On-line measurement of some selected soil properties using a VIS–NIR sensor

The on-line measurement of soil properties is essential for site-specific application of different inputs into agricultural soils. Using a previously developed soil sensor for on-line measurement of soil properties, primary results about carbon (C), moisture content (MC), pH and phosphorous (P) were reported. The on-line sensor consisted of a soil penetration unit (subsoiler), to which the optical probe to acquire soil spectra from the bottom of the trench opened by the subsoiler chisel is attached. A mobile, fibre-type, visible (VIS) and near infrared (NIR) spectrophotometer (Zeiss Corona 45 visnir fibre, Germany), with a measurement range of 306.5–1710.9 nm was used to measure soil spectra in reflectance mode. General calibration models were established under non-mobile laboratory conditions, on the basis of two sample sets collected from large geographical areas covering Belgium and northern France. These models were developed using partial least squares (PLS) regression coupled with the full cross-validation technique. On the basis of the values of coefficient of multiple determination (R2) and the ratio of standard deviation of calibration set (S.D.) to root mean square error of cross-validation (RMSECV), the prediction of MC was evaluated as good, whereas the predictions of C, pH and P were evaluated as only possible to provide quantitative approximations. When these models were validated using on-line measured spectra, they provided moderately similar maps to those measured with the reference methods. The best similarity was obtained for MC maps. Mean error values of 5.97, 0.37, 27.48 and 5.10% were found between the on-line and reference measurements of total carbon (C-tot), MC, pH and available phosphorous (P-avl), respectively, suggesting the potential use of the VIS–NIR sensing system for on-line measurement of soil properties.