The understanding of soil fertility is fundamental to agricultural production. However, soils can show great variation both spatially and temporally. This chapter outlines the key challenges faced in monitoring such variation and the solutions that have been developed and further employed to aid in optimizing crop production. Sampling methods are described, with a focus on proximal soil sensing where a sensor is placed in the soil or at a very short distance from the soil surface. These types of sensors provide information which is particularly useful in developing high-resolution soil maps for use in precision agriculture. A wide array of sensors is presented followed by a section on how such sensors may be integrated into agricultural management. An overview of future trends in research and development in this field is also included.