<p>Spectral vegetation indices are used for non-invasive monitoring of plant health and evaluation of plant nitrogen status, chlorophyll content, and green leaf biomass. In particular, normalized difference vegetative index (NDVI) is commonly used to evaluate plant health by measuring reflected near infrared (NIR) light against visible red light. This information is applied to improve the precision of fertilizer application, irrigation, and other field treatment activities. However, commercially available spectral sensors are cost-intensive, which limits its accessibility to small-scale farmers and further spectral vegetation index studies in developing countries. </p> <p>Thus, we developed a portable, low cost, open source spectral sensor, and validated its accuracy against a more established but expensive reference spectral sensor (SKR 1840(ND) with data logger component CR1000). Our low cost spectral sensor consists of SparkFun RedBoard microcontroller that is programmable in Arduino IDE. The device is mounted on a 1.8 pole to ensure similar sensor height from the reference sensor used. The program was developed and uploaded using open source Arduino Software (IDE) version 1.8.15 Additional components include Sparkfun Qwiic OpenLog (compatible from 64 MB to 32 GB microSD card) and SparkFun Real Time Clock Module (RV-1805 Qwiic). For the spectral sensors, both upward and downward facing SparkFun Spectral Sensor Breakout AS7263 NIR and AS7262 VIS spectral sensors were used. Here, we present the field validation of our portable low cost spectral sensor and the result of the year-long NDVI measurement of pineapple plants under various pineapple residue treatments in our Philippine field site. Plant height, number of leaves and leaf chlorophyll concentration were also measured in parallel to compare NDVI measurements from various pineapple residue treatments. Similar low cost sensor was developed and deployed for NDVI measurements in the field site in Southern Benin to evaluate rice crop development under various N fertilizer amount and water management practices.</p>