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Variable rate application is the process of applying different rates of crop inputs according to the variability within an agricultural field. Variable rate fertilizer application is a technology that regulates the fertilizer application rate based on site-specific needs within a field. A GPS-based variable rate fertilizer application (VRFA) system was developed, which consisted of a differential global positioning system (DGPS), micro-processor, micro-controller, DC motor actuator, power supply, threaded screw arrangement and fluted roller metering mechanism. The digital soil nutrient availability and application maps for targeted yield were also developed. DGPS was used for real-time identification of grids. Based on the microcontroller algorithm, application rates were varied by changing the feed roller exposure length. The observed fertilizer application rate was 5 and 300 kg/ha for exposure length of 0 and 44 mm respectively. The results indicate that the fertilizer application rate changes according to the prescribed application rate at the identified grid with coefficient of variation of 11.7-15%. The values of root mean square error and relative difference of the system for different levels of application rates were 2.62 and 3.71 respectively. It can be concluded that the developed VRFA system closely meets the target fertilizer application rate at the selected grid point.

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Spectral reflectance in the near‐infrared (NIR) and short‐wave–infrared (SWIR) regions shows decreasing water absorption troughs upon crop water stress. In this study, decreasing water absorption troughs were identified from reflectance of irrigated wheat crop. The crop was grown under two different irrigation methods, sprinkler and flood irrigation. Crop reflectance was measured using an ASD FieldSpec 3 (350–2,500 nm) hand‐held spectroradiometer at 1 week before the critical growth stages. Among different growth stages, wheat crop was found to be under stress in crown root initiation, jointing, and flowering. Results based on crop yield, and raw and first derivative spectral analysis indicated that flood‐irrigated wheat had more stress compared with sprinkler‐irrigated wheat. Five water stress‐sensitive wavelengths (974; 1,195; 1,455; 1,791; and 1,935 nm) were identified from flood‐irrigated wheat. Among them, two wavelengths (974 and 1,195 nm) were found to be highly water sensitive and good indicators for water stress. Detecting crop water stress prior to the critical growth stages of wheat helps in proper irrigation scheduling.

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