Submitted to: Trade Journal Publication
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/12/2018
Publication Date: 2/21/2018
Citation: Elkin, K.R., Bryant, R.B., Kleinman, P.J., Moore Jr, P.A., Cade-Menun, B.J. 2018. Characterizing the phosphorus forms extracted from soil by the Mehlich III soil test. Geochemical Transactions. 19:7. https://doi.org/10.1186/s12932-018-0052-9.
Interpretive Summary: Soil testing is primarily used to provide food producers with soil fertility information to optimize crop production while minimizing environmental risks to nearby water. One of the most widely used soil test methods in North America is the Mehlich III test. This test uses a cocktail of different chemicals to extract plant nutrients from the soil with the same strength that plants are supposed to be able to. Once the soil is extracted with these chemicals, they are then analyzed by 2 different methods, one that uses a color change reaction and one that “burns” the sample and looks at the color of the flames to determine what is in it. The main problem with this test, as with many other laboratory methods is that they do not mimic the plant nutrient gathering strength exactly. We took a closer look at the extract process to characterize what it was actually extracting by exposing the extract to a variety of highly specific methods. What we found was that the Mehlich III extraction was removing nutrients from the soil that the plants could not make direct use of and showing them as being plant available. This finding suggests that we have to be cautious about how we analyze these extracts or use several methods to show the variation in non-plant available nutrients.
Technical Abstract: Phosphorus (P) can limit crop production in many soils, but P loss from soils may impair water quality; soil testing can guide fertilizer recommendations to optimize crop growth while minimizing P loss. The Mehlich III (M3) soil test is widely used in North America, followed by colorimetric analysis for P, or by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP) for P and cations. However, differences have been observed in M3 P concentrations measured by these methods. Using 31P nuclear magnetic resonance (P-NMR) and mass spectrometry (MS), we characterized P forms in M3 extracts treated with cation-exchange resin. In addition to the orthophosphate that would be detected during colorimetric analysis, several organic P forms were present in M3 extracts that would be unreactive colorimetrically but measured by ICP. Extraction of these P forms by M3 was confirmed by P-NMR and MS in NaOH-ethylenediaminetetraacetic acid (NE) extracts of whole soils and residue soils after M3 extraction. The most abundant P form in M3 extracts was myo-inositol hexaphosphate (myo-IHP, phytate), a compound that may not contribute to plant-available P if tightly sorbed in soil. The concentrations of myo-IHP and other organic P forms varied among soils, and even among treatment plots on the same soil. Extraction of myo-IHP in M3 appeared to be linked to cations, with substantially more myo-IHP extracted from alum-poultry litter than untreated poultry litter. These results suggest that ICP analysis may substantially over-estimate plant-available P in samples with high MUP concentrations, but there is no way at present to determine MUP concentrations without analysis by colorimetry and ICP. Treating acid extracts with a cation exchange resin prior to pH adjustment preserved organic P compounds that might otherwise be removed by precipitation; this could be used to expand the extraction procedures for soil P-NMR. Comparing NE extracts of whole soils with extracted residues after pre-treatments also provided information that will be useful to refine other soil extractions. This study also demonstrated that techniques such as P-NMR and MS should be seen as complimentary, each yielding additional information that analysis by a single technique may not provide.