Development of Mid-Infrared-based Calibration Equations for Predicting Soil Nitrate, Phosphate, and Organic Matter Concentrations

The purpose of this paper was to develop calibration equations for predicting nitrate, phosphate, and organic matter concentrations in soil pastes using mid-infrared attenuated total reflection (ATR) spectroscopy. Wavelet analysis was used to locate a wavenumber at which the absorbance was due to nitrate alone and not influenced by carbonate or other components in the soil. The absorbance at this location was correlated to the nitrate content. Results of the field experiments gave R2-values as high as 0.99 and standard errors as low as 8 ppm NO3-N for individual experiments. Pooling the absorbance values from all eight laboratory and six field soil spectra together gave a R2-value of 0.98 suggesting that a unique calibration curve exists regardless of soil type. Also, a filter-based, rugged, mid-infrared spectrometer capable of use in a field environment was utilized to develop similar calibration equations for predicting nitrate, phosphorus, and organic matter content in soils. These preliminary results are also presented in this paper.