Investigations Into the Nature of Phosphorus in Soil Humic Acids Using 31P NMR Spectroscopy

Abstract

Phosphorus-31 NMR was used to analyse humic acids (HAs) extracted using different conditions. Two soil samples from Western Australia were studied (ROl and RO3) and both had significant iron and aluminium contents. The HAs isolated from these soils in 0.1M neutral sodium pyrophosphate solution had high ash contents. A further sample of HAs isolated from RO3, using a sodium hydroxide solution (0.1M, under N2), had a much lower ash content. A HA sample isolated from the Bh horizon of a podzol from Co. Mayo, Ireland, using the neutral sodium pyrophosphate solution (following an acid wash of the soil) had lower ash and phosphorus contents than the samples from the RO. The nature of the phosphorus in these HA samples, and in a special NaOH/HCl extract of RO3, was studied by 31P NMR. A link was observed between the ash contents and the residual pyrophosphate contents. It is concluded that when pyrophosphate is used as an extractant the HA fractions will be contaminated with the pyrophosphate unless the metal complexes and the (hydr)oxides associated with the HAs are removed.