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

Abstract

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.

Correlation of extractable soil phosphorus (P) with plant P uptake: 14 extraction methods applied to 50 agricultural soils from Central Europe

“…Basic soil properties were analysed according to standard methods, which are described in detail in Wuenscher et al (2015). Table 1 summarizes selected soil properties and demonstrates the diversity of the soils used in this study; more details are presented in Wuenscher et al (2015).…”

“…Soil P was extracted with 14 different extraction methods that utilize various mechanisms of extraction and, hence, target different soil P pools; a brief summary of the 14 applied methods is given in Table 2, for more details see Wuenscher et al (2015).…”

“…The photometric determination of P extracted from soils and after plant digestion was conducted with the molybdenum blue method according to Murphy and Riley (1962). This method was further modified to allow detection of low P concentrations (detection limit of 25 µg P/L) needed for some of the samples/ extracts; the modifications are described in detail in Wuenscher et al (2015).…”

“…In a previous paper, the results of 14 different soil P extraction methods applied to 50 agricultural soils from Central Europe were compared (Wuenscher et al 2015). It was shown that the different methods extract distinct pools of soil P, and that the extractability of a given pool is influenced by different soil properties to different extents.…”

Microwave induced plasma optical emission spectrometry in agricultural analysis

“…Comparison studies between different extractants have shown that M-3 can be adapted as a suitable extractant for routine soil analysis (Alva 1993). The Mehlich 3 solution (Wuenscher et al 2015) is an agrochemical extractant used for determination of bioavailable P, Ca, Mg and K for soil fertility assessment in agriculture (with additional potential to determine content of some micronutrients). Extractability of phosphorus is improved by addition of ammonium fluoride and acetic acid.…”