Importantly, conventional classification of organic phosphorus bioavailability based on chemical solubility is misleading, because plants can obtain phosphorus from supposedly ‘stable’ fractions of the soil organic phosphorus [65] and [66]. Furthermore, the unextractable fraction is often assumed to be organic phosphorus, yet there is no direct evidence for this. This can be investigated in future studies by subjecting the residual fraction to hypobromite oxidation and solution 31P NMR spectroscopy to determine the presence of higher-order inositol phosphates [67] and [68], or by solid-state 31P NMR to determine the possible presence of phosphonates [69].