Spectral analysis of total phosphorus in soils based on its diagnostic reflectance spectra

•Proposed a new way to reveal the diagnostic reflectance spectra of soil P.•Collect the reflectance spectra of different P fractions using soil residues.•Each P fractions had specific spectra, contributing the diagnostic spectra to total P.•A better spectral P model was set up using these diagnostic spectra. Visible and near infrared spectroscopy (VNIR) has been successfully used to determine many soil parameters rapidly. However, total phosphorous (P) cannot be well quantified by VNIR, mainly because its absorption spectrum or reflectance spectrum was too weak to be studied. Here, we explored a new way to reveal the reflectance spectrum of total P in soils. Firstly, six types of soil residues were prepared after the original soil was sequentially extracted Ca2-P, Ca8-P, Al-P, Fe-P, O-P, and Ca10-P, respectively. Secondly, the reflectance spectra of these P fractions were obtained using these soil residues by QE65000 spectrometer (Ocean Optics). Specifically, if we intended to get the spectrum of a certain P fraction, we would measure the reflectance spectrum of the soil residues without its neighboring former P fraction, using the soil residues after extracting this P fraction as the reference spectrum. Results showed that these P fractions had different characteristics of reflectance spectrum and can be well analyzed qualitatively and quantitatively. After summarizing the diagnostic spectra of these P fractions, the total P diagnostic spectrum was found to be distributed in 250–750 nm. Using these diagnostic bands, the spectral model of total P was established and showed a better result. This study provided an important evidence for understanding the characteristics of total P reflectance spectrum in soils and predicting total P concentrations.