Improving Phosphorus Sensing by Eliminating Soil Particle Size Effect in Spectral Measurement

This study investigated the effects of soil particle size on the reflectance spectra of sandy soils using ultraviolet, visible, and near-infrared spectroscopy in sensing phosphorus (P) concentration. Pure sandy soil was graded into three particle sizes. Sieve sizes were 125, 250, and 600 m for fine, medium, and coarse, respectively. Phosphorus application rates for the soil samples were 0.0, 12.5, 62.5, 175.0, 375.0, 750.0, and 1000.0 mg kg−1. Concentrations of P in the soil samples were analyzed. The reflectance of the samples was measured between 225 and 2525 nm at 1 nm intervals. Overall, soils with coarse particles absorbed light more than those with medium and fine particles. Detection analysis for soil particle sizes was conducted using ratio and discriminant analysis methods. Prediction analyses for P concentration were performed using multiple linear regression (MLR; stepwise and maximum R2 methods) and linear partial least squares (PLS). Results showed that detection of the particle size in a spectrum and then the prediction of P using individual calibration models for each soil particle size produced lower prediction errors. For the maximum R2 MLR, stepwise MLR, and linear PLS analyses, respectively, the standard errors of prediction (SEPs) for determining P concentration without removing the particle size effect were 105.8, 106.2, and 69.8 mg kg−1 and after removing the particle size effect were 52.8, 73.4, and 64.4 mg kg−1.