Iron oxide impregnated filter paper (Pi test): a review of its development and methodological research

Iron oxide impregnated filter paper (FeO paper) has been used to study the availability of phosphorus (P) to plants and algae, P desorption kinetics and P dynamics in the field. Since its initial development a number of differences in the method of preparation of the paper and its application have been implemented. The purpose of this article is to (i) critically review studies on the various aspects of both preparation and use of the FeO paper and (ii) to suggest a standardized procedure. Type of filter paper, FeO impregnation of the filter paper, neutralization of FeCl3, washing and drying of the impregnated paper, all have a significant effect on the amount of P sorbed. It is suggested that the method for neutralization of FeCl3 is checked on the release of acid when FeO paper is used, because the acid can release P compounds that would otherwise not desorb. Washing after neutralization is essential to remove loose FeO particles, which otherwise will cause incomplete P recovery. Modifications of the desorption procedure itself also affect the amount of P sorbed onto the paper. The ionic strength of the shaking solution, surface area of the FeO paper and the number and position of the paper in the shaking solution, and shaking strength and time all may introduce variations in the amount of P on the FeO paper. Differences in the dissolution of the Fe compounds on the filter paper and the subsequent determination of P in the solute may contribute to variations in P recovery. It is illustrated that the existing variation in the preparation and use of the FeO paper will cause differences in the amount of P extracted from soils, impeding the comparison of different studies. Finally, we recommend a standardized procedure for the preparation and use of the FeO paper. Such a procedure will allow comparison between studies and may reduce or circumvent possible artefacts.