Common types of soil moisture sensors used in VRI systems include tensiometers, resistive sensors, and capacitance sensors. Tensiometers measure soil water tension, while resistive sensors measure the electrical resistance of the soil, which is affected by moisture levels. Capacitance sensors measure the dielectric constant of the soil, which is related to its moisture content. These sensors provide real-time data on soil moisture levels, enabling farmers to make informed decisions about irrigation.