Spectral Measurement of Common Soil Phosphates

Compounds of Al, Fe, Ca, and Mg phosphates were particularly found in the Lake Okeechobee drainage basin in Florida. Identification of the spectral characteristics of these compounds would improve sensing of phosphorus (P) concentration in soil samples. This study investigated the effects of common soil P compounds on reflectance spectra of sandy soils using ultraviolet (UV), visible (VIS), and near-infrared (NIR) reflectance spectroscopy. Pure sandy soil was leached to remove all nutrients and organic matter. Effects of four different P compounds (CaPO4, AlPO4, FePO42H2O, and Mg3(PO4)22H2O) at 0.0, 12.5, 62.5, 175.0, 375.0, 750.0, and 1000.0 mg kg−1 were investigated. Actual P concentrations of the soil samples were analyzed. Reflectance of the samples was measured between 175 and 2550 nm with 1 nm intervals. The highest absorbance peaks were found at 286, 2548, 2516, and 228 nm for FePO42H2O, Mg3(PO4)22H2O, CaPO4, and AlPO4, respectively. These wavelengths may be used for a calibration to detect or determine phosphates of soils. Correlation coefficients, standard deviations, and first derivatives of absorbance spectra were computed. Wavelengths were selected using a stepwise discriminant analysis to build calibration models for P prediction. Classification of P compounds was tested using discriminant analysis. The results indicated that the Fe, Ca, Mg, and Al associated phosphates could be detected with a classification error of 1.9%. Partial least squares (PLS) analysis results for the dry soil samples with four compounds yielded R2 of 0.48 to 0.75 and root mean square error (RMSE) of 27 to 43 mg/kg.