Optimum three-point linkage set up for improving the quality of soil spectra and the accuracy of soil phosphorus measured using an on-line visible and near infrared sensor

• Published in: Soil & tillage research Vol. 103; no. 1; pp. 144 - 152
• Main Authors: Mouazen, A.M., Maleki, M.R., Cockx, L., Van Meirvenne, M., Van Holm, L.H.J., Merckx, R., De Baerdemaeker, J., Ramon, H.
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 01-04-2009; [Amsterdam]: Elsevier Science; Elsevier
• Subjects: accuracy; Agronomy. Soil science and plant productions; angle variations; Biological and medical sciences; Fundamental and applied biological sciences. Psychology; measurement; methodology; Near infrared; On-line; phosphorus; prediction; sensors; soil analysis; soil properties; Soil science; Soil sensor; soil-to-sensor distance; spectral analysis; spectral quality; Spectrophotometer; spectrophotometers; spectroscopy; Visible; Visible; On-line; Soil sensor; Spectrophotometer; Near infrared; Near infrared radiation; Soils; Accuracy; Measurement sensor; Phosphorus; Soil science; Measurement method
• ISSN: 0167-1987; 1879-3444
• DOI: 10.1016/j.still.2008.10.006

On-line measurement of soil properties using the visible (Vis) and near infrared (NIR) spectroscopy is sensitive to soil-to-sensor distance (
D) and angle (
α) variations, which have prevented the successful development of on-line soil sensors so far. This study was undertaken to minimise these variations through optimising the three-point linkage of the tractor to improve the quality of soil spectra and the accuracy of plant available phosphorus (P-avl) measured with an on-line soil sensor. The sensor consisted of a tine, to the back of which an optical probe was attached to acquire soil spectra in diffuse reflectance mode from the bottom of the trench opened by the tine. A mobile, fibre-type, Vis–NIR spectrophotometer (Zeiss Corona 45 visnir fibre, Germany), with a measurement range of 306.5–1710.9
nm was used. Five lengths of the third point link (
L) of the tractor of 545, 550, 555, 560 and 565
mm were selected to evaluate the quality of spectra collected on-line at 0.15
m tine depth. The on-line measured spectra were corrected to remove the effect of
D and
α. The correction was evaluated by estimating the accuracy of predicting P-avl using on-line measured spectra and a previously developed P-avl calibration model.
Results showed that the best quality of spectra measured on-line was obtained for
L of 555
mm, at which
D and
α vanished. This finding was supported by the maximum value of average maximum reflectance (AMR) of 75.7% obtained and by 100% successfully collected spectra. The worst quality of spectra was obtained at
L of 545
mm, with the largest
D of 6
mm and the largest
α of 0.6°. Values of
L of 560 and 565
mm led to a decrease in the AMR (43.3 and 33.2%, respectively), while recording 100% successful spectra. Correction of on-line measured spectra led to clear improvements in the accuracy of on-line measured P-avl. A lower root mean square error (RMSE) of 1.07
mg 100
g
−1 and higher ratio of prediction deviations (RPD) of 1.42 were obtained with corrected spectra as compared to uncorrected spectra (RMSE
=
1.15 and RPD
=
1.39). In addition, the correction of spectra resulted in an increase in the degree of similarity between laboratory and on-line measured P-avl maps by 29.6%. These results suggest the need for optimising the tractor hydraulic three-point linkage set up, and for spectra correction in order to improve the accuracy of on-line measured soil properties.