically these components would make up 5 to 10% by volume of the filter media mixture. A number of granular amendments have been demonstrated to improve nutrient removal from discharge water in BMPs such as bioretention systems, stormwater planters, absorbent landscapes, sand filters or green roofs.

There are two primary processes involved, chemical precipitation and adsorption. Both mechanisms are ultimately finite, but have been shown in come cases to make significant improvements on the discharged water quality over several years.

In our effort to make this guide as functional as possible, we have decided to include proprietary systems and links to manufacturers websites.
Inclusion of such links does not constitute endorsement by the Sustainable Technologies Evaluation Program.
Lists are ordered alphabetically; link updates are welcomed using the form below.

Filter Media Additives
Material Benefits Potential concerns
Biochar Renewable
Enhances soil aggregation, water holding capacity and organic carbon content
Currently expensive
Energy intensive to produce
Some sources say ineffective for phosphorus removal
Bold & GoldTM Documented total phosphorus removal of up to 71%[1] Proprietary
Iron filings or Zero valent iron (ZVI) Proven phosphorus retention
Retained phosphorus is stable
May harm plants[2]
Removal efficiency declines with increased concentration of incoming phosphorus
Red sand or Iron-enriched sand Proven phosphorus removal
Also removes TSS
Poor orthophosphate removal in hypoxic or anoxic conditions
Smart SpongeTM Removes phosphorus, as well as TSS, fecal coliform bacteria and heavy metals
1-3 year lifespan, after which the product is removed as solid waste
Sorbtive MediaTM High phosphorus removal efficiency Proprietary
Water treatment residuals Waste product reuse Quality control (capabilities depend on source, treatment methods, storage time)

Phosphorus testing in media[edit]

To help ensure LID BMPs sustain healthy vegetation cover while not contributing substantially to nutrient loading of receiving waters, the quantity of extractable (i.e., available) P in the soil component needs to be measured and compared to design specifications or acceptance criteria. For BMPs including: bioretention, enhanced grass swales, vegetated filter strips, absorbent landscapes, green roofs and, bulk materials including: compost, and topsoil, phosphorus (P) should be measured as extractable phosphorus. Extractable P is the portion that is easily available to organisms like plants and algae i.e. is the measure of immediate concern to water quality.

The quantity of extractable P is determined through acid or base extraction of a sample and testing the concentration in solution by a soil testing laboratory. Commonly used extraction methods on soil samples are the Bray and Kurtz P-1 procedure for non-calcareous soil [3]or the Sodium Bicarbonate (Olsen) method for calcareous soil [4].

The Olsen method is recommended as the default to use for typical Ontario soils [5]. Calcareous soils are mostly or partly composed of calcium carbonate (i.e., lime or limestone). The Olsen extraction method should be used if the soil contains more than 2% calcium carbonate [6]

For green roof media, the Saturated Media Extract (SME) method should be used [7]. In this extraction procedure, a sample of the media is saturated with deionized water containing a small amount of Pentetic acid (DTPA) to enhance extraction of micro-nutrients [8]. This SME procedure should also be used to measure concentrations of soluble salts and nitrogen in green roof growing media.


Construction inspections[edit]

If laboratory testing indicates the extractable phosphorus concentration is not within the design or product specification range, notify the supplier, issue a “do not install” order to the construction site supervisor and contact the design professionals and property owner or project manager to determine corrective actions.

Assumption and Verification inspections[edit]

For media found through testing to be below the design or product specification range, or Acceptance Criteria range, corrective actions are only needed if problems with vegetation cover, condition or composition (i.e., dominance by weeds) are also detected through visual inspection. Where vegetation cover is poor, unhealthy or dominated by weeds and soil P is lower than the design specification or Acceptance Criteria, schedule investigative work to do further sampling and testing to determine the affected area and depth and decide on corrective actions. Depending on the findings, corrective action could involve amending the soil with compost or other fertilizer.

Amendments to green roof growing media to address P deficiency should be prescribed by the media manufacturer or product vendor. Where soil P concentration is found to be higher than the Acceptance Criteria range, and the BMP drains to a nutrient sensitive receiving water, continuous monitoring during natural or simulated storm events should be undertaken that includes sampling and testing of nutrient concentrations (i.e., Phosphorus and Nitrogen) in sub-drain or surface flows from the BMP to evaluate if the exceedance is negatively impacting effluent quality and if corrective actions are warranted. Corrective action could involve incorporating a soil additive that increases phosphorus retention, or replacement of part or all of the media or topsoil with material that is within the design or product specification.


  1. Hood A, Chopra M, Wanielista M. Assessment of Biosorption Activated Media Under Roadside Swales for the Removal of Phosphorus from Stormwater. Water. 2013;5(1):53-66. doi:10.3390/w5010053.
  2. Logsdon SD, Sauer PA. Iron Filings Cement Engineered Soil Mix. Agron J. 2016;108(4):1753. doi:10.2134/agronj2015.0427.
  3. Bray, R.H. and Kurtz, L.T. 1945. Determination of total, organic, and available forms of phosphorus in soils. Soil Science, 59: 39-45.
  4. Olsen, S.R., Cole, C.V., Watanabe, F.S., and Dean, L.A. 1954. Estimation of available phosphorus in soils by extraction with sodium bicarbonate. USDA Circular 939:1-19. Gov. Printing Office Washington D.C.
  5. Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). 2006. Soil Fertility Handbook. Publication #611. Toronto, Ontario.
  6. Frank, K., Beegle, D., Denning, J. 2012. “Phosphorus” In Recommended Chemical Soil Test Procedures for the North Central Region. North Central Regional Research Publication No. 221. Missouri Agricultural Experimental Station.
  7. Green Roofs for Healthy Cities. 2011. Advanced Green Roof Maintenance: Participant’s Manual. Toronto, ON.
  8. Warnacke, D. 1995. “Recommended Test Procedures for Greenhouse Growth Media.” In J. Thomas Sims and A. Wolf, eds., Recommended Soil Testing Procedures for the Northeastern United States. Northeast Regional Bulletin #493. Newark: Agricultural Experiment Station, University of Delaware.