Nutrients - Nitrogen, Phosphorus, and Silica

Nutrients is the general term given to certain compounds essential for growth of living organisms. The most important of these are nitrogen and phosphorus compounds. They are also excreted by living organisms and can build up in the environment as waste products, food residues, industrial and agricultural by-products, and contaminants. They can make their way into water ways by run-off and other means.  This can contribute to the growth of photosynthetic aquatic micro and macro organisms which can cause significant health and environmental issues. 


All of the forms of nitrogen described here are components of the nitrogen cycle and are biochemically interconvertible.

“Total Kjeldahl Nitrogen” (TKN) is a high temperature acid-digest process that measures organic nitrogen and ammonia. Add nitrate and nitrite (NOx) and we get what is termed “Total Nitrogen”.

  • Ammonia (NH3) occurs in natural waters and in wastewaters.  Ammonia is a component of the nitrogen cycle and is produced largely by deamination of organic nitrogen-containing compounds and by hydrolysis of urea.  Its concentration is generally low in groundwater because it adsorbs to soil particles and clays and is not leached readily from soils.  At some water treatment plants ammonia is added to react with chlorine to form a combined chlorine residual. 
  • Nitrite (NO2) occurs in natural waters, water distribution systems and wastewater treatment plants as a result of chemical oxidation and reduction processes.  Nitrite is an intermediate oxidation state of nitrogen, being produced when ammonia is oxidised and when nitrate is being reduced, and thus does not usually occur in large quantities or for very long.  Nitrite can enter a water supply system through its use as a corrosion inhibitor in industrial process water. 
  • Nitrate (NO3) also occurs in natural waters, water distribution systems and wastewater treatment plants as a result of chemical oxidation and reduction processes.  Nitrate generally occurs in trace quantities in surface water but may attain higher levels in some ground waters.  It is an essential nutrient for many photosynthetic autotrophs and in some cases has been identified as the growth-limiting nutrient. 

Phosphorus occurs in natural waters and in wastewaters almost solely as phosphates (PO4).  Dissolved reactive phosphorus (DRP) is analysed on the filtered part of a water sample.

At AST our dissolved nutrients suite includes ammonia (NH3), dissolved reactive phosphorus (DRP), nitrate (NO3), nitrite (NO2), and nitrate + nitrite (NOx). All of these analytes are reported as either mgN/L or mgP/L. We utilise modern flow injection analyser (FIA) instrumentation for these automated colourimetric analyses.


AST’s NATA-accredited low-level methods for environmental monitoring programs deliver the following reporting limits:

Water AnalyteReporting limit concentration (mg/L)Measurement Uncertainty​
Ammonia (NH3)0.005 mgN/L​±15% or ±0.005 mgN/L, whichever is greater
Nitrate (NO3)0.002 mgN/L​±15% or ±0.002 mgN/L, whichever is greater
Nitrite (NO2)0.002 mgN/L±10%​ ±0.002 mgN/L, whichever is greater
Nitrate + Nitrite (NOx)0.002 mgN/L​±15% ±0.002 mgN/L, whichever is greater
Dissolved Reactive Phosphorus (DRP)0.003 mgP/L​±20% ±0.003 mgP/L, whichever is greater
Total Kjeldahl Nitrogen (TKN)0.10 mgN/L​±23% ±0.10 mgN/L, whichever is greater
Total Nitrogen (TN)0.10 mgN/L​±23% ±0.10 mgN/L, whichever is greater
Total Phosphorus (TP)0.01 mgP/L​±28% ±0.01 mgP/L, whichever is greater

Recommended requirements for taking nutrient samples (sample bottle types, preservation, filtration etc.) is available on our Nutrients Information Sheet.

Silica (SiO2) is important to phytoplankton communities.  Silica is a food source for zooplankton and filter feeders and can become depleted in waters if eutrophication occurs.  Silica can be also be found in ground waters, depending on the parent rock material.

At AST we offer NATA-accredited SiO2 colourimetrically by automated discrete analyser. Ideally samples should not be frozen before analysis. Results are reported as mg SiO2/L with a reporting limit of 0.1 SiO2/L. Measurement uncertainty for Silica has been calculated to be ±12% or ±0.05mg/L SiO2, whichever is greater.  

Samples for silica analysis should be taken in a blue labelled bottle or vial.

Nutrients in soils and sediments

AST offers NATA accredited nutrient analyses in soils, sediments, and biosolids. The tests on offer are similar to the waters suite: Ammonia, Nitrate, Nitrite, Nitrate + Nitrite, Total Kjeldahl Nitrogen, Total Nitrogen, Total Phosphorus, Bicarbonate extractable phosphorus, and water extractable Phosphorus.

We offer potassium chloride (KCl) extractions (1:10 or 1:5) or water extractions (1:10 or 1:5) for the nitrogen species, and bicarbonate extractions (1:100) or water extractions for phosphorous. The total nutrients analysis process comprises a high temperature acid-digest (Kjeldahl) followed by flow injection (FIA) automated colourimetric analysis. Other methodology may be accommodated or tailored for your specific requirements, please contact us if tailored analyses may be of use to your testing program.

Reporting limits for soil/sediment analytes vary, depending on the moisture content of the sample. Samples can be taken in glass sample jars and kept chilled while in transit to the laboratory.


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