Analytical Evaluation of Mobile In Situ Soil Nitrate Infrared Sensor Designs for Precision Agriculture

One approach to improving nitrogen fertilizer use efficiency in crop production is to use infrared (IR) soil nitrate sensors to analyze soil nitrate and then spatially modulate fertilizer application rates. The most commonly used Fourier transform IR (FTIR) spectrometers are sensitive to vibration and hence not suitable for real-time measurements in agricultural fields when mounted on tractors that experience severe vibrations as they are driven across a field. In this paper, we propose IR sensor designs to address the vibration issues currently limiting the use of IR spectroscopy for precision nitrogen fertilizer management in agriculture. We evaluate the performance of different nitrate sensor configurations based on diamond attenuated total reflection (D-ATR) spectrometers, that do not have moving parts and less sensitive to vibrations than FTIRs. We study sensor configurations based on the following infrared light sources: commercially available tunable quantum cascade lasers (QCLs), thermal sources, superluminescent diodes (SLDs), and superlattice light emitting diodes (SLEDs). Performance of the different sensor systems is evaluated and optimized to achieve maximum signal-to-noise ratio (SNR). We also propose a sensing limit variable, S, to characterize different sensor configurations. The work presented in this manuscript provides design options for agricultural nitrate sensors that can be rugged in the field and collect spectral data at fast rates with high SNR.