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Soil phosphorus tests need to be improved

Scientists from Aarhus University have investigated one of the most common methods for analysing soil phosphorus in agricultural soils and recommend much-needed improvements.

[Translate to English:] De danske analyser af fosfor i jorden kan forbedres. Foto: Janne Hansen

There is a need for a robust and valid method to analyse soil phosphorus in agricultural soils. The current method in Denmark is a bicarbonate extraction method, often called the Olsen P method. It has long been recognised that results obtained from this method vary too much and apparently systematically between labs and over time.  


DCA – Danish Centre for Food and Agriculture at Aarhus University has published a report that outlines recommendations on how to improve the bicarbonate extraction method in Denmark.


Chemical extraction of soil phosphorus with the aim to roughly estimate plant availability of soil phosphorus has for many years formed the cornerstone for recommendations on how to fertilise agricultural soils. Increasing awareness of the role of soil phosphorus as a contributor to surface water eutrophication together with the renewed focus on phosphate rock as a valuable non-renewable resource has also shone a new light on the robustness and validity of the established soil phosphorus tests for both agronomic and environmental purposes.


The bicarbonate extraction method (known in Denmark as “P-tallet” or “fosfortallet”) is the recommended soil phosphorus test method in Denmark and several other countries. However, close examination of the results from the Danish proficiency test programme has clearly demonstrated that the method lacks robustness; Test results on reference soils vary systematically between labs and over time. Moreover, an increasing body of evidence indicates that this method on some Danish soil types does not reflect plant availability of soil phosphorus.


It is therefore clear that initiatives leading to better soil phosphorus tests in Denmark with high laboratory precision and valid information on soil phosphorus status for use by farmers, researchers and authorities are highly needed.


The report prepared by scientists from the Department of Agroecology on commission from the Environmental Protection Agency and prepared under supervision of a board of stakeholders gives recommendations on how to improve soil phosphorus testing in Denmark and includes:

·         Recommendations for how the quality of soil phosphorus analyses and other soil analyses can be improved and maintained

·         A draught for an updated method description for the Danish version of the bicarbonate extractable soil phosphorus (the present one is outdated and imprecise)

·         Evaluation of the suitability of the Olsen soil P test and other corresponding methods for routine analyses and their validity as advisory tools for crop production, including a description of a potential new method

·         A description of how the P test result can be corrected using reference soils in case the problems with systematic variation of results between labs and over time remains unsolved.


The report “Validity and analytical robustness of the Olsen soil P test and other agronomic soil P tests used in the northern Europe”, DCA report no. 71, 2015 is published by DCA – Danish Centre for Food and Agriculture and can be downloaded here.


You can also read the article “The quality of soil analyses can be improved”.

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