Spectral Signatures of Common Phosphates in Soils and Their Effect on Absorbance Spectra of Soil Samples with Different Phosphorus Concentrations

Abstract. Phosphorus in the environment is mostly bound with iron, magnesium, calcium, and aluminum (Fe, Mg, Ca, and Al, respectively). This study aimed to determine both spectral signatures of Fe, Mg, Ca, and Al associated phosphates and analyze their spectra with soil samples. Obtaining most common spectra of the phosphates and their spectral signatures within a soil sample can be used for determining soil inorganic phosphorus (P) concentration. Four different P-compounds (CaPO4, AlPO4, FePO42H2O, Mg3(PO4)22H2O) at 0,12.5, 62.5, 175, 375, 750, and 1000 mg/kg were investigated. Reflectance spectra of the four individual phosphates and different concentrations of the compounds within leached sandy soil samples were measured using UV, VIS, and NIR spectroscopy in a range of 225-2525 nm with an interval of 1 nm. In order to build a calibration model and reduce the number of selected wavelengths, correlation coefficient, stepwise multiple linear regression, and stepwise discriminat analysis were applied to data set. With selected wavelengths, discriminant analysis was conducted to classify the types of phosphates in soils.