The on-line measurement of soil properties is essential for site specific application of different input into agricultural soils. This paper reports primary results on on-line measurement of total carbon (C-tot), moisture content (MC), pH and available phosphorus(p-avl) using a soil sensor developed for on-line measurement od soil properties. The sensor consisted of a soil penetration unit (subsoiler), to which the optical unit to acquire spectra from the bottom of the trench opened by the subsoiler is attached. A mobile, fiber-type, visible (VIS) and near infrared (NIR) spectrophotometer (Zeiss Corona 45 visnir fiber) with measurement range of 306.5-1710.9 nm was used to measure soil spectra in reflectancy mode. Global calibration models were established under non-mobile laboratory environment, considering samples collected from large areas covering Belgium and northern France. These models were developed using partial least squares (PLS) regression analysis coupled with the leave one-out cross-validation technique. On the basis of the values of determination coefficient (R2) and the ratio of standard deviation of calibration set (SD) to root mean square error of cross validation (RMSECV), the prediction of MC was evaluated as good, whereas the prediction of C-to, pH and P-avl were evaluated as only possible to provide quantitative approximations. When these models were used for on-line measurement methods. Mean error value of 5.97%, 0.3%, 5.10% and 27.48% were found between the on-line and reference measurements of C-tot, MC, pH and p-avl, respectively, suggesting the potential use of the VIS-NIR sensing system for on-line measurement of soil properties.
