Phosphorus testing in media

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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 [1]or the Sodium Bicarbonate (Olsen) method for calcareous soil [2].

The Olsen method is recommended as the default to use for typical Ontario soils [3]. 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 [4]

For green roof media, the Saturated Media Extract (SME) method should be used [5]. 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 [6]. This SME procedure should also be used to measure concentrations of soluble salts and nitrogen in green roof growing media.

Evaluation in media[edit]

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. Bray, R.H. and Kurtz, L.T. 1945. Determination of total, organic, and available forms of phosphorus in soils. Soil Science, 59: 39-45.
  2. 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.
  3. Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA). 2006. Soil Fertility Handbook. Publication #611. Toronto, Ontario.
  4. 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.
  5. Green Roofs for Healthy Cities. 2011. Advanced Green Roof Maintenance: Participant’s Manual. Toronto, ON.
  6. 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.