Characterizing the phosphorus forms extracted from soil by the Mehlich III soil test

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.

The structural composition of soil phosphomonoesters as determined by solution 31P NMR spectroscopy and transverse relaxation (T2) experiments

“…Whilst this signal would be considered to have a relatively slow decay due to its narrow line-width, the peak is similar to that reported in large molecular weight fractions (> 10 kDa) by McLaren et al (2015a). Its presence has also been suggested in unfractionated soil extracts (Cade-Menun et al, 2018). Our results support its association with organic P in large molecular weight fractions and suggest that it should be considered in relation to the broad signal.…”

Phosphorus Leaching from Soil Cores from a Twenty‐Year Study Evaluating Alum Treatment of Poultry Litter

“…However, the ICP measures all forms of P, including the colorless organic phytate compound, which explains the higher M3‐P ICP values for alum‐treated litter. The presence of phytate and other inositol hexakisphosphate compounds in soil M3‐P extracts was recently confirmed by Cade‐Menun et al (2018).…”

“…However, the ICP measures all forms of P, including the colorless organic phytate compound, which explains the higher M3-P ICP values for alum-treated litter. The presence of phytate and other inositol hexakisphosphate compounds in soil M3-P extracts was recently confirmed by Cade-Menun et al (2018). Another explanation for the differences observed in P concentrations between colorimetry and ICP is the presence of colloidal-sized Al-P minerals in soil.…”

Kinetics Growth and Recovery of Valuable Nutrients from Selangor Peat Swamp and Pristine Forest Soils Using Different Extraction Methods as Potential Microalgae Growth Enhancers

“…This implies that soil extraction condition significantly affected the recovery of nitrogen. Moreover, [ 33 ] in their study has reported TNP recovery are 39% to 97%. DOC studied by [ 34 ] implies a higher recovery accounted for almost 58.8–84.5% of the total DOC.…”