An examination of potential extraction methods to assess plant-available organic phosphorus in soil

The role of soil organic phosphorus (P) in plant nutrition was assessed using data from a glasshouse pot experiment carried out on seven soil types using two contrasting plant species (Lolium perenne, Pinus radiata) and 12 different extractants (five salts (0.025 M ethylenediaminetetraacetic acid (EDTA), 0.025 M EDTA pH 7, Olsen, Mehlich-III, and 6% NaOCl pH 7.5) and seven exchange resins (Hampton chelating resin, Bio-Rad Chelex-100, Dow MAC-3, Amberlite IRC76, Diaion WT01S, Lewatit MP500A, Diaion WA30)). The contribution from mineralization of soil organic P was inferred by consistent increases in correlation coefficients between extractable P and plant P uptake when organic P was considered in addition to inorganic P. The best correlated extractants for combined inorganic and organic P were NaOCl (r = 0.84), Hampton chelating resin (r = 0.78), and MP500A resin (r = 0.73), which compared favorably with Olsen P (r = 0.66) and EDTA (r = 0.72). ³¹P nuclear magnetic resonance analysis of selected extracts from two soils confirmed that the Hampton-chelating-resin-extractable P was mainly monoester and diester forms of organic P, while there was no monoester or diester organic P in the IRC76 resin extract—poorly correlated with plant uptake. The findings of this study suggest that readily extractable forms of organic P in soil contribute to short-term plant P uptake, and this P should be considered for inclusion in routine tests for soil P availability.