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The Danish protocol for an Olsen soil phosphorus test gets a boost

Researchers from Aarhus University have proposed concrete improvements to the present Danish protocol for the Olsen soil phosphorus test.

[Translate to English:] Der er kommet nye anbefalinger til analyse af fosfor i jordprøver. Foto: Colourbox

The method for determining the phosphorus status of soil must be robust and provide reliable and reproducible results no matter who carries out the analysis and when the analysis is carried out. Researchers from, among others, the Department of Agroecology at Aarhus University have previously pointed out that the protocol that is presently used in Denmark does not live up to these requirements.

At the request of the Ministry of Environment and Food of Denmark, the researchers have therefore collaborated with the three major soil laboratories in Denmark, Seges, University of Copenhagen and representatives from the authorities on preparing a new protocol for bicarbonate extraction of soil phosphorus, often referred to as the “Olsen P test”. The results have been published in a report from DCA – Danish Centre for Food and Agriculture at Aarhus University.

The report describes how variations in key elements of the analytical procedure influence the test results and presents concrete recommendations as well as a new and updated protocol for the analysis. The key elements comprise the following:


  • The temperature at which the soil sample is dried prior to analysis
  • The amount of soil used for one analysis
  • How long and how the sample should be shaken
  • The temperature in the extraction process 
  • Handling time at certain steps in the procedure

Analysis of soil phosphorus using bicarbonate extraction as described in a Danish protocol, almost identical with the original Olsen P test, has since 1987 been the basis for phosphorus fertiliser recommendations in Denmark. Increasing awareness of the role that soil phosphorus plays for crop growth and the risk of loss to and eutrophication of surface water has led to increased focus on the applicability and quality of this method and the way it is currently carried out.

For years it has been known that the results of the Olsen P analysis in Denmark vary systematically depending on when and by which laboratory the analysis is carried out.  

In Denmark we obviously needs initiatives that can lead to an improved quality of the Olsen P test – not least now, when the analysis is used not only for recommendations but also in connection with regulation of phosphorus application in Denmark. Farmers, advisers, researchers and the authorities are all calling for this, says Associate Professor Gitte Rubæk from the Department of Agroecology, Aarhus University.

The report “Protocol for bicarbonate extraction of inorganic phosphate from agricultural soils”, DCA rapport no. 102, 2017 is published by DCA – Danish Centre for Food and Agriculture and can be downloaded here.  

For more information please contact: Associate Professor Gitte Holton Rubæk, Department of Agroecology, email: gitte.rubaek@agro.au.dk, telephone: +45 8715 7686, mobile: +45 9350 8076


Sustainable Nutrient Management is one of the research areas in which the Department of Agroecology is particularly strong and from which results are delivered in line with national and global societal challenges and goals.