Phosphorus is a naturally-occurring element that is critical for animal and plant life functions.  Phosphorous compounds form the helical skeleton of DNA! It is found in animal urine and excreta, in bones, guano, in mineral rock, and is a key ingredient in many fertilizers. It is also found in innumerable manufactured compounds we use every day, from detergents to herbicides to steel.

In the environment, excess phosphorus run-off from fertilized lawns and farmland and sewage discharge can lead to too much plant and algae growth in streams, rivers, lakes, and the ocean. This "eutrophication" uses up available oxygen dissolved in the water, killing off higher forms of aquatic animal life.

Monitoring phosphorus discharge is commonly required for industrial and municipal waste water treatment plants.  Phosphorus tests are also frequently used by lakes and streams monitoring programs as a general measure of health (see EPA's Monitoring and Assessing - Phosphorus)

What MEL Tests for:

TOTAL PHOSPHORUS (TP): Includes the total amount of all forms of phosphorus in solution (reactive) and in particulate form. Only a portion of the TP is readily available for use in plant and algal growth.

ORTHO-PHOSPHATE (OP): The form of phosphorus that is immediately available for use by algae and other aquatic plants for growth, aka "Reactive Phosphorus", is
dissolved inorganic phosphate (orthophosphate).

TOTAL HYDROLYZABLE PHOSPHOROUS  - phosphorus in the sample as measured by the sulfuric acid hydrolysis procedure, and minus predetermined orthophosphates. This hydrolyzable phosphorus includes polyphosphates plus some organic phosphorus.

TOTAL ORGANIC PHOSPHOROUS - phosphorus (inorganic plus oxidizable organic) in the sample as measured by the persulfate digestion procedure, and minus hydrolyzable phosphorus and orthophosphate.

DISSOLVED PHOSPHOROUS (P-D) - all of the phosphorus present in the filtrate of a sample filtered through a phosphorus-free filter of 0.45 micron pore size and measured by the persulfate digestion procedure.


DISSOLVED HYDROLYZABLE PHOSPHOROUS  - as measured by the sulfuric acid hydrolysis procedure and minus predetermined dissolved orthophosphates.

DISSOLVED ORGANIC PHOSPHOROUS  - as measured by the persulfate digestion procedure, and minus dissolved hydrolyzable phosphorus and orthophosphate.





AVAILABLE PHOSPHORUS  Available P = (Total P) – (Citrate insoluble P)

Diagram from Stable Isotope Ecology Laboratory, University of Georgia (USA).

-some definitions reprinted from "Southern Minnesota Tile Monitoring Project," Pallardy/Jarcho, Minnesota State University, Mankato, 2008 and from EPA Method 365.3

Health Limits

Maine Maximum Exposure Guideline for Drinking Water (MEG): 0.1 ppb (= 0.1 ug/L)

United States Environmental Protection Agency EPA Health Advisory (DW): 0.1 ppb (= 0.1 ug/L)

Additional Resources

National Aquatic Resource Surveys / Indicators: Phosphorus

Water Research Center: "Total Phosphorus and Phosphate Impact on Surface Waters" (website)

Toxic Substances Portal: "White Phosphorus" (website)

COOL MAPS! EPA's Nitrogen and Phosphorus Pollution Data Access Tool (NPDAT):

BACK TO BASICS: "The Phosphorous Cycle", Khan Academy (video, 5:30)

MEL Test Methods for Phosphorus

Aqueous:  EPA 365.3 (NPDES compliant)

Available Phosphorous (solids)

EPA 6010B or C (RCRA compliant)

Sample Requirements

Container: plastic, glass

Hold time:

Total P:  28 days

Ortho-P: 48 hours

Available-P (solid): 6 months,


Total P: Preserved with 1:1 H2SO4 to pH < 2 and cooled to <6°C. Add approximately 2 mL per 500 mL sample.

Ortho-P: field filtered and cooled to <6°C.

Available-P: Cool to to <6°C.

Filtration in Field: Dissolved forms of phosphorus and ortho-phosphate are analyzed after filtration with a 0.45 um disc filter (preferably within 15 minutes of sampling and prior to preservation).

Image result for eutrophication maine sabattus

Algal blooms in Sabattus Pond. By Natural Resources Council of Maine, 2013.

Fluorapatite from Mount Apatite, Auburn, Androscoggin County, Maine

Phosphorus-containing fluorapatite from Mount Apatite in Auburn, Maine (from JohnBetts Fine Minerals)