Phosphorus (P) is a central element to life on Earth. Living organisms are dependent on a persistent supply of P as it is crucially involved in most major metabolic processes, e.g. in energy transfer as adenosine triphosphate (ATP). Likewise, plants rely on P to secure energy production in photosynthesis (Smil 2000, Ruttenberg 2009). Therefore, the fundamental significance of P in agriculture and food production is beyond question.In nature, we can identify several P pools (Smil 2000, Ruttenberg 2009). The largest P reservoir in the lithosphere occurs in marine and freshwater sediments (about 800-4000 × 10 6 Mt P). The entire phytomass (terrestrial and marine; about 570-625 Mt P) amounts to over 90% of P stored in the totality of all living organisms. Gaseous compounds of P are not stable; therefore, only an insignificant amount (0.028 Mt P) exists in the atmosphere as dust and sea spray picked up by wind erosion. The flows of P between these reservoirs are slow (one cycle may take more than 1 million years), which is caused by the low solubility of phosphate in water and the lack of an airborne reservoir.In soils, P derives mainly from weathering of the primary mineral apatite (Schlesinger 1997). The average total P in soils ranges from 200 mg/kg (in older/highly weathered soils) to 800 mg/kg (in younger/less developed soils) (Cross and
ABSTRACTPhosphorus (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: H 2 O, CaCl 2 , LiCl, Olsen, Bray and Kurtz II (Bray II), Mehlich 3, calcium-acetate-lactate (CAL), iron oxide impregnated filter papers (Fe-oxide P i ), 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 H 2 O < CaCl 2 < LiCl < Fe-oxide P i < 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.
