Measurement of Nitrate-Nitrogen Concentration in Soil based on Absorption Spectroscopy

Abstract

The nitrate-nitrogen concentration in soil is estimated based on the nitrate-nitrogen 200 nm absorption peak. In one embodiment, a device measures the attenuation spectrum of a soil-extractant mixture over a wavelength range that includes wavelengths in the vicinity of the 200 nm absorption peak (the spectral operating range) and then determines the nitrate-nitrogen concentration based on the attenuation spectrum.

1 - 36 . (canceled) 
     
     
         37 . A device for measuring a nitrate-nitrogen concentration in soil based on attenuation over a spectral operating range, the device comprising:
 a light source that generates light that spans a spectral operating range;   a detector having a sensitivity that spans the spectral operating range;   a sample chamber configured to contain a soil-extractant mixture, the light propagating from the light source to the detector and attenuated by the soil-extractant mixture in the sample chamber, the detector generating a soil spectral signal that indicates the light received by the detector at different wavelengths across the spectral operating range;   a processor that is coupled to the detector, and that estimates an attenuation spectrum of the soil-extractant mixture over the spectral operating range based on the soil spectral signal,   wherein the processor, when estimating the attenuation spectrum, removes interference in the soil-extractant mixture using one of spectral deconvolution and curve-fitting, and wherein the processor estimates the nitrate-nitrogen concentration based on the attenuation spectrum.   
     
     
         38 . The device of claim  1 , wherein the spectral operating range includes wavelengths proximate a nitrate-nitrogen absorption peak. 
     
     
         39 . The device of claim  2 , wherein the nitrate-nitrogen absorption peak includes a peak wavelength and estimate attenuation values for wavelengths greater than the peak wavelength. 
     
     
         40 . The device of claim  2 , wherein the peak wavelength is 200 nm. 
     
     
         41 . The device of claim  4 , wherein the 200 nm nitrate-nitrogen absorption peak includes estimated attenuation values for wavelengths greater than 200 nm. 
     
     
         42 . The device of claim  4 , wherein the processor:
 curve-fits the attenuation spectrum based on the nitrate-nitrogen 200 nm absorption peak;   receives a soil type and estimates the nitrate-nitrogen concentration further based on the soil type;   receives a soil conductivity and estimates the nitrate-nitrogen concentration further based on the soil conductivity;   applies a partial least squares regression to the attenuation spectrum to estimate the nitrate-nitrogen concentration;   is trained based on a set of absorption spectra and their corresponding nitrate-nitrogen concentrations;   estimates the nitrate-nitrogen concentration based on its training;   estimates the nitrate-nitrogen concentration as a function of a measurement time; and   extrapolates the estimates to a final estimated nitrate-nitrogen concentration.   
     
     
         43 . The device of claim  1 , wherein the processor removes interference in the soil-extractant mixture using at least one of spectral deconvolution or curve-fitting when estimating the attenuation spectrum. 
     
     
         44 . The device of claim  1 , wherein the processor estimates the attenuation spectrum based on the soil spectral signal, a reference spectral signal and a dark spectral signal; wherein the reference spectral signal is generated when the sample chamber contains extractant without soil; wherein the dark spectral signal is generated without light from a light source incident on the detector. 
     
     
         45 . The device of claim  1 , wherein, at different times, the sample chamber contains the soil-extractant mixture or extractant without soil; wherein the detector generates the soil spectral signal when the sample chamber contains the soil-extractant mixture; wherein the detector generates a reference spectral signal when the sample chamber contains extractant without soil; wherein the processor estimates the attenuation spectrum based on the soil spectral signal and the reference spectral signal. 
     
     
         46 . The device of claim  1 , further comprising a second sample chamber configured to contain extractant without soil; wherein the processor estimates the attenuation spectrum based on the soil spectral signal and a reference spectral signal; wherein the reference spectral signal is generated from the second sample chamber containing extractant without soil. 
     
     
         47 . The device of claim  1 , wherein further comprising a reference optical path from the light source to the detector; wherein the reference optical path is not attenuated by the soil-extractant mixture; wherein the processor estimates the attenuation spectrum based on the soil spectral signal and a reference spectral signal; wherein the reference spectral signal is generated based on the reference optical path. 
     
     
         48 . The device of claim  1 , wherein the light source includes two bulbs, one of which has a relatively stronger UV spectrum than the other, and which can be separately controlled. 
     
     
         49 . The device of claim  1 , further comprising a centrifuge for separating the soil-extractant mixture, the light propagating through the separated soil-extractant mixture. 
     
     
         50 . A method for measuring a nitrate-nitrogen concentration in soil based on attenuation over a spectral operating range, the method comprising:
 generating light that spans a spectral operating range;   providing a soil-extractant mixture that attenuates the light;   detecting the attenuated light;   generating a soil spectral signal that indicates the light detected at different wavelengths across the spectral operating range;   estimating an attenuation spectrum of the soil-extractant mixture over the spectral operating range based on the soil spectral signal;   estimating the nitrate-nitrogen concentration based on the attenuation spectrum, wherein estimating the attenuation spectrum includes removing interference in the soil-extractant mixture using one of spectral deconvolution and curve-fitting.   
     
     
         51 . The method of claim  14 , wherein estimating the attenuation spectrum includes removing interference in the soil-extractant mixture using at least one of spectral deconvolution and curve-fitting. 
     
     
         52 . The method of claim  14 , further comprising estimating the attenuation spectrum based on the soil spectral signal, a reference spectral signal and a dark spectral signal; wherein the reference spectral signal is generated when a sample chamber contains extractant without soil and the dark spectral signal is generated without light from a light source incident on a detector. 
     
     
         53 . The method of claim  14 , wherein:
 the spectral operating range includes wavelengths at least as short as 230 nm; and   the nitrate-nitrogen concentration is estimated based on the attenuation spectrum.   
     
     
         54 . A device for measuring a nitrate-nitrogen concentration in soil based on attenuation over a spectral operating range, the device comprising:
 a light source that generates light that spans a spectral operating range, the spectral operating range including wavelengths proximate a 200 nm nitrate-nitrogen absorption peak;   a detector having a sensitivity that spans the spectral operating range;   a sample chamber configured to contain a soil-extractant mixture, the light propagating from the light source to the detector and attenuated by the soil-extractant mixture in the sample chamber, the detector generating a soil spectral signal that indicates the light received by the detector at different wavelengths across the spectral operating range;   a processor that is coupled to the detector, and that estimates an attenuation spectrum of the soil-extractant mixture over the spectral operating range based on the soil spectral signal,   wherein the processor, when estimating the attenuation spectrum, removes interference in the soil-extractant mixture; wherein the 200 nm nitrate-nitrogen absorption peak includes estimated attenuation values for wavelengths greater than 200 nm, and wherein the processor estimates the nitrate-nitrogen concentration based on the attenuation spectrum.   
     
     
         55 . The device of claim  18 , wherein the processor removes interference in the soil-extractant mixture using spectral deconvolution. 
     
     
         56 . The device of claim  18 , wherein the processor removes interference in the soil-extractant mixture using curve-fitting.