As the inherent spatial variability of soil phosphorus (P) within a field is considerable, variable-rate (VR) technology of P fertilisation should be implemented over small areas (e.g. 1m2). The objective of this study was to design and implement a soil sensor-based VR fertilisation system for on-the-go application of phosphate (P2O5) during maize planting. An on-the-go visible (VIS) and near-infrared (NIR) soil sensor with a measurement range of 305–1711nm was installed at the front of a planter-applicator for on-the-go measurement of soil P. A previously developed VIS–NIR model was used to predict the extractable phosphorous (P-ext) and a custom-built LabVIEW programme was developed to record soil spectra, predict soil P-ext, calculate phosphate during on-the-go measurement and provide the signal to the fertiliser applicator to adjust the application rate. Alternate plots were used for VR application and for uniform-rate (UR) treatment. The number of plant leaves and grain yield were measured as growth indices that may be influenced by P deficiency. The coefficient of variation (CV) of P-ext measured on-the-go ranged from 5% to 51% while variation of phosphate ranged from 36% to 76% over the experimental plots. The average phosphate applied on VR plots was 28.75, 1.25 kg ha1 less than the UR (30 kg ha1) recommended according to the standard soil test. The application rate of the phosphate ranged from 0 to 100 kg ha1 in the VR plots. Lower variation in plant leaves was observed in plots with VR treatment, possibly indicating better P distribution over the VR plots. The number of plant leaves variations were 25% and 31% for VR and UR plots, respectively. However, there was no significant difference between VR and UR plots. The maize yield was significantly higher (336 kg ha1) and less variable on plots that received VR treatment.
