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Characterizing the phosphorus forms extracted from soil by the Mehlich III soil test
Barbara J Cade-Menun1, Kyle R Elkin2, Corey W Liu3
1Swift Current Research and Development Centre, Agriculture and Agri-Food Canada, Box 1030, Gate 4, Airport Drive, Swift Current, SK, S9H 3X2, Canada. barbara.cade-menun@agr.gc.ca.
Geochemical Transactions
|February 23, 2018
Summary
The Mehlich III soil test may overestimate plant-available phosphorus (P) when using ICP analysis due to unreactive organic P forms. New methods using P-NMR and MS offer more accurate P characterization in soils.
Area of Science:
- Soil Science
- Analytical Chemistry
- Agricultural Science
Background:
- Soil phosphorus (P) is crucial for crop production, with soil testing guiding fertilizer application.
- The Mehlich III (M3) test is standard in North America, but colorimetric and ICP-MS analyses yield differing P results.
- Organic P forms in M3 extracts are not fully characterized, leading to potential inaccuracies in plant-available P assessment.
Purpose of the Study:
- To characterize phosphorus (P) forms in Mehlich III (M3) soil extracts using advanced analytical techniques.
- To investigate the presence and impact of molybdate unreactive P (MUP) on P measurements.
- To evaluate the role of soil amendments, like poultry litter, on the extraction of organic P forms.
Main Methods:
- Utilized 31P nuclear magnetic resonance (P-NMR) and mass spectrometry (MS) to identify and quantify P forms in M3 extracts.
- Confirmed M3 extraction efficiency for various P forms using NaOH-ethylenediaminetetraacetic acid (EDTA) extracts.
- Analyzed P forms in soils with varying fertilization histories, including alum-treated poultry litter.
Main Results:
- Identified significant concentrations of organic P forms, particularly myo-inositol hexaphosphate (myo-IHP), in M3 extracts alongside orthophosphate.
- Demonstrated that molybdate unreactive P (MUP), including myo-IHP, is measured by ICP but not colorimetry, potentially inflating plant-available P estimates.
- Observed variations in myo-IHP extraction linked to soil properties and cation content, with increased extraction from soils treated with alum-treated poultry litter.
Conclusions:
- ICP analysis of M3 extracts may overestimate plant-available P in soils with high MUP concentrations.
- P-NMR and MS are complementary techniques for comprehensive soil P characterization, providing insights beyond single-method analysis.
- Accurate soil P assessment requires considering both inorganic and organic P fractions and the limitations of current analytical methods.
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