Spectral Phosphorus Mapping using Diffuse Reflectance of Soils and Grass

Phosphorus (P) concentration was determined from reflectance spectra of grass and soils in a total of 150 samples of each collected from three different sites in the Lake Okeechobee drainage basin. The reflectance spectra of both fresh and dried samples for grass and soil were measured in the ultraviolet (UV), visible (VIS), and near infrared (NIR) regions from 225 to 2550 nm with an interval of 1 nm. Phosphorus concentrations of the samples were correlated with the absorbance of the same samples. Two-thirds of both vegetation and soil samples were used for calibration and the remaining one-third of vegetation and soils were used for validation. Stepwise multiple linear regressions (SMLR), and partial least-squares (PLS) analyses were applied to the data sets in order to predict P concentrations for soil and grass. Actual and predicted P concentration maps of the fields for vegetation and soil were plotted. Strong relationships (coefficient of determination R2=0·778, 0·914, and 0·922) in PLS for the validation data sets were obtained between absorbance and P concentrations in soils. However, a weak relationship (R2=0·425) in PLS for the validation data set was produced from absorbance and P concentrations in vegetation samples. Spatial variation in actual and predicted maps showed that P variability could be represented using diffuse reflectance spectroscopy in the UV, VIS, and NIR regions.