This study was undertaken to optimise the position of an "on-the-go" soil sensor installed on a variable rate (VR) granular fertiliser application system. Optimisation was carried out by measuring the lag time between acquiring the information on soil phosphorus (P) from the soil sensor and changing the fertiliser rate on the fertilisation system. The sensing system consisted of a soil sensor coupled with a Ziess spectrophotometer (CORONA fibre VISNIR 1.7). The applicator was a four-row planter- applicator from AMAZONE (ED302). The overall lag time consisted of a series of sub lag times consisting of the times required to collect soil spectra, compile data, change the fertiliser rate and the travel time of granules from the fertiliser metering devices to furrow openers. The static and dynamic tests were carried out to measure these lag times. For the dynamic test, a hydraulic shaker (Four Poster Vibratory Stands) was used to simulate the working movement under field conditions. The results revealed that the important factor for positioning the soil sensor at the front of the "on-the-go" fertiliser application system is tractor speed. The results showed that the soil sensor should be installed at the front of the tractor and an extra time delay should be artificially included. (c) 2008 lAgrE. Published by Elsevier Ltd. All rights reserved.