Marín González, Omar and Kuang, Boyan and Quraishi, Mohammed Z. and Muñoz-García, Miguel Angel and Mouazen, Abdul Mounem
On-line measurement of soil properties without direct spectral response in near infrared spectral range.
"Soil & Tillage Research"
So far, the majority of reports on on-line measurement considered soil properties with direct spectral
responses in near infrared spectroscopy (NIRS). This work reports on the results of on-line measurement
of soil properties with indirect spectral responses, e.g. pH, cation exchange capacity (CEC), exchangeable
calcium (Caex) and exchangeable magnesium (Mgex) in one field in Bedfordshire in the UK. The on-line
sensor consisted of a subsoiler coupled with an AgroSpec mobile, fibre type, visible and near infrared
(vis–NIR) spectrophotometer (tec5 Technology for Spectroscopy, Germany), with a measurement range
305–2200 nm to acquire soil spectra in diffuse reflectance mode. General calibration models for the
studied soil properties were developed with a partial least squares regression (PLSR) with one-leave-out
cross validation, using spectra measured under non-mobile laboratory conditions of 160 soil samples
collected from different fields in four farms in Europe, namely, Czech Republic, Denmark, Netherland and
UK. A group of 25 samples independent from the calibration set was used as independent validation set.
Higher accuracy was obtained for laboratory scanning as compared to on-line scanning of the 25
independent samples. The prediction accuracy for the laboratory and on-line measurements was
classified as excellent/very good for pH (RPD = 2.69 and 2.14 and r2 = 0.86 and 0.78, respectively), and
moderately good for CEC (RPD = 1.77 and 1.61 and r2 = 0.68 and 0.62, respectively) and Mgex (RPD = 1.72
and 1.49 and r2 = 0.66 and 0.67, respectively). For Caex, very good accuracy was calculated for laboratory
method (RPD = 2.19 and r2 = 0.86), as compared to the poor accuracy reported for the on-line method
(RPD = 1.30 and r2 = 0.61). The ability of collecting large number of data points per field area (about
12,800 point per 21 ha) and the simultaneous analysis of several soil properties without direct spectral
response in the NIR range at relatively high operational speed and appreciable accuracy, encourage the
recommendation of the on-line measurement system for site specific fertilisation.