A comparison of 14 soil phosphorus extraction methods applied to 50 agricultural soils from Central Europe

Abstract

Phosphorus (P) fertilization is commonly based on soil testing, for which a variety of different soil P extraction methods are in use. The aim of this study was to compare 14 soil P extraction methods in terms of their extraction yield and their relation to soil properties. Fifty contrasting agricultural topsoils were sampled from Austria and Germany. The soils were extracted with the following methods/extractants: H2O, CaCl2, LiCl, Olsen, Bray and Kurtz II (Bray II), Mehlich 3, calcium-acetate-lactate (CAL), iron oxide impregnated filter papers (Fe-oxide Pi), cation and anion exchange membranes (CAEM), acid ammonium oxalate, citrate-bicarbonate-dithionite, HCl, organic P and total P. The extracted P varied over three orders of magnitude and increased in the order H2O < CaCl2 < Fe-oxide Pi < Olsen < CAL < CAEM < Mehlich 3 < Bray II < dithionite < organic P < HCl < oxalate < total P. This sequence is in accordance with previous studies and reflects different extraction mechanisms and P pools. The different extraction methods were generally well correlated, especially when P extraction was achieved by a similar mechanism. The soil properties most influential on P extractability were pH, carbonate content, texture as well as iron oxide content and crystallinity. Our results show that the different extraction methods extract distinct pools of soil P with strongly varying extractability, and that the extractability of a given pool may be influenced by different soil properties to different extents. If and how these relationships translate to plant P uptake requires further examination.