This paper presents an IoT sensing system for agricultural applications, which can monitor at real time the moisture content of grains stored in silo bags located in remote locations thus allowing to avoid costly grain losses and to optimize the logistics of post-harvesting operations. The system comprises a customized gateway that collects the sensor data and sends it to a satellite network as well as multiple sensing units, which can measure both the grain moisture content with a resolution of 0.1% and the air temperature and air humidity inside the silo bags that are used together to assess the quality and condition of grains. The details about the modeling, design and fabrication of the proposed system together with the experimental results obtained during field tests are reported.