Inositol phosphates are the dominant class of organic phosphorus (P) compounds in most soils, but are poorly understood because they are not easily identified in soil extracts. This study reports a relatively simple technique using solution31P NMR spectroscopy and spectral deconvolution for the quantification ofmyo-inositol hexakisphosphate (phytic acid), the most abundant soil inositol phosphate, in alkaline soil extracts. An authenticmyo-inositol hexakisphosphate standard added to a re-dissolved soil extract gave signals at 5.85, 4.92, 4.55, and 4.43 ppm in the ratio 1:2:2:1. Spectral deconvolution quantified these signals accurately (102 ± 4%) in solutions containing a mixture of model P compounds by resolving the envelope of signals in the orthophosphate monoester region. In NaOH-EDTA extracts from a range of lowland permanent pasture soils in England and Wales, concentrations ofmyo-inositol hexakisphosphate determined by spectral deconvolution ranged between 26 and 189 mg P kg−1soil, equivalent to between 11 and 35% of the extracted organic P. Concentrations were positively correlated with oxalate-extractable aluminum and iron but were not correlated with total carbon, total nitrogen, clay, or the microbial biomass. This suggests thatmyo-inositol hexakisphosphate accumulates in soils by mechanisms at least partially independent of those controlling organic matter stabilization and dynamics. Furthermore,myo-inositol hexakisphosphate concentrations were positively correlated with plant-available inorganic P and negatively correlated with the carbon-to-organic P ratio, suggesting that biological P availability may, in part, regulatemyo-inositol hexakisphosphate concentrations in soils, perhaps because organisms capable of degrading this compound are favored in more P-limited environments. Solution31P NMR spectroscopy and spectral deconvolution offers a relatively simple method of quantifyingmyo-inositol hexakisphosphate in soil extracts.