The longrange goal of this study was to develop a portable, soil nitrate sensor to determine soil nitrate contentinsitu. The immediate objective of this study was to develop a rapid technique to determine soil nitrate content using diffusereflectance spectroscopy in the midinfrared (MIR) range. A Fourier Transform Infrared (FTIR) spectrophotometer was usedto determine the MIR response of various concentrations of calcium nitrate solution in water. The results clearly showed theexistence of a strong nitrate absorption peak at 7194 nm (1390 cm 1 ). For KBrdiluted soil samples, the ratio of the area underthe nitrate absorbance peak (13601390 cm 1 ) to the water absorbance peak (16401660 cm 1 ) was proportional to nitrateconcentration. Midinfrared diffuse reflectance spectroscopy detected soil nitrate content at low concentrations inKBrdiluted soil samples. However, when undiluted soil samples containing varying amounts of nitrate content were testedin the MIR range, it was necessary to further condition the data with noise filtering techniques. Three noise removaltechniques were compared to improve the spectral quality: (1) the Savitzky and Golay, (2) FFT filtering, and (3) waveletdenoising. The wavelet denoising technique resulted in the highest coefficient of determination (R 2 = 0.856) for nitrateprediction. These noise removal techniques could not remove specular reflection effects and band overlap. To overcome theseproblems, a continuous wavelet transform, which could remove specular reflection as well as minimize noise, was used todeconvolute the soil spectral data, resulting in a high correlation (R 2 = 0.878) between soil nitrate content and MIR diffusespectral reflectance.
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Ehsani et al. (2001) studied this question.