Determination of Soil Nitrate and Organic Matter Content Using Portable, Filter-Based Mid-Infrared Spectroscopy

Abstract

Soil nitrate content and organic matter are two important parameters that determine the amount of nitrogen available for plant growth. The goal of this study was to develop soil nitrate and organic matter sensing techniques, since traditional methods of determining soil organic matter and nitrate content are tedious, time-consuming, and expensive. Studies conducted at UC Davis have clearly shown that unmistakable nitrate peaks exist in the mid-infrared (MIR) spectra of soil pastes. In particular, the peak located at 1,390 cm⁻¹ wave number is nitrate sensitive. However, this peak is broad and is influenced by carbonate/bicarbonate and organic matter peaks, which exist in the vicinity of the nitrate peak. Using wavelet analysis, we were able to isolate a single wave number, 1,350 cm⁻¹, at which these interferences were minimal. However, if the soil contained a large amount of carbonates, a second wavelength located at 1,500 cm⁻¹ was necessary to obtain a unique calibration equation. Using the absorbance values at these two wave numbers, nitrate-N concentrations of 14 different soils (6 Californian soils and 8 Israeli ones) could be determined using a single calibration curve with a coefficient of multiple determination (R ² value) of 0.98. When only six Californian field soils were considered, the calibration equation resulted in an R ² value of 0.95 with a standard error of 8 ppm. Moreover, preliminary tests indicated that soil organic matter can also be determined using MIR spectral response at two wave numbers (1,383 and 1,452 cm⁻¹). The R ² value for soil organic matter determination was 0.95 and the standard error was 0.25%. These results indicate that both soil nitrate content and organic matter can be determined using just a few wavelengths in the MIR region with sufficient accuracy for use in site-specific nutrient management.