The United Nations marks responsible consumption and production as one of the 17 key goals to fulfill the 2030 Agenda for Sustainable Development. In this context, affordable precision instruments can play a significant role in the optimization of crops in developing countries where precision agriculture tools are barely available. In this work, a simple to use, cost-effective instrument for spectral analysis of plants and fruits based on open source hardware and software has been developed. The instrument is a 7-band spectrophotometer equipped with a microprocessor that allows one to acquire the reflectance spectrum of samples and compute up to 9 vegetation indices. The accuracy in reflectance measurements is between 0.4% and 1.4% full scale, just above that of high-end spectrophotometers, while the precision at determining the normalized difference vegetation index (NDVI) is 0.61%, between 3 and 6 times better than more expensive commercial instruments. Some use cases of this instrument have been tested, and the prototype has proven to be able to precisely monitor minute spectral changes of different plants and fruits under different conditions, most of them before they were perceptible to the bare eye. This kind of information is essential in the decision-making process regarding harvesting, watering, or pest control, allowing precise control of crops. Given the low cost (less than USD 100) and open source architecture of this instrument, it is an affordable tool to bring precision agriculture techniques to small farmers in developing countries.