Soil nitrate nitrogen sensing for tea garden based on diffuse reflectance spectroscopy

Authors researched the method of soil nitrate nitrogen content determination with diffuse reflectance spectroscopy for its rapidness and accuracy. The portable spectroradiometer, FieldSpec 3 with a full spectral wavelength of 400-2500 nm, was used to scan diffuse reflectance spectra of soil samples. Eighty-nine soil samples were selected according to different soil fertility, depth and sites, which covered wide range of nitrate nitrogen content. Soil samples were air-dried and sieved through 1-mm screen holes after grinding. Data validity of original spectra was checked and averaged. Sixty-three samples were used to establish the calibration model with the methods of standing-wave ratio after the cluster analysis by partial least squares. Ten samples were used to establish the prediction set and the calibration model was validated. After being preprocessed by the combination of first-order derivative and moving average filter, the calibration model with fifteen principal component factors was regarded as the best with the algorithm of principal constituent analysis and artificial neural networks, and the correlation coefficient of the calibration model was 0.9908. The root mean square error of calibration was 1.4528. The correlation coefficient between predicted values and real values was 0.7179. The results show that soil nitrate nitrogen content can be determined precisely with visible-near infrared spectra.