Characterizing the phosphorus forms extracted from soil by the Mehlich III soil test
Citation
Cade-Menun, B.J., Elkin, K.R., Liu, C.W., Bryant, R.B., Kleinman, P.J.A., Moore, P.A. (2018). Characterizing the phosphorus forms extracted from soil by the Mehlich III soil test. Geochemical Transactions, [online] 19(1), http://dx.doi.org/10.1186/s12932-018-0052-9
Plain language summary
Phosphorus (P) is an essential nutrient for plant growth, but excess P from fertilizers can be lost in runoff from agricultural soils, which can cause water quality problems. Soil testing is used to guide fertilizer recommendations, and a widely used test for P in North America is the Mehlich III (M3) soil test. This involves extracting soil and then measuring the concentration of P in the extracts with one of two methods. However, these methods can give differences in M3 P concentrations, but the reasons for this were not clear. The current study used advanced chemistry techniques to identify the specific forms of P in M3 extracts, instead of just the P concentrations measured by the simpler methods. This showed that that a lot of the P in M3 extracts is not in a form that plants can use, and one of the simple ways to measure P in M3 extracts is better than the other method for measuring the amount of P that plants can use. Fertilizer recommendations should be based on the method that was better for measuring the concentration of P that plants can use, to avoid applying too much or too little P fertilizer.
Abstract
Phosphorus (P) can limit crop production in many soils, and soil testing is used to guide fertilizer recommendations. The Mehlich III (M3) soil test is widely used in North America, followed by colorimetric analysis for P, or by inductively coupled plasma-based 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. 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 (molybdate unreactive P, MUP). Extraction of these P forms by M3 was confirmed by P-NMR and MS in NaOH-ethylenediaminetetraacetic acid extracts of whole soils and residues 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. 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 soils fertilized with alum-treated poultry litter than untreated 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 both colorimetry and ICP. This study also tested procedures that will improve future soil P-NMR studies, such as treatment of acid extracts, and demonstrated that techniques such as P-NMR and MS are complimentary, each yielding additional information that analysis by a single technique may not provide.